[PATCH] block: Limit number of items taken from the I/O scheduler in one go

[Salman Qazi <sqazi@google.com>]

Flushes bypass the I/O scheduler and get added to hctx->dispatch
in blk_mq_sched_bypass_insert.  This can happen while a kworker is running
hctx->run_work work item and is past the point in
blk_mq_sched_dispatch_requests where hctx->dispatch is checked.

The blk_mq_do_dispatch_sched call is not guaranteed to end in bounded time,
because the I/O scheduler can feed an arbitrary number of commands.

Since we have only one hctx->run_work, the commands waiting in
hctx->dispatch will wait an arbitrary length of time for run_work to be
rerun.

A similar phenomenon exists with dispatches from the software queue.

The solution is to poll hctx->dispatch in blk_mq_do_dispatch_sched and
blk_mq_do_dispatch_ctx and return from the run_work handler and let it
rerun.

Signed-off-by: Salman Qazi <sqazi@google.com>
---
 block/blk-mq-sched.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index ca22afd47b3d..d1b8b31bc3d4 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -97,6 +97,9 @@ static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
 		if (e->type->ops.has_work && !e->type->ops.has_work(hctx))
 			break;
 
+		if (!list_empty_careful(&hctx->dispatch))
+			break;
+
 		if (!blk_mq_get_dispatch_budget(hctx))
 			break;
 
@@ -140,6 +143,9 @@ static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
 	do {
 		struct request *rq;
 
+		if (!list_empty_careful(&hctx->dispatch))
+			break;
+
 		if (!sbitmap_any_bit_set(&hctx->ctx_map))
 			break;
 
-- 
2.25.0.341.g760bfbb309-goog