On Tue, Feb 1, 2022 at 4:34 AM David Jeffery <djeffery@xxxxxxxxxx> 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. > > Signed-off-by: David Jeffery <djeffery@xxxxxxxxxx> > --- > block/blk-mq.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > > diff --git a/block/blk-mq.c b/block/blk-mq.c > index f3bf3358a3bb..ae46eb4bf547 100644 > --- a/block/blk-mq.c > +++ b/block/blk-mq.c > @@ -2177,6 +2177,14 @@ void blk_mq_delay_run_hw_queues(struct 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 <ming.lei@xxxxxxxxxx> Thanks,