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[dm-devel] Re: [PATCH 20/23] io-controller: Per cgroup request descriptor support



On Fri, Aug 28, 2009 at 2:31 PM, Vivek Goyal <vgoyal redhat com> wrote:
> o Currently a request queue has got fixed number of request descriptors for
>  sync and async requests. Once the request descriptors are consumed, new
>  processes are put to sleep and they effectively become serialized. Because
>  sync and async queues are separate, async requests don't impact sync ones
>  but if one is looking for fairness between async requests, that is not
>  achievable if request queue descriptors become bottleneck.
>
> o Make request descriptor's per io group so that if there is lots of IO
>  going on in one cgroup, it does not impact the IO of other group.
>
> o This patch implements the per cgroup request descriptors. request pool per
>  queue is still common but every group will have its own wait list and its
>  own count of request descriptors allocated to that group for sync and async
>  queues. So effectively request_list becomes per io group property and not a
>  global request queue feature.
>
> o Currently one can define q->nr_requests to limit request descriptors
>  allocated for the queue. Now there is another tunable q->nr_group_requests
>  which controls the requests descriptr limit per group. q->nr_requests
>  supercedes q->nr_group_requests to make sure if there are lots of groups
>  present, we don't end up allocating too many request descriptors on the
>  queue.
>
> Signed-off-by: Nauman Rafique <nauman google com>
> Signed-off-by: Vivek Goyal <vgoyal redhat com>
> ---
>  block/blk-core.c             |  317 +++++++++++++++++++++++++++++++++---------
>  block/blk-settings.c         |    1 +
>  block/blk-sysfs.c            |   59 ++++++--
>  block/elevator-fq.c          |   36 +++++
>  block/elevator-fq.h          |   29 ++++
>  block/elevator.c             |    7 +-
>  include/linux/blkdev.h       |   47 ++++++-
>  include/trace/events/block.h |    6 +-
>  kernel/trace/blktrace.c      |    6 +-
>  9 files changed, 421 insertions(+), 87 deletions(-)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 47cce59..18b400b 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -460,20 +460,53 @@ void blk_cleanup_queue(struct request_queue *q)
>  }
>  EXPORT_SYMBOL(blk_cleanup_queue);
>
> -static int blk_init_free_list(struct request_queue *q)
> +struct request_list *
> +blk_get_request_list(struct request_queue *q, struct bio *bio)
> +{
> +#ifdef CONFIG_GROUP_IOSCHED
> +       /*
> +        * Determine which request list bio will be allocated from. This
> +        * is dependent on which io group bio belongs to
> +        */
> +       return elv_get_request_list_bio(q, bio);
> +#else
> +       return &q->rq;
> +#endif
> +}
> +
> +static struct request_list *rq_rl(struct request_queue *q, struct request *rq)
> +{
> +#ifdef CONFIG_GROUP_IOSCHED
> +       int priv = rq->cmd_flags & REQ_ELVPRIV;
> +
> +       return elv_get_request_list_rq(q, rq, priv);
> +#else
> +       return &q->rq;
> +#endif
> +}
> +
> +void blk_init_request_list(struct request_list *rl)
>  {
> -       struct request_list *rl = &q->rq;
>
>        rl->count[BLK_RW_SYNC] = rl->count[BLK_RW_ASYNC] = 0;
> -       rl->starved[BLK_RW_SYNC] = rl->starved[BLK_RW_ASYNC] = 0;
> -       rl->elvpriv = 0;
>        init_waitqueue_head(&rl->wait[BLK_RW_SYNC]);
>        init_waitqueue_head(&rl->wait[BLK_RW_ASYNC]);
> +}
>
> -       rl->rq_pool = mempool_create_node(BLKDEV_MIN_RQ, mempool_alloc_slab,
> -                               mempool_free_slab, request_cachep, q->node);
> +static int blk_init_free_list(struct request_queue *q)
> +{
> +       /*
> +        * In case of group scheduling, request list is inside group and is
> +        * initialized when group is instanciated.
