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authorSalman Qazi <sqazi@google.com>2020-04-24 08:03:21 -0700
committerJens Axboe <axboe@kernel.dk>2020-04-24 09:16:56 -0600
commit28d65729b050977d8a9125e6726871e83bd22124 (patch)
tree7327602a47637499b372cf7e95c5a432c5ea6940 /block/blk-mq-sched.c
parent895d47759bdc4a8452e36ad3f7bc4324c9251932 (diff)
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 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> Reviewed-by: Ming Lei <ming.lei@redhat.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
Diffstat (limited to 'block/blk-mq-sched.c')
-rw-r--r--block/blk-mq-sched.c64
1 files changed, 51 insertions, 13 deletions
diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index eca81bd4010c..702ce525933c 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -86,12 +86,16 @@ 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 -EAGAIN 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 int 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);
+ int ret = 0;
do {
struct request *rq;
@@ -99,6 +103,11 @@ 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 = -EAGAIN;
+ break;
+ }
+
if (!blk_mq_get_dispatch_budget(hctx))
break;
@@ -123,6 +132,8 @@ 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,
@@ -140,16 +151,25 @@ 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 -EAGAIN if hctx->dispatch was found non-empty and run_work has to
+ * 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 int 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);
+ int ret = 0;
do {
struct request *rq;
+ if (!list_empty_careful(&hctx->dispatch)) {
+ ret = -EAGAIN;
+ break;
+ }
+
if (!sbitmap_any_bit_set(&hctx->ctx_map))
break;
@@ -183,21 +203,17 @@ 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)
+int __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;
+ int ret = 0;
LIST_HEAD(rq_list);
- /* RCU or SRCU read lock is needed before checking quiesced flag */
- if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)))
- return;
-
- hctx->run++;
-
/*
* If we have previous entries on our dispatch list, grab them first for
* more fair dispatch.
@@ -226,19 +242,41 @@ 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);
+ ret = blk_mq_do_dispatch_sched(hctx);
else
- blk_mq_do_dispatch_ctx(hctx);
+ ret = blk_mq_do_dispatch_ctx(hctx);
}
} else if (has_sched_dispatch) {
- blk_mq_do_dispatch_sched(hctx);
+ ret = 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);
+ ret = blk_mq_do_dispatch_ctx(hctx);
} else {
blk_mq_flush_busy_ctxs(hctx, &rq_list);
blk_mq_dispatch_rq_list(q, &rq_list, false);
}
+
+ return ret;
+}
+
+void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
+{
+ struct request_queue *q = hctx->queue;
+
+ /* RCU or SRCU read lock is needed before checking quiesced flag */
+ if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)))
+ return;
+
+ hctx->run++;
+
+ /*
+ * A return of -EAGAIN is an indication that hctx->dispatch is not
+ * empty and we must run again in order to avoid starving flushes.
+ */
+ if (__blk_mq_sched_dispatch_requests(hctx) == -EAGAIN) {
+ if (__blk_mq_sched_dispatch_requests(hctx) == -EAGAIN)
+ blk_mq_run_hw_queue(hctx, true);
+ }
}
bool blk_mq_sched_try_merge(struct request_queue *q, struct bio *bio,