| Message ID | 20220510131629.1964415-3-yukuai3@huawei.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | block, bfq: make bfq_has_work() more accurate | expand |
On Tue 10-05-22 21:16:29, Yu Kuai wrote: > bfq_has_work() is using busy_queues currently, which is not accurate > because bfq_queue is busy doesn't represent that it has requests. Since > bfqd aready has a counter 'queued' to record how many requests are in > bfq, use it instead of busy_queues. > > Noted that bfq_has_work() can be called with 'bfqd->lock' held, thus the > lock can't be held in bfq_has_work() to protect 'bfqd->queued'. > > Signed-off-by: Yu Kuai <yukuai3@huawei.com> So did you find this causing any real problem? Because bfq queue is accounted among busy queues once bfq_add_bfqq_busy() is called. And that happens once a new request is inserted into the queue so it should be very similar to bfqd->queued. Honza > --- > block/bfq-iosched.c | 4 ++-- > 1 file changed, 2 insertions(+), 2 deletions(-) > > diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c > index 61750696e87f..1d2f8110c26b 100644 > --- a/block/bfq-iosched.c > +++ b/block/bfq-iosched.c > @@ -5063,11 +5063,11 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx) > struct bfq_data *bfqd = hctx->queue->elevator->elevator_data; > > /* > - * Avoiding lock: a race on bfqd->busy_queues should cause at > + * Avoiding lock: a race on bfqd->queued should cause at > * most a call to dispatch for nothing > */ > return !list_empty_careful(&bfqd->dispatch) || > - bfq_tot_busy_queues(bfqd) > 0; > + READ_ONCE(bfqd->queued); > } > > static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx) > -- > 2.31.1 >
On 2022/05/11 22:08, Jan Kara wrote: > On Tue 10-05-22 21:16:29, Yu Kuai wrote: >> bfq_has_work() is using busy_queues currently, which is not accurate >> because bfq_queue is busy doesn't represent that it has requests. Since >> bfqd aready has a counter 'queued' to record how many requests are in >> bfq, use it instead of busy_queues. >> >> Noted that bfq_has_work() can be called with 'bfqd->lock' held, thus the >> lock can't be held in bfq_has_work() to protect 'bfqd->queued'. >> >> Signed-off-by: Yu Kuai <yukuai3@huawei.com> > > So did you find this causing any real problem? Because bfq queue is > accounted among busy queues once bfq_add_bfqq_busy() is called. And that > happens once a new request is inserted into the queue so it should be very > similar to bfqd->queued. > > Honza Hi, The related problem is described here: https://lore.kernel.org/all/20220510112302.1215092-1-yukuai3@huawei.com/ The root cause of the panic is a linux-block problem, however, it can be bypassed if bfq_has_work() is accurate. On the other hand, unnecessary run_work will be triggered if bfqq stays busy: __blk_mq_run_hw_queue __blk_mq_sched_dispatch_requests __blk_mq_do_dispatch_sched if (!bfq_has_work()) break; blk_mq_delay_run_hw_queues -> run again after 3ms Thanks, Kuai > >> --- >> block/bfq-iosched.c | 4 ++-- >> 1 file changed, 2 insertions(+), 2 deletions(-) >> >> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c >> index 61750696e87f..1d2f8110c26b 100644 >> --- a/block/bfq-iosched.c >> +++ b/block/bfq-iosched.c >> @@ -5063,11 +5063,11 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx) >> struct bfq_data *bfqd = hctx->queue->elevator->elevator_data; >> >> /* >> - * Avoiding lock: a race on bfqd->busy_queues should cause at >> + * Avoiding lock: a race on bfqd->queued should cause at >> * most a call to dispatch for nothing >> */ >> return !list_empty_careful(&bfqd->dispatch) || >> - bfq_tot_busy_queues(bfqd) > 0; >> + READ_ONCE(bfqd->queued); >> } >> >> static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx) >> -- >> 2.31.1 >>
On Thu 12-05-22 09:30:16, yukuai (C) wrote: > On 2022/05/11 22:08, Jan Kara wrote: > > On Tue 10-05-22 21:16:29, Yu Kuai wrote: > > > bfq_has_work() is using busy_queues currently, which is not accurate > > > because bfq_queue is busy doesn't represent that it has requests. Since > > > bfqd aready has a counter 'queued' to record how many requests are in > > > bfq, use it instead of busy_queues. > > > > > > Noted that bfq_has_work() can be called with 'bfqd->lock' held, thus the > > > lock can't be held in bfq_has_work() to protect 'bfqd->queued'. > > > > > > Signed-off-by: Yu Kuai <yukuai3@huawei.com> > > > > So did you find this causing any real problem? Because bfq queue is > > accounted among busy queues once bfq_add_bfqq_busy() is called. And that > > happens once a new request is inserted