> +        */
> +#ifndef CONFIG_GROUP_IOSCHED
> +       blk_init_request_list(&q->rq);
> +#endif
> +       q->rq_data.rq_pool = mempool_create_node(BLKDEV_MIN_RQ,
> +                               mempool_alloc_slab, mempool_free_slab,
> +                               request_cachep, q->node);
>
> -       if (!rl->rq_pool)
> +       if (!q->rq_data.rq_pool)
>                return -ENOMEM;
>
>        return 0;
> @@ -581,6 +614,9 @@ blk_init_queue_node(request_fn_proc *rfn, spinlock_t *lock, int node_id)
>        q->queue_flags          = QUEUE_FLAG_DEFAULT;
>        q->queue_lock           = lock;
>
> +       /* init starved waiter wait queue */
> +       init_waitqueue_head(&q->rq_data.starved_wait);
> +
>        /*
>         * This also sets hw/phys segments, boundary and size
>         */
> @@ -615,14 +651,14 @@ static inline void blk_free_request(struct request_queue *q, struct request *rq)
>  {
>        if (rq->cmd_flags & REQ_ELVPRIV)
>                elv_put_request(q, rq);
> -       mempool_free(rq, q->rq.rq_pool);
> +       mempool_free(rq, q->rq_data.rq_pool);
>  }
>
>  static struct request *
>  blk_alloc_request(struct request_queue *q, struct bio *bio, int flags, int priv,
>                                        gfp_t gfp_mask)
>  {
> -       struct request *rq = mempool_alloc(q->rq.rq_pool, gfp_mask);
> +       struct request *rq = mempool_alloc(q->rq_data.rq_pool, gfp_mask);
>
>        if (!rq)
>                return NULL;
> @@ -633,7 +669,7 @@ blk_alloc_request(struct request_queue *q, struct bio *bio, int flags, int priv,
>
>        if (priv) {
>                if (unlikely(elv_set_request(q, rq, bio, gfp_mask))) {
> -                       mempool_free(rq, q->rq.rq_pool);
> +                       mempool_free(rq, q->rq_data.rq_pool);
>                        return NULL;
>                }
>                rq->cmd_flags |= REQ_ELVPRIV;
> @@ -676,18 +712,18 @@ static void ioc_set_batching(struct request_queue *q, struct io_context *ioc)
>        ioc->last_waited = jiffies;
>  }
>
> -static void __freed_request(struct request_queue *q, int sync)
> +static void __freed_request(struct request_queue *q, int sync,
> +                                       struct request_list *rl)
>  {
> -       struct request_list *rl = &q->rq;
> -
> -       if (rl->count[sync] < queue_congestion_off_threshold(q))
> +       if (q->rq_data.count[sync] < queue_congestion_off_threshold(q))
>                blk_clear_queue_congested(q, sync);
>
> -       if (rl->count[sync] + 1 <= q->nr_requests) {
> +       if (q->rq_data.count[sync] + 1 <= q->nr_requests)
> +               blk_clear_queue_full(q, sync);
> +
> +       if (rl->count[sync] + 1 <= q->nr_group_requests) {
>                if (waitqueue_active(&rl->wait[sync]))
>                        wake_up(&rl->wait[sync]);
> -
> -               blk_clear_queue_full(q, sync);
>        }
>  }
>
> @@ -695,63 +731,130 @@ static void __freed_request(struct request_queue *q, int sync)
>  * A request has just been released.  Account for it, update the full and
>  * congestion status, wake up any waiters.   Called under q->queue_lock.
>  */
> -static void freed_request(struct request_queue *q, int sync, int priv)
> +static void freed_request(struct request_queue *q, int sync, int priv,
> +                                       struct request_list *rl)
>  {
> -       struct request_list *rl = &q->rq;
> +       /*
> +        * There is a window during request allocation where request is
> +        * mapped to one group but by the time a queue for the group is
> +        * allocated, it is possible that original cgroup/io group has been
> +        * deleted and now io queue is allocated in a different group (root)
> +        * altogether.
> +        *
> +        * One solution to the problem is that rq should take io group
> +        * reference. But it looks too much to do that to solve this issue.
> +        * The only side affect to the hard to hit issue seems to be that
> +        * we will try to decrement the rl->count for a request list which
> +        * did not allocate that request. Chcek for rl->count going less than
> +        * zero and do not decrement it if that's the case.
> +        */
> +
> +       if (priv && rl->count[sync] > 0)
> +               rl->count[sync]--;
> +
> +       BUG_ON(!q->rq_data.count[sync]);
> +       q->rq_data.count[sync]--;
>
> -       rl->count[sync]--;
>        if (priv)
> -               rl->elvpriv--;
> +               q->rq_data.elvpriv--;
>
> -       __freed_request(q, sync);
> +       __freed_request(q, sync, rl);
>
>        if (unlikely(rl->starved[sync ^ 1]))
> -               __freed_request(q, sync ^ 1);
> +               __freed_request(q, sync ^ 1, rl);
> +
> +       /* Wake up the starved process on global list, if any */
> +       if (unlikely(q->rq_data.starved)) {
> +               if (waitqueue_active(&q->rq_data.starved_wait))
> +                       wake_up(&q->rq_data.starved_wait);
> +               q->rq_data.starved--;
> +       }
> +}
> +
> +/*
> + * Returns whether one can sleep on this request list or not. There are
> + * cases (elevator switch) where request list might not have allocated
> + * any request descriptor but we deny request allocation due to gloabl
> + * limits. In that case one should sleep on global list as on this request
> + * list no wakeup will take place.
> + *
> + * Also sets the request list starved flag if there are no requests pending
> + * in the direction of rq.
> + *
> + * Return 1 --> sleep on request list, 0 --> sleep on global list
> + */
> +static int can_sleep_on_request_list(struct request_list *rl, int is_sync)
> +{
> +       if (unlikely(rl->count[is_sync] == 0)) {
> +               /*
> +                * If there is a request pending in other direction
> +                * in same io group, then set the starved flag of
> +                * the group request list. Otherwise, we need to
> +                * make this process sleep in global starved list
> +                * to make sure it will not sleep indefinitely.
