From: David Jeffery <hidden> Date: 2022-01-31 20:33:59
When blk_mq_delay_run_hw_queues sets an hctx to run in the future, it can
reset the delay length for an already pending delayed work run_work. This
creates a scenario where multiple hctx may have their queues set to run,
but if one runs first and finds nothing to do, it can reset the delay of
another hctx and stall the other hctx's ability to run requests.
To avoid this I/O stall when an hctx's run_work is already pending,
leave it untouched to run at its current designated time rather than
extending its delay. The work will still run which keeps closed the race
calling blk_mq_delay_run_hw_queues is needed for while also avoiding the
I/O stall.
Signed-off-by: David Jeffery <redacted>
---
block/blk-mq.c | 8 ++++++++
1 file changed, 8 insertions(+)
On Mon, 2022-01-31 at 15:33 -0500, David Jeffery wrote:
quoted hunk
When blk_mq_delay_run_hw_queues sets an hctx to run in the future, it
can
reset the delay length for an already pending delayed work run_work.
This
creates a scenario where multiple hctx may have their queues set to
run,
but if one runs first and finds nothing to do, it can reset the delay
of
another hctx and stall the other hctx's ability to run requests.
To avoid this I/O stall when an hctx's run_work is already pending,
leave it untouched to run at its current designated time rather than
extending its delay. The work will still run which keeps closed the
race
calling blk_mq_delay_run_hw_queues is needed for while also avoiding
the
I/O stall.
Signed-off-by: David Jeffery <redacted>
---
block/blk-mq.c | 8 ++++++++
1 file changed, 8 insertions(+)
request_queue *q, unsigned long msecs)
queue_for_each_hw_ctx(q, hctx, i) {
if (blk_mq_hctx_stopped(hctx))
continue;
+ /*
+ * If there is already a run_work pending, leave the
+ * pending delay untouched. Otherwise, a hctx can stall
+ * if another hctx is re-delaying the other's work
+ * before the work executes.
+ */
+ if (delayed_work_pending(&hctx->run_work))
+ continue;
/*
* Dispatch from this hctx either if there's no hctx
preferred
* by IO scheduler or if it has requests that bypass
the
Ming is aware of this patch and had asked David to submit it.
David already explained his reasoning internally.
It's for an already reported issue by a customer.
Reviewed-by:
Laurence Oberman [off-list ref]
On Tue, Feb 1, 2022 at 4:34 AM David Jeffery [off-list ref] wrote:
quoted hunk
When blk_mq_delay_run_hw_queues sets an hctx to run in the future, it can
reset the delay length for an already pending delayed work run_work. This
creates a scenario where multiple hctx may have their queues set to run,
but if one runs first and finds nothing to do, it can reset the delay of
another hctx and stall the other hctx's ability to run requests.
To avoid this I/O stall when an hctx's run_work is already pending,
leave it untouched to run at its current designated time rather than
extending its delay. The work will still run which keeps closed the race
calling blk_mq_delay_run_hw_queues is needed for while also avoiding the
I/O stall.
Signed-off-by: David Jeffery <redacted>
---
block/blk-mq.c | 8 ++++++++
1 file changed, 8 insertions(+)
The issue is triggered on BFQ, since BFQ's has_work() may return true,
however its ->dispatch_request() may return NULL, so
blk_mq_delay_run_hw_queues()
is run for delay schedule.
In case of multiple hw queue, the described issue may be triggered, and cause io
stall for long time. And there are only 3 in-tree callers of
blk_mq_delay_run_hw_queues(),
David's fix works well for the 3 users, so this patch looks fine:
Reviewed-by: Ming Lei <redacted>
Thanks,
From: John Pittman <hidden> Date: 2022-02-14 14:50:52
This patch has now been tested in the customer environment and results
were good (fixed the hangs).
