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[dm-devel] [patch] block: fix flush machinery for stacking drivers with differring flush flags



Hi,

Commit ae1b1539622fb46e51b4d13b3f9e5f4c713f86ae, block: reimplement
FLUSH/FUA to support merge, introduced a performance regression when
running any sort of fsyncing workload using dm-multipath and certain
storage (in our case, an HP EVA).  The test I ran was fs_mark, and it
dropped from ~800 files/sec on ext4 to ~100 files/sec.  It turns out
that dm-multipath always advertised flush+fua support, and passed
commands on down the stack, where those flags used to get stripped off.
The above commit changed that behavior:

static inline struct request *__elv_next_request(struct request_queue *q)
{
        struct request *rq;

        while (1) {
-               while (!list_empty(&q->queue_head)) {
+               if (!list_empty(&q->queue_head)) {
                        rq = list_entry_rq(q->queue_head.next);
-                       if (!(rq->cmd_flags & (REQ_FLUSH | REQ_FUA)) ||
-                           (rq->cmd_flags & REQ_FLUSH_SEQ))
-                               return rq;
-                       rq = blk_do_flush(q, rq);
-                       if (rq)
-                               return rq;
+                       return rq;
                }

Note that previously, a command would come in here, have
REQ_FLUSH|REQ_FUA set, and then get handed off to blk_do_flush:

struct request *blk_do_flush(struct request_queue *q, struct request *rq)
{
        unsigned int fflags = q->flush_flags; /* may change, cache it */
        bool has_flush = fflags & REQ_FLUSH, has_fua = fflags & REQ_FUA;
        bool do_preflush = has_flush && (rq->cmd_flags & REQ_FLUSH);
        bool do_postflush = has_flush && !has_fua && (rq->cmd_flags &
        REQ_FUA);
        unsigned skip = 0;
...
        if (blk_rq_sectors(rq) && !do_preflush && !do_postflush) {
                rq->cmd_flags &= ~REQ_FLUSH;
		if (!has_fua)
			rq->cmd_flags &= ~REQ_FUA;
	        return rq;
	}

So, the flush machinery was bypassed in such cases (q->flush_flags == 0
&& rq->cmd_flags & (REQ_FLUSH|REQ_FUA)).

Now, however, we don't get into the flush machinery at all.  Instead,
__elv_next_request just hands a request with flush and fua bits set to
the scsi_request_fn, even if the underlying request_queue does not
support flush or fua.

The agreed upon approach is to fix the flush machinery to allow
stacking.  While this isn't used in practice (since there is only one
request-based dm target, and that target will now reflect the flush
flags of the underlying device), it does future-proof the solution, and
make it function as designed.

In order to make this work, I had to add a field to the struct request,
inside the flush structure (to store the original req->end_io).  Shaohua
had suggested overloading the union with rb_node and completion_data,
but the completion data is used by device mapper and can also be used by
other drivers.  So, I didn't see a way around the additional field.

I chose to short-circuit empty flush requests (when the flush flags
don't advertise flush) in blk_insert_cloned_request.  I don't see a huge
advantage to doing this inside blk_insert_flush, though it could be done
there as well.

I tested this patch on an HP EVA with both ext4 and xfs, and it recovers
the lost performance.  Comments and other testers, as always, are
appreciated.

Cheers,
Jeff

Signed-off-by: Jeff Moyer <jmoyer redhat com>

diff --git a/block/blk-core.c b/block/blk-core.c
index b850bed..7ee03c6 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -39,6 +39,8 @@ EXPORT_TRACEPOINT_SYMBOL_GPL(block_rq_remap);
 EXPORT_TRACEPOINT_SYMBOL_GPL(block_bio_complete);
 
 static int __make_request(struct request_queue *q, struct bio *bio);
+static bool blk_end_bidi_request(struct request *rq, int error,
+				 unsigned int nr_bytes, unsigned int bidi_bytes);
 
 /*
  * For the allocated request tables
@@ -1700,6 +1702,7 @@ EXPORT_SYMBOL_GPL(blk_rq_check_limits);
 int blk_insert_cloned_request(struct request_queue *q, struct request *rq)
 {
 	unsigned long flags;
+	int where = ELEVATOR_INSERT_BACK;
 
 	if (blk_rq_check_limits(q, rq))
 		return -EIO;
@@ -1708,6 +1711,20 @@ int blk_insert_cloned_request(struct request_queue *q, struct request *rq)
 	    should_fail_request(&rq->rq_disk->part0, blk_rq_bytes(rq)))
 		return -EIO;
 