> +                */
> +               if (rl->count[is_sync ^ 1] != 0) {
> +                       rl->starved[is_sync] = 1;
> +                       return 1;
> +               } else
> +                       return 0;
> +       }
> +
> +       return 1;
>  }
>
>  /*
>  * Get a free request, queue_lock must be held.
> - * Returns NULL on failure, with queue_lock held.
> + * Returns NULL on failure, with queue_lock held. Also sets the "reason" field
> + * in case of failure. This reason field helps caller decide to whether sleep
> + * on per group list or global per queue list.
> + * reason = 0 sleep on per group list
> + * reason = 1 sleep on global list
> + *
>  * Returns !NULL on success, with queue_lock *not held*.
>  */
>  static struct request *get_request(struct request_queue *q, int rw_flags,
> -                                  struct bio *bio, gfp_t gfp_mask)
> +                                       struct bio *bio, gfp_t gfp_mask,
> +                                       struct request_list *rl, int *reason)
>  {
>        struct request *rq = NULL;
> -       struct request_list *rl = &q->rq;
>        struct io_context *ioc = NULL;
>        const bool is_sync = rw_is_sync(rw_flags) != 0;
>        int may_queue, priv;
> +       int sleep_on_global = 0;
>
>        may_queue = elv_may_queue(q, rw_flags);
>        if (may_queue == ELV_MQUEUE_NO)
>                goto rq_starved;
>
> -       if (rl->count[is_sync]+1 >= queue_congestion_on_threshold(q)) {
> -               if (rl->count[is_sync]+1 >= q->nr_requests) {
> -                       ioc = current_io_context(GFP_ATOMIC, q->node);
> -                       /*
> -                        * The queue will fill after this allocation, so set
> -                        * it as full, and mark this process as "batching".
> -                        * This process will be allowed to complete a batch of
> -                        * requests, others will be blocked.
> -                        */
> -                       if (!blk_queue_full(q, is_sync)) {
> -                               ioc_set_batching(q, ioc);
> -                               blk_set_queue_full(q, is_sync);
> -                       } else {
> -                               if (may_queue != ELV_MQUEUE_MUST
> -                                               && !ioc_batching(q, ioc)) {
> -                                       /*
> -                                        * The queue is full and the allocating
> -                                        * process is not a "batcher", and not
> -                                        * exempted by the IO scheduler
> -                                        */
> -                                       goto out;
> -                               }
> +       if (q->rq_data.count[is_sync]+1 >= queue_congestion_on_threshold(q))
> +               blk_set_queue_congested(q, is_sync);
> +
> +       /* queue full seems redundant now */
> +       if (q->rq_data.count[is_sync]+1 >= q->nr_requests)
> +               blk_set_queue_full(q, is_sync);
> +
> +       if (rl->count[is_sync]+1 >= q->nr_group_requests) {
> +               ioc = current_io_context(GFP_ATOMIC, q->node);
> +               /*
> +                * The queue request descriptor group will fill after this
> +                * allocation, so set it as full, and mark this process as
> +                * "batching". This process will be allowed to complete a
> +                * batch of requests, others will be blocked.
> +                */
> +               if (rl->count[is_sync] <= q->nr_group_requests)
> +                       ioc_set_batching(q, ioc);
> +               else {
> +                       if (may_queue != ELV_MQUEUE_MUST
> +                                       && !ioc_batching(q, ioc)) {
> +                               /*
> +                                * The queue is full and the allocating
> +                                * process is not a "batcher", and not
> +                                * exempted by the IO scheduler
> +                                */
> +                               goto out;
>                        }
>                }
> -               blk_set_queue_congested(q, is_sync);
>        }
>
>        /*
> @@ -759,21 +862,60 @@ static struct request *get_request(struct request_queue *q, int rw_flags,
>         * limit of requests, otherwise we could have thousands of requests
>         * allocated with any setting of ->nr_requests
>         */
> -       if (rl->count[is_sync] >= (3 * q->nr_requests / 2))
> +
> +       if (q->rq_data.count[is_sync] >= (3 * q->nr_requests / 2)) {
> +               /*
> +                * Queue is too full for allocation. On which request queue
> +                * the task should sleep? Generally it should sleep on its
> +                * request list but if elevator switch is happening, in that
> +                * window, request descriptors are allocated from global
> +                * pool and are not accounted against any particular request
> +                * list as group is going away.
> +                *
> +                * So it might happen that request list does not have any
> +                * requests allocated at all and if process sleeps on per
> +                * group request list, it will not be woken up. In such case,
> +                * make it sleep on global starved list.
> +                */
> +               if (test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags)
> +                   || !can_sleep_on_request_list(rl, is_sync))
> +                       sleep_on_global = 1;
> +               goto out;
> +       }
> +
> +       /*
> +        * Allocation of request is allowed from queue perspective. Now check
> +        * from per group request list
> +        */
> +
> +       if (rl->count[is_sync] >= (3 * q->nr_group_requests / 2))
>                goto out;
>
> -       rl->count[is_sync]++;
>        rl->starved[is_sync] = 0;
>
> +       q->rq_data.count[is_sync]++;
> +
>        priv = !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
> -       if (priv)
> -               rl->elvpriv++;
> +       if (priv) {
> +               q->rq_data.elvpriv++;
> +               /*
> +                * Account the request to request list only if request is
> +                * going to elevator. During elevator switch, there will
> +                * be small window where group is going away and new group
> +                * will not be allocated till elevator switch is complete.
