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

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

Signed-off-by: Salman Qazi <>
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))

+ if (!list_empty_careful(&hctx->dispatch))
+ break;
if (!blk_mq_get_dispatch_budget(hctx))

@@ -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))

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