[dm-devel] [PATCH 2/7] dm: add reserved_rq_based_ios module parameter

[Mikulas Patocka]

I think this is too complicated.

dm-bufio uses similar approach like this patch - but in dm-bufio the code 
path is performance critical, we don't want to recalculate memory size 
with each i/o request.

Here it is not performance critical, so I'd drop the mutex, drop the 
latch, drop the function __reserved_request_based_ios_refresh and add only 
these lines:

pool_size = ACCESS_ONCE(reserved_rq_based_ios);
if (!pool_size) pool_size = RESERVED_REQUEST_BASED_IOS;

Mikulas


On Thu, 12 Sep 2013, Mike Snitzer wrote:

> Allow user to change the number of IOs that are reserved by
> request-based DM's mempools by writing to this file:
> /sys/module/dm_mod/parameters/reserved_rq_based_ios
> 
> The default value is RESERVED_REQUEST_BASED_IOS (256).
> 
> Signed-off-by: Mike Snitzer <snitzer at redhat.com>
> ---
>  drivers/md/dm.c | 49 ++++++++++++++++++++++++++++++++++++++++++++++++-
>  1 file changed, 48 insertions(+), 1 deletion(-)
> 
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index 47bac14..8553d03 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -216,6 +216,38 @@ struct dm_md_mempools {
>  static struct kmem_cache *_io_cache;
>  static struct kmem_cache *_rq_tio_cache;
>  
> +/*
> + * Request-based DM's mempools' reserved IOs set by the user
> + */
> +static unsigned reserved_rq_based_ios;
> +
> +/*
> + * A copy of reserved_rq_based_ios because it can change anytime.
> + * If values disagree, the user has changed reserved_rq_based_ios.
> + */
> +static unsigned reserved_rq_based_ios_latch;
> +
> +/*
> + * This mutex protects reserved_rq_based_ios_latch.
> + */
> +static DEFINE_MUTEX(dm_mempools_lock);
> +
> +static void __reserved_request_based_ios_refresh(void)
> +{
> +	BUG_ON(!mutex_is_locked(&dm_mempools_lock));
> +
> +	reserved_rq_based_ios_latch = ACCESS_ONCE(reserved_rq_based_ios);
> +
> +	/*
> +	 * If the user uses "0", it means default.  Modify
> +	 * reserved_rq_based_ios to report the default to the user.
> +	 */
> +	if (!reserved_rq_based_ios_latch) {
> +		(void)cmpxchg(&reserved_rq_based_ios, 0, RESERVED_REQUEST_BASED_IOS);
> +		reserved_rq_based_ios_latch = reserved_rq_based_ios;
> +	}
> +}
> +
>  static int __init local_init(void)
>  {
>  	int r = -ENOMEM;
> @@ -241,6 +273,10 @@ static int __init local_init(void)
>  	if (!_major)
>  		_major = r;
>  
> +	mutex_lock(&dm_mempools_lock);
> +	__reserved_request_based_ios_refresh();
> +	mutex_unlock(&dm_mempools_lock);
> +
>  	return 0;
>  
>  out_uevent_exit:
> @@ -2867,7 +2903,14 @@ struct dm_md_mempools *dm_alloc_md_mempools(unsigned type, unsigned integrity, u
>  		front_pad = roundup(per_bio_data_size, __alignof__(struct dm_target_io)) + offsetof(struct dm_target_io, clone);
>  	} else if (type == DM_TYPE_REQUEST_BASED) {
>  		cachep = _rq_tio_cache;
> -		pool_size = RESERVED_REQUEST_BASED_IOS;
> +
> +		mutex_lock(&dm_mempools_lock);
> +		/* Check if reserved_rq_based_ios changed. */
> +		if (reserved_rq_based_ios != reserved_rq_based_ios_latch)
> +			__reserved_request_based_ios_refresh();
> +		pool_size = reserved_rq_based_ios_latch;
> +		mutex_unlock(&dm_mempools_lock);
> +
>  		front_pad = offsetof(struct dm_rq_clone_bio_info, clone);
>  		/* per_bio_data_size is not used. See __bind_mempools(). */
>  		WARN_ON(per_bio_data_size != 0);
> @@ -2925,6 +2968,10 @@ module_exit(dm_exit);
>  
>  module_param(major, uint, 0);
>  MODULE_PARM_DESC(major, "The major number of the device mapper");
> +
> +module_param(reserved_rq_based_ios, uint, S_IRUGO | S_IWUSR);
> +MODULE_PARM_DESC(reserved_rq_based_ios, "Reserved IOs in request-based mempools");
> +
>  MODULE_DESCRIPTION(DM_NAME " driver");
>  MODULE_AUTHOR("Joe Thornber <dm-devel at redhat.com>");
>  MODULE_LICENSE("GPL");
> -- 
> 1.8.1.4
>