> +                * So till then instead of slowing down the application,
> +                * we will continue to allocate request from total common
> +                * pool instead of per group limit
> +                */
> +               rl->count[is_sync]++;
> +       }
>
>        if (blk_queue_io_stat(q))
>                rw_flags |= REQ_IO_STAT;
>        spin_unlock_irq(q->queue_lock);
>
>        rq = blk_alloc_request(q, bio, rw_flags, priv, gfp_mask);
> +
>        if (unlikely(!rq)) {
>                /*
>                 * Allocation failed presumably due to memory. Undo anything
> @@ -783,7 +925,7 @@ static struct request *get_request(struct request_queue *q, int rw_flags,
>                 * wait queue, but this is pretty rare.
>                 */
>                spin_lock_irq(q->queue_lock);
> -               freed_request(q, is_sync, priv);
> +               freed_request(q, is_sync, priv, rl);
>
>                /*
>                 * in the very unlikely event that allocation failed and no
> @@ -793,9 +935,8 @@ static struct request *get_request(struct request_queue *q, int rw_flags,
>                 * rq mempool into READ and WRITE
>                 */
>  rq_starved:
> -               if (unlikely(rl->count[is_sync] == 0))
> -                       rl->starved[is_sync] = 1;
> -
> +               if (!can_sleep_on_request_list(rl, is_sync))
> +                       sleep_on_global = 1;
>                goto out;
>        }
>
> @@ -810,6 +951,8 @@ rq_starved:
>
>        trace_block_getrq(q, bio, rw_flags & 1);
>  out:
> +       if (reason && sleep_on_global)
> +               *reason = 1;
>        return rq;
>  }
>
> @@ -823,16 +966,39 @@ static struct request *get_request_wait(struct request_queue *q, int rw_flags,
>                                        struct bio *bio)
>  {
>        const bool is_sync = rw_is_sync(rw_flags) != 0;
> +       int sleep_on_global = 0;
>        struct request *rq;
> +       struct request_list *rl = blk_get_request_list(q, bio);
>
> -       rq = get_request(q, rw_flags, bio, GFP_NOIO);
> +       rq = get_request(q, rw_flags, bio, GFP_NOIO, rl, &sleep_on_global);
>        while (!rq) {
>                DEFINE_WAIT(wait);
>                struct io_context *ioc;
> -               struct request_list *rl = &q->rq;
>
> -               prepare_to_wait_exclusive(&rl->wait[is_sync], &wait,
> -                               TASK_UNINTERRUPTIBLE);
> +               if (sleep_on_global) {
> +                       /*
> +                        * Task failed allocation and needs to wait and
> +                        * try again. There are no requests pending from
> +                        * the io group hence need to sleep on global
> +                        * wait queue. Most likely the allocation failed
> +                        * because of memory issues.
> +                        */
> +
> +                       q->rq_data.starved++;
> +                       prepare_to_wait_exclusive(&q->rq_data.starved_wait,
> +                                       &wait, TASK_UNINTERRUPTIBLE);
> +               } else {
> +                       /*
> +                        * We are about to sleep on a request list and we
> +                        * drop queue lock. After waking up, we will do
> +                        * finish_wait() on request list and in the mean
> +                        * time group might be gone. Take a reference to
> +                        * the group now.