+	if (rq->cmd_flags & (REQ_FLUSH|REQ_FUA)) {
+		/*
+		 * Filter empty flush requests here.  REQ_FLUSH_SEQ will
+		 * ensure that no I/O accounting is done for this request.
+		 */
+		if (!q->flush_flags && !blk_rq_sectors(rq)) {
+			blk_end_bidi_request(rq, 0, 0, 0);
+			return 0;
+		}
+		where = ELEVATOR_INSERT_FLUSH;
+		/* REQ_FLUSH_SEQ will be set again by the flush machinery */
+		rq->cmd_flags &= ~REQ_FLUSH_SEQ;
+	}
+
 	spin_lock_irqsave(q->queue_lock, flags);
 
 	/*
@@ -1716,7 +1733,7 @@ int blk_insert_cloned_request(struct request_queue *q, struct request *rq)
 	 */
 	BUG_ON(blk_queued_rq(rq));
 
-	add_acct_request(q, rq, ELEVATOR_INSERT_BACK);
+	add_acct_request(q, rq, where);
 	spin_unlock_irqrestore(q->queue_lock, flags);
 
 	return 0;
diff --git a/block/blk-flush.c b/block/blk-flush.c
index 2d162bd..2633a08 100644
--- a/block/blk-flush.c
+++ b/block/blk-flush.c
@@ -123,7 +123,7 @@ static void blk_flush_restore_request(struct request *rq)
 
 	/* make @rq a normal request */
 	rq->cmd_flags &= ~REQ_FLUSH_SEQ;
-	rq->end_io = NULL;
+	rq->end_io = rq->flush.saved_end_io;
 }
 
 /**
@@ -301,7 +301,6 @@ void blk_insert_flush(struct request *rq)
 	unsigned int fflags = q->flush_flags;	/* may change, cache */
 	unsigned int policy = blk_flush_policy(fflags, rq);
 
-	BUG_ON(rq->end_io);
 	BUG_ON(!rq->bio || rq->bio != rq->biotail);
 
 	/*
@@ -320,6 +319,7 @@ void blk_insert_flush(struct request *rq)
 	if ((policy & REQ_FSEQ_DATA) &&
 	    !(policy & (REQ_FSEQ_PREFLUSH | REQ_FSEQ_POSTFLUSH))) {
 		list_add_tail(&rq->queuelist, &q->queue_head);
+		blk_run_queue_async(q);
 		return;
 	}
 
@@ -330,6 +330,7 @@ void blk_insert_flush(struct request *rq)
 	memset(&rq->flush, 0, sizeof(rq->flush));
 	INIT_LIST_HEAD(&rq->flush.list);
 	rq->cmd_flags |= REQ_FLUSH_SEQ;
+	rq->flush.saved_end_io = rq->end_io; /* Usually NULL */
 	rq->end_io = flush_data_end_io;
 
 	blk_flush_complete_seq(rq, REQ_FSEQ_ACTIONS & ~policy, 0);
diff --git a/include/linux/blk_types.h b/include/linux/blk_types.h
index 6395692..60cfd24 100644
--- a/include/linux/blk_types.h
+++ b/include/linux/blk_types.h
@@ -168,7 +168,18 @@ enum rq_flag_bits {
 #define REQ_COMMON_MASK \
 	(REQ_WRITE | REQ_FAILFAST_MASK | REQ_SYNC | REQ_META | REQ_DISCARD | \
 	 REQ_NOIDLE | REQ_FLUSH | REQ_FUA | REQ_SECURE)
-#define REQ_CLONE_MASK		REQ_COMMON_MASK
+/*
+ * Cloned requests are inserted into the elevator via blk_insert_cloned_request.
+ * Because the flush flags exported by the request-based dm target may in
+ * theory be different from the flush flags of the underlying request_queue,
+ * we need to pass along information regarding whether a particular request
+ * is part of a flush sequence.  This is primarily used to complete I/Os early
+ * that would otherwise not be necessary (such as an empty flush for a request
+ * queue that does not support flush).  In such a case, the end_io path for
+ * the request would try to account the I/O instead of ignoring it, resulting
+ * in a null pointer dereference.
+ */
+#define REQ_CLONE_MASK		(REQ_COMMON_MASK | REQ_FLUSH_SEQ)
 
 #define REQ_RAHEAD		(1 << __REQ_RAHEAD)
 #define REQ_THROTTLED		(1 << __REQ_THROTTLED)
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index 0e67c45..7c12781 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -117,6 +117,7 @@ struct request {
 		struct {
 			unsigned int		seq;
 			struct list_head	list;
+			rq_end_io_fn *saved_end_io;
 		} flush;
 	};
 


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