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@xxxxxxxxxx>
Reviewed-by: Ming Lei <ming.lei@xxxxxxxxxx>
---
block/blk-mq-sched.c | 49 ++++++++++++++++++++++++++++++++++++++------
1 file changed, 43 insertions(+), 6 deletions(-)
diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index ca22afd47b3d..3e78c5bbb4d9 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -84,12 +84,17 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
* Only SCSI implements .get_budget and .put_budget, and SCSI restarts
* its queue by itself in its completion handler, so we don't need to
* restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
+ *
+ * Returns true if hctx->dispatch was found non-empty and
+ * run_work has to be run again. This is necessary to avoid
+ * starving flushes.
*/
-static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
+static bool blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
{
struct request_queue *q = hctx->queue;
struct elevator_queue *e = q->elevator;
LIST_HEAD(rq_list);
+ bool ret = false;
do {
struct request *rq;
@@ -97,6 +102,11 @@ 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)) {
+ ret = true;
+ break;
+ }
+
if (!blk_mq_get_dispatch_budget(hctx))
break;
@@ -113,6 +123,8 @@ static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
*/
list_add(&rq->queuelist, &rq_list);
} while (blk_mq_dispatch_rq_list(q, &rq_list, true));
+
+ return ret;
}
static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
@@ -130,16 +142,26 @@ static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
* Only SCSI implements .get_budget and .put_budget, and SCSI restarts
* its queue by itself in its completion handler, so we don't need to
* restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
+ *
+ * Returns true if hctx->dispatch was found non-empty and
+ * run_work has to be run again. This is necessary to avoid
+ * starving flushes.
*/
-static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
+static bool blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
{
struct request_queue *q = hctx->queue;
LIST_HEAD(rq_list);
struct blk_mq_ctx *ctx = READ_ONCE(hctx->dispatch_from);
+ bool ret = false;
do {
struct request *rq;
+ if (!list_empty_careful(&hctx->dispatch)) {
+ ret = true;
+ break;
+ }
+
if (!sbitmap_any_bit_set(&hctx->ctx_map))
break;
@@ -165,6 +187,7 @@ static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
} while (blk_mq_dispatch_rq_list(q, &rq_list, true));
WRITE_ONCE(hctx->dispatch_from, ctx);
+ return ret;
}
void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
@@ -172,6 +195,8 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
struct request_queue *q = hctx->queue;
struct elevator_queue *e = q->elevator;
const bool has_sched_dispatch = e && e->type->ops.dispatch_request;
+ bool run_again;
+ bool restarted = false;
LIST_HEAD(rq_list);
/* RCU or SRCU read lock is needed before checking quiesced flag */
@@ -180,6 +205,9 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
hctx->run++;
+again:
+ run_again = false;
+
/*
* If we have previous entries on our dispatch list, grab them first for
* more fair dispatch.
@@ -208,19 +236,28 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
blk_mq_sched_mark_restart_hctx(hctx);
if (blk_mq_dispatch_rq_list(q, &rq_list, false)) {
if (has_sched_dispatch)
- blk_mq_do_dispatch_sched(hctx);
+ run_again = blk_mq_do_dispatch_sched(hctx);
else
- blk_mq_do_dispatch_ctx(hctx);
+ run_again = blk_mq_do_dispatch_ctx(hctx);
}
} else if (has_sched_dispatch) {
- blk_mq_do_dispatch_sched(hctx);
+ run_again = blk_mq_do_dispatch_sched(hctx);
} else if (hctx->dispatch_busy) {
/* dequeue request one by one from sw queue if queue is busy */
- blk_mq_do_dispatch_ctx(hctx);
+ run_again = blk_mq_do_dispatch_ctx(hctx);
} else {
blk_mq_flush_busy_ctxs(hctx, &rq_list);
blk_mq_dispatch_rq_list(q, &rq_list, false);
}
+
+ if (run_again) {
+ if (!restarted) {
+ restarted = true;
+ goto again;
+ }
+
+ blk_mq_run_hw_queue(hctx, true);
+ }
}
bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio,
--
2.25.0.341.g760bfbb309-goog