> +                        */
> +                       prepare_to_wait_exclusive(&rl->wait[is_sync], &wait,
> +                                       TASK_UNINTERRUPTIBLE);
> +                       elv_get_rl_iog(rl);
> +               }
>
>                trace_block_sleeprq(q, bio, rw_flags & 1);
>
> @@ -850,9 +1016,25 @@ static struct request *get_request_wait(struct request_queue *q, int rw_flags,
>                ioc_set_batching(q, ioc);
>
>                spin_lock_irq(q->queue_lock);
> -               finish_wait(&rl->wait[is_sync], &wait);
>
> -               rq = get_request(q, rw_flags, bio, GFP_NOIO);
> +               if (sleep_on_global) {
> +                       finish_wait(&q->rq_data.starved_wait, &wait);
> +                       sleep_on_global = 0;
> +               } else {
> +                       /*
> +                        * We had taken a reference to the rl/iog. Put that now
> +                        */
> +                       finish_wait(&rl->wait[is_sync], &wait);
> +                       elv_put_rl_iog(rl);
> +               }
> +
> +               /*
> +                * After the sleep check the rl again in case cgrop bio
> +                * belonged to is gone and it is mapped to root group now
> +                */
> +               rl = blk_get_request_list(q, bio);
> +               rq = get_request(q, rw_flags, bio, GFP_NOIO, rl,
> +                                       &sleep_on_global);
>        };
>
>        return rq;
> @@ -861,14 +1043,16 @@ static struct request *get_request_wait(struct request_queue *q, int rw_flags,
>  struct request *blk_get_request(struct request_queue *q, int rw, gfp_t gfp_mask)
>  {
>        struct request *rq;
> +       struct request_list *rl;
>
>        BUG_ON(rw != READ && rw != WRITE);
>
>        spin_lock_irq(q->queue_lock);
> +       rl = blk_get_request_list(q, NULL);
>        if (gfp_mask & __GFP_WAIT) {
>                rq = get_request_wait(q, rw, NULL);
>        } else {
> -               rq = get_request(q, rw, NULL, gfp_mask);
> +               rq = get_request(q, rw, NULL, gfp_mask, rl, NULL);
>                if (!rq)
>                        spin_unlock_irq(q->queue_lock);
>        }
> @@ -1085,12 +1269,13 @@ void __blk_put_request(struct request_queue *q, struct request *req)
>        if (req->cmd_flags & REQ_ALLOCED) {
>                int is_sync = rq_is_sync(req) != 0;
>                int priv = req->cmd_flags & REQ_ELVPRIV;
> +               struct request_list *rl = rq_rl(q, req);
>
>                BUG_ON(!list_empty(&req->queuelist));
>                BUG_ON(!hlist_unhashed(&req->hash));
>
>                blk_free_request(q, req);
> -               freed_request(q, is_sync, priv);
> +               freed_request(q, is_sync, priv, rl);

We have a potential memory bug here. freed_request should be called
before blk_free_request as blk_free_request might result in release of
cgroup, and request_list. Calling freed_request after blk_free_request
would result in operations on freed memory.

>        }
>  }
>  EXPORT_SYMBOL_GPL(__blk_put_request);
> diff --git a/block/blk-settings.c b/block/blk-settings.c
> index 476d870..c3102c7 100644
> --- a/block/blk-settings.c
> +++ b/block/blk-settings.c
> @@ -149,6 +149,7 @@ void blk_queue_make_request(struct request_queue *q, make_request_fn *mfn)
>         * set defaults
>         */
>        q->nr_requests = BLKDEV_MAX_RQ;
> +       q->nr_group_requests = BLKDEV_MAX_GROUP_RQ;
>
>        q->make_request_fn = mfn;
>        blk_queue_dma_alignment(q, 511);
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index 418d636..f3db7f0 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -38,42 +38,67 @@ static ssize_t queue_requests_show(struct request_queue *q, char *page)
>  static ssize_t
>  queue_requests_store(struct request_queue *q, const char *page, size_t count)
>  {
> -       struct request_list *rl = &q->rq;
> +       struct request_list *rl;
>        unsigned long nr;
>        int ret = queue_var_store(&nr, page, count);
>        if (nr < BLKDEV_MIN_RQ)
>                nr = BLKDEV_MIN_RQ;
>
>        spin_lock_irq(q->queue_lock);
> +       rl = blk_get_request_list(q, NULL);
>        q->nr_requests = nr;
>        blk_queue_congestion_threshold(q);
>
> -       if (rl->count[BLK_RW_SYNC] >= queue_congestion_on_threshold(q))
> +       if (q->rq_data.count[BLK_RW_SYNC] >= queue_congestion_on_threshold(q))
>                blk_set_queue_congested(q, BLK_RW_SYNC);
> -       else if (rl->count[BLK_RW_SYNC] < queue_congestion_off_threshold(q))
> +       else if (q->rq_data.count[BLK_RW_SYNC] <
> +                               queue_congestion_off_threshold(q))
>                blk_clear_queue_congested(q, BLK_RW_SYNC);
>
> -       if (rl->count[BLK_RW_ASYNC] >= queue_congestion_on_threshold(q))