On Mon, Feb 7, 2022 at 9:45 PM Ming Lei [off-list ref] wrote:
On Tue, Feb 1, 2022 at 4:34 AM David Jeffery [off-list ref] wrote:
quoted
When blk_mq_delay_run_hw_queues sets an hctx to run in the future, it can
reset the delay length for an already pending delayed work run_work. This
creates a scenario where multiple hctx may have their queues set to run,
but if one runs first and finds nothing to do, it can reset the delay of
another hctx and stall the other hctx's ability to run requests.
To avoid this I/O stall when an hctx's run_work is already pending,
leave it untouched to run at its current designated time rather than
extending its delay. The work will still run which keeps closed the race
calling blk_mq_delay_run_hw_queues is needed for while also avoiding the
I/O stall.
Signed-off-by: David Jeffery <redacted>
---
block/blk-mq.c | 8 ++++++++
1 file changed, 8 insertions(+)
The issue is triggered on BFQ, since BFQ's has_work() may return true,
however its ->dispatch_request() may return NULL, so
blk_mq_delay_run_hw_queues()
is run for delay schedule.
In case of multiple hw queue, the described issue may be triggered, and cause io
stall for long time. And there are only 3 in-tree callers of
blk_mq_delay_run_hw_queues(),
David's fix works well for the 3 users, so this patch looks fine:
Reviewed-by: Ming Lei <redacted>
Thanks,
On Mon, 31 Jan 2022 15:33:37 -0500, David Jeffery wrote:
When blk_mq_delay_run_hw_queues sets an hctx to run in the future, it can
reset the delay length for an already pending delayed work run_work. This
creates a scenario where multiple hctx may have their queues set to run,
but if one runs first and finds nothing to do, it can reset the delay of
another hctx and stall the other hctx's ability to run requests.
To avoid this I/O stall when an hctx's run_work is already pending,
leave it untouched to run at its current designated time rather than
extending its delay. The work will still run which keeps closed the race
calling blk_mq_delay_run_hw_queues is needed for while also avoiding the
I/O stall.
[...]
Applied, thanks!
[1/1] blk-mq: avoid extending delays of active hctx from blk_mq_delay_run_hw_queues
commit: 8f5fea65b06de1cc51d4fc23fb4d378d1abd6ed7
Best regards,
--
Jens Axboe
On Mon, 2022-02-14 at 09:50 -0500, John Pittman wrote:
This patch has now been tested in the customer environment and
results
were good (fixed the hangs).
On Mon, Feb 7, 2022 at 9:45 PM Ming Lei [off-list ref] wrote:
quoted
On Tue, Feb 1, 2022 at 4:34 AM David Jeffery [off-list ref]
wrote:
quoted
When blk_mq_delay_run_hw_queues sets an hctx to run in the
future, it can
reset the delay length for an already pending delayed work
run_work. This
creates a scenario where multiple hctx may have their queues set
to run,
but if one runs first and finds nothing to do, it can reset the
delay of
another hctx and stall the other hctx's ability to run requests.
To avoid this I/O stall when an hctx's run_work is already
pending,
leave it untouched to run at its current designated time rather
than
extending its delay. The work will still run which keeps closed
the race
calling blk_mq_delay_run_hw_queues is needed for while also
avoiding the
I/O stall.
request_queue *q, unsigned long msecs)
queue_for_each_hw_ctx(q, hctx, i) {
if (blk_mq_hctx_stopped(hctx))
continue;
+ /*
+ * If there is already a run_work pending, leave
the
+ * pending delay untouched. Otherwise, a hctx can
stall
+ * if another hctx is re-delaying the other's
work
+ * before the work executes.
+ */
+ if (delayed_work_pending(&hctx->run_work))
+ continue;
The issue is triggered on BFQ, since BFQ's has_work() may return
true,
however its ->dispatch_request() may return NULL, so
blk_mq_delay_run_hw_queues()
is run for delay schedule.
In case of multiple hw queue, the described issue may be triggered,
and cause io
stall for long time. And there are only 3 in-tree callers of
blk_mq_delay_run_hw_queues(),
David's fix works well for the 3 users, so this patch looks fine:
Reviewed-by: Ming Lei <redacted>
Thanks,
Hello
Jens, gentle ping, can we get this in please
Sincerely
Laurence and the RH team