> +       if (q->rq_data.count[BLK_RW_ASYNC] >= queue_congestion_on_threshold(q))
>                blk_set_queue_congested(q, BLK_RW_ASYNC);
> -       else if (rl->count[BLK_RW_ASYNC] < queue_congestion_off_threshold(q))
> +       else if (q->rq_data.count[BLK_RW_ASYNC] <
> +                               queue_congestion_off_threshold(q))
>                blk_clear_queue_congested(q, BLK_RW_ASYNC);
>
> -       if (rl->count[BLK_RW_SYNC] >= q->nr_requests) {
> +       if (q->rq_data.count[BLK_RW_SYNC] >= q->nr_requests) {
>                blk_set_queue_full(q, BLK_RW_SYNC);
> -       } else if (rl->count[BLK_RW_SYNC]+1 <= q->nr_requests) {
> +       } else if (q->rq_data.count[BLK_RW_SYNC]+1 <= q->nr_requests) {
>                blk_clear_queue_full(q, BLK_RW_SYNC);
>                wake_up(&rl->wait[BLK_RW_SYNC]);
>        }
>
> -       if (rl->count[BLK_RW_ASYNC] >= q->nr_requests) {
> +       if (q->rq_data.count[BLK_RW_ASYNC] >= q->nr_requests) {
>                blk_set_queue_full(q, BLK_RW_ASYNC);
> -       } else if (rl->count[BLK_RW_ASYNC]+1 <= q->nr_requests) {
> +       } else if (q->rq_data.count[BLK_RW_ASYNC]+1 <= q->nr_requests) {
>                blk_clear_queue_full(q, BLK_RW_ASYNC);
>                wake_up(&rl->wait[BLK_RW_ASYNC]);
>        }
>        spin_unlock_irq(q->queue_lock);
>        return ret;
>  }
> +#ifdef CONFIG_GROUP_IOSCHED
> +static ssize_t queue_group_requests_show(struct request_queue *q, char *page)
> +{
> +       return queue_var_show(q->nr_group_requests, (page));
> +}
> +
> +static ssize_t
> +queue_group_requests_store(struct request_queue *q, const char *page,
> +                                       size_t count)
> +{
> +       unsigned long nr;
> +       int ret = queue_var_store(&nr, page, count);
> +
> +       if (nr < BLKDEV_MIN_RQ)
> +               nr = BLKDEV_MIN_RQ;
> +
> +       spin_lock_irq(q->queue_lock);
> +       q->nr_group_requests = nr;
> +       spin_unlock_irq(q->queue_lock);
> +       return ret;
> +}
> +#endif
>
>  static ssize_t queue_ra_show(struct request_queue *q, char *page)
>  {
> @@ -240,6 +265,14 @@ static struct queue_sysfs_entry queue_requests_entry = {
>        .store = queue_requests_store,
>  };
>
> +#ifdef CONFIG_GROUP_IOSCHED
> +static struct queue_sysfs_entry queue_group_requests_entry = {
> +       .attr = {.name = "nr_group_requests", .mode = S_IRUGO | S_IWUSR },
> +       .show = queue_group_requests_show,
> +       .store = queue_group_requests_store,
> +};
> +#endif
> +
>  static struct queue_sysfs_entry queue_ra_entry = {
>        .attr = {.name = "read_ahead_kb", .mode = S_IRUGO | S_IWUSR },
>        .show = queue_ra_show,
> @@ -314,6 +347,9 @@ static struct queue_sysfs_entry queue_iostats_entry = {
>
>  static struct attribute *default_attrs[] = {
>        &queue_requests_entry.attr,
> +#ifdef CONFIG_GROUP_IOSCHED
> +       &queue_group_requests_entry.attr,
> +#endif
>        &queue_ra_entry.attr,
>        &queue_max_hw_sectors_entry.attr,
>        &queue_max_sectors_entry.attr,
> @@ -393,12 +429,11 @@ static void blk_release_queue(struct kobject *kobj)
>  {
>        struct request_queue *q =
>                container_of(kobj, struct request_queue, kobj);
> -       struct request_list *rl = &q->rq;
>
>        blk_sync_queue(q);
>
> -       if (rl->rq_pool)
> -               mempool_destroy(rl->rq_pool);
> +       if (q->rq_data.rq_pool)
> +               mempool_destroy(q->rq_data.rq_pool);
>
>        if (q->queue_tags)
>                __blk_queue_free_tags(q);
> diff --git a/block/elevator-fq.c b/block/elevator-fq.c
> index 9c8783c..39896c2 100644
> --- a/block/elevator-fq.c
> +++ b/block/elevator-fq.c
> @@ -925,6 +925,39 @@ static struct io_cgroup *cgroup_to_io_cgroup(struct cgroup *cgroup)
>                            struct io_cgroup, css);
>  }
>
> +struct request_list *
> +elv_get_request_list_bio(struct request_queue *q, struct bio *bio)
> +{
> +       struct io_group *iog;
> +
> +       if (!elv_iosched_fair_queuing_enabled(q->elevator))
> +               iog = q->elevator->efqd->root_group;
> +       else
> +               iog = elv_io_get_io_group_bio(q, bio, 1);
> +
> +       BUG_ON(!iog);
> +       return &iog->rl;
> +}
> +
> +struct request_list *
> +elv_get_request_list_rq(struct request_queue *q, struct request *rq, int priv)
> +{
> +       struct io_group *iog;
> +
> +       if (!elv_iosched_fair_queuing_enabled(q->elevator))
> +               return &q->elevator->efqd->root_group->rl;
> +
> +       BUG_ON(priv && !rq->ioq);
> +
> +       if (priv)
> +               iog = ioq_to_io_group(rq->ioq);
> +       else
> +               iog = q->elevator->efqd->root_group;
> +
> +       BUG_ON(!iog);
> +       return &iog->rl;
> +}
> +
>  /*
>  * Search the io_group for efqd into the hash table (by now only a list)
>  * of bgrp.  Must be called under rcu_read_lock().
> @@ -1281,6 +1314,8 @@ io_group_chain_alloc(struct request_queue *q, void *key, struct cgroup *cgroup)
>                elv_get_iog(iog);
>                io_group_path(iog);
>
> +               blk_init_request_list(&iog->rl);
> +
>                if (leaf == NULL) {
>                        leaf = iog;
>                        prev = leaf;
> @@ -1502,6 +1537,7 @@ static struct io_group *io_alloc_root_group(struct request_queue *q,
>        for (i = 0; i < IO_IOPRIO_CLASSES; i++)
>                iog->sched_data.service_tree[i] = ELV_SERVICE_TREE_INIT;
>
> +       blk_init_request_list(&iog->rl);
>        spin_lock_irq(&iocg->lock);
>        rcu_assign_pointer(iog->key, key);
>        hlist_add_head_rcu(&iog->group_node, &iocg->group_data);
> diff --git a/block/elevator-fq.h b/block/elevator-fq.h
> index 9fe52fa..989102e 100644
> --- a/block/elevator-fq.h
> +++ b/block/elevator-fq.h
> @@ -128,6 +128,9 @@ struct io_group {
>
>        /* Single ioq per group, used for noop, deadline, anticipatory */
>        struct io_queue *ioq;
> +
> +       /* request list associated with the group */
> +       struct request_list rl;
>  };
>
>  struct io_cgroup {
> @@ -425,11 +428,31 @@ static inline void elv_get_iog(struct io_group *iog)
>        atomic_inc(&iog->ref);
>  }
>
> +static inline struct io_group *rl_iog(struct request_list *rl)
> +{
> +       return container_of(rl, struct io_group, rl);
> +}
> +
> +static inline void elv_get_rl_iog(struct request_list *rl)
> +{
> +       elv_get_iog(rl_iog(rl));
> +}
> +
> +static inline void elv_put_rl_iog(struct request_list *rl)
> +{
> +       elv_put_iog(rl_iog(rl));
> +}
> +
>  extern int elv_set_request_ioq(struct request_queue *q, struct request *rq,
>                                        struct bio *bio, gfp_t gfp_mask);
>  extern void elv_reset_request_ioq(struct request_queue *q, struct request *rq);
>  extern struct io_queue *elv_lookup_ioq_bio(struct request_queue *q,
>                                                struct bio *bio);
> +struct request_list *
> +elv_get_request_list_bio(struct request_queue *q, struct bio *bio);
> +
> +struct request_list *
> +elv_get_request_list_rq(struct request_queue *q, struct request *rq, int priv);
>
>  #else /* !GROUP_IOSCHED */
>
> @@ -469,6 +492,9 @@ elv_lookup_ioq_bio(struct request_queue *q, struct bio *bio)
>        return NULL;
>  }
>
> +static inline void elv_get_rl_iog(struct request_list *rl) { }
> +static inline void elv_put_rl_iog(struct request_list *rl) { }
> +
>  #endif /* GROUP_IOSCHED */
>
>  extern ssize_t elv_slice_sync_show(struct elevator_queue *q, char *name);
> @@ -578,6 +604,9 @@ static inline struct io_queue *elv_lookup_ioq_bio(struct request_queue *q,
>        return NULL;
>  }
>
> +static inline void elv_get_rl_iog(struct request_list *rl) { }
> +static inline void elv_put_rl_iog(struct request_list *rl) { }
> +
>  #endif /* CONFIG_ELV_FAIR_QUEUING */
>  #endif /* _ELV_SCHED_H */
>  #endif /* CONFIG_BLOCK */
> diff --git a/block/elevator.c b/block/elevator.c
> index 4ed37b6..b23db03 100644
> --- a/block/elevator.c
> +++ b/block/elevator.c
> @@ -678,7 +678,7 @@ void elv_quiesce_start(struct request_queue *q)
>         * make sure we don't have any requests in flight
>         */
>        elv_drain_elevator(q);
> -       while (q->rq.elvpriv) {
> +       while (q->rq_data.elvpriv) {
>                __blk_run_queue(q);
>                spin_unlock_irq(q->queue_lock);
>                msleep(10);
> @@ -777,8 +777,9 @@ void elv_insert(struct request_queue *q, struct request *rq, int where)
>        }
>
>        if (unplug_it && blk_queue_plugged(q)) {
> -               int nrq = q->rq.count[BLK_RW_SYNC] + q->rq.count[BLK_RW_ASYNC]
> -                               - queue_in_flight(q);
> +               int nrq = q->rq_data.count[BLK_RW_SYNC] +
> +                               q->rq_data.count[BLK_RW_ASYNC] -
> +                               queue_in_flight(q);
>
>                if (nrq >= q->unplug_thresh)
>                        __generic_unplug_device(q);
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index 7cff5f2..74deb17 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -32,21 +32,51 @@ struct request;
>  struct sg_io_hdr;
>
>  #define BLKDEV_MIN_RQ  4
> +
> +#ifdef CONFIG_GROUP_IOSCHED
> +#define BLKDEV_MAX_RQ  512     /* Default maximum for queue */
> +#define BLKDEV_MAX_GROUP_RQ    128      /* Default maximum per group*/
> +#else
>  #define BLKDEV_MAX_RQ  128     /* Default maximum */
> +/*
> + * This is eqivalent to case of only one group present (root group). Let
> + * it consume all the request descriptors available on the queue .
> + */
> +#define BLKDEV_MAX_GROUP_RQ    BLKDEV_MAX_RQ      /* Default maximum */
> +#endif
>
>  struct request;
>  typedef void (rq_end_io_fn)(struct request *, int);
>
>  struct request_list {
>        /*
> -        * count[], starved[], and wait[] are indexed by
> +        * count[], starved and wait[] are indexed by
>         * BLK_RW_SYNC/BLK_RW_ASYNC
>         */
>        int count[2];
>        int starved[2];
> +       wait_queue_head_t wait[2];
> +};
> +
> +/*
> + * This data structures keeps track of mempool of requests for the queue
> + * and some overall statistics.
> + */
> +struct request_data {
> +       /*
> +        * Per queue request descriptor count. This is in addition to per
> +        * cgroup count
> +        */
> +       int count[2];
>        int elvpriv;
>        mempool_t *rq_pool;
> -       wait_queue_head_t wait[2];
> +       int starved;
> +       /*
> +        * Global list for starved tasks. A task will be queued here if
> +        * it could not allocate request descriptor and the associated
> +        * group request list does not have any requests pending.
> +        */
> +       wait_queue_head_t starved_wait;
>  };
>
>  /*
> @@ -339,10 +369,17 @@ struct request_queue
>        struct request          *last_merge;
>        struct elevator_queue   *elevator;
>
> +#ifndef CONFIG_GROUP_IOSCHED
>        /*
>         * the queue request freelist, one for reads and one for writes
> +        * In case of group io scheduling, this request list is per group
> +        * and is present in group data structure.
>         */
>        struct request_list     rq;
> +#endif
> +
> +       /* Contains request pool and other data like starved data */
> +       struct request_data     rq_data;
>
>        request_fn_proc         *request_fn;
>        make_request_fn         *make_request_fn;
> @@ -405,6 +442,8 @@ struct request_queue
>         * queue settings
>         */
>        unsigned long           nr_requests;    /* Max # of requests */
> +       /* Max # of per io group requests */
> +       unsigned long           nr_group_requests;
>        unsigned int            nr_congestion_on;
>        unsigned int            nr_congestion_off;
>        unsigned int            nr_batching;
> @@ -784,6 +823,10 @@ extern int scsi_cmd_ioctl(struct request_queue *, struct gendisk *, fmode_t,
>  extern int sg_scsi_ioctl(struct request_queue *, struct gendisk *, fmode_t,
>                         struct scsi_ioctl_command __user *);
>
> +extern void blk_init_request_list(struct request_list *rl);
> +
> +extern struct request_list *blk_get_request_list(struct request_queue *q,
> +                                                       struct bio *bio);
>  /*
>  * A queue has just exitted congestion.  Note this in the global counter of
>  * congested queues, and wake up anyone who was waiting for requests to be
> diff --git a/include/trace/events/block.h b/include/trace/events/block.h
> index 9a74b46..af6c9e5 100644
> --- a/include/trace/events/block.h
> +++ b/include/trace/events/block.h
> @@ -397,7 +397,8 @@ TRACE_EVENT(block_unplug_timer,
>        ),
>
>        TP_fast_assign(
> -               __entry->nr_rq  = q->rq.count[READ] + q->rq.count[WRITE];
> +               __entry->nr_rq  = q->rq_data.count[READ] +
> +                                       q->rq_data.count[WRITE];
>                memcpy(__entry->comm, current->comm, TASK_COMM_LEN);
>        ),
>
> @@ -416,7 +417,8 @@ TRACE_EVENT(block_unplug_io,
>        ),
>
>        TP_fast_assign(
> -               __entry->nr_rq  = q->rq.count[READ] + q->rq.count[WRITE];
> +               __entry->nr_rq  = q->rq_data.count[READ] +
> +                                       q->rq_data.count[WRITE];
>                memcpy(__entry->comm, current->comm, TASK_COMM_LEN);
>        ),
>
> diff --git a/kernel/trace/blktrace.c b/kernel/trace/blktrace.c
> index 7a34cb5..9a03980 100644
> --- a/kernel/trace/blktrace.c
> +++ b/kernel/trace/blktrace.c
> @@ -786,7 +786,8 @@ static void blk_add_trace_unplug_io(struct request_queue *q)
>        struct blk_trace *bt = q->blk_trace;
>
>        if (bt) {
> -               unsigned int pdu = q->rq.count[READ] + q->rq.count[WRITE];
> +               unsigned int pdu = q->rq_data.count[READ] +
> +                                       q->rq_data.count[WRITE];
>                __be64 rpdu = cpu_to_be64(pdu);
>
>                __blk_add_trace(bt, 0, 0, 0, BLK_TA_UNPLUG_IO, 0,
> @@ -799,7 +800,8 @@ static void blk_add_trace_unplug_timer(struct request_queue *q)
>        struct blk_trace *bt = q->blk_trace;
>
>        if (bt) {
> -               unsigned int pdu = q->rq.count[READ] + q->rq.count[WRITE];
> +               unsigned int pdu = q->rq_data.count[READ] +
> +                                       q->rq_data.count[WRITE];
>                __be64 rpdu = cpu_to_be64(pdu);
>
>                __blk_add_trace(bt, 0, 0, 0, BLK_TA_UNPLUG_TIMER, 0,
> --
> 1.6.0.6
>
>


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