into the queue so it should be very > > similar to bfqd->queued. > > > > Honza > > Hi, > > The related problem is described here: > > https://lore.kernel.org/all/20220510112302.1215092-1-yukuai3@huawei.com/ > > The root cause of the panic is a linux-block problem, however, it can > be bypassed if bfq_has_work() is accurate. On the other hand, > unnecessary run_work will be triggered if bfqq stays busy: > > __blk_mq_run_hw_queue > __blk_mq_sched_dispatch_requests > __blk_mq_do_dispatch_sched > if (!bfq_has_work()) > break; > blk_mq_delay_run_hw_queues -> run again after 3ms Ah, I see. So it is the other way around than I thought. Due to idling bfq_tot_busy_queues() can be greater than 0 even if there are no requests to dispatch. Indeed. OK, the patch makes sense. But please use WRITE_ONCE for the updates of bfqd->queued. Otherwise the READ_ONCE does not really make sense (it can still result in some bogus value due to compiler optimizations on the write side). Honza > > > --- > > > block/bfq-iosched.c | 4 ++-- > > > 1 file changed, 2 insertions(+), 2 deletions(-) > > > > > > diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c > > > index 61750696e87f..1d2f8110c26b 100644 > > > --- a/block/bfq-iosched.c > > > +++ b/block/bfq-iosched.c > > > @@ -5063,11 +5063,11 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx) > > > struct bfq_data *bfqd = hctx->queue->elevator->elevator_data; > > > /* > > > - * Avoiding lock: a race on bfqd->busy_queues should cause at > > > + * Avoiding lock: a race on bfqd->queued should cause at > > > * most a call to dispatch for nothing > > > */ > > > return !list_empty_careful(&bfqd->dispatch) || > > > - bfq_tot_busy_queues(bfqd) > 0; > > > + READ_ONCE(bfqd->queued); > > > } > > > static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx) > > > -- > > > 2.31.1 > > >
在 2022/05/13 1:10, Jan Kara 写道: > On Thu 12-05-22 09:30:16, yukuai (C) wrote: >> On 2022/05/11 22:08, Jan Kara wrote: >>> On Tue 10-05-22 21:16:29, Yu Kuai wrote: >>>> bfq_has_work() is using busy_queues currently, which is not accurate >>>> because bfq_queue is busy doesn't represent that it has requests. Since >>>> bfqd aready has a counter 'queued' to record how many requests are in >>>> bfq, use it instead of busy_queues. >>>> >>>> Noted that bfq_has_work() can be called with 'bfqd->lock' held, thus the >>>> lock can't be held in bfq_has_work() to protect 'bfqd->queued'. >>>> >>>> Signed-off-by: Yu Kuai <yukuai3@huawei.com> >>> >>> So did you find this causing any real problem? Because bfq queue is >>> accounted among busy queues once bfq_add_bfqq_busy() is called. And that >>> happens once a new request is inserted into the queue so it should be very >>> similar to bfqd->queued. >>> >>> Honza >> >> Hi, >> >> The related problem is described here: >> >> https://lore.kernel.org/all/20220510112302.1215092-1-yukuai3@huawei.com/ >> >> The root cause of the panic is a linux-block problem, however, it can >> be bypassed if bfq_has_work() is accurate. On the other hand, >> unnecessary run_work will be triggered if bfqq stays busy: >> >> __blk_mq_run_hw_queue >> __blk_mq_sched_dispatch_requests >> __blk_mq_do_dispatch_sched >> if (!bfq_has_work()) >> break; >> blk_mq_delay_run_hw_queues -> run again after 3ms > > Ah, I see. So it is the other way around than I thought. Due to idling > bfq_tot_busy_queues() can be greater than 0 even if there are no requests > to dispatch. Indeed. OK, the patch makes sense. But please use WRITE_ONCE > for the updates of bfqd->queued. Otherwise the READ_ONCE does not really > make sense (it can still result in some bogus value due to compiler > optimizations on the write side). Thanks for you adivce, I'll send a new version. Kuai
diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c index 61750696e87f..1d2f8110c26b 100644 --- a/block/bfq-iosched.c +++ b/block/bfq-iosched.c @@ -5063,11 +5063,11 @@ static bool bfq_has_work(struct blk_mq_hw_ctx *hctx) struct bfq_data *bfqd = hctx->queue->elevator->elevator_data; /* - * Avoiding lock: a race on bfqd->busy_queues should cause at + * Avoiding lock: a race on bfqd->queued should cause at * most a call to dispatch for nothing */ return !list_empty_careful(&bfqd->dispatch) || - bfq_tot_busy_queues(bfqd) > 0; + READ_ONCE(bfqd->queued); } static struct request *__bfq_dispatch_request(struct blk_mq_hw_ctx *hctx)
bfq_has_work() is using busy_queues currently, which is not accurate because bfq_queue is busy doesn't represent that it has requests. Since bfqd aready has a counter 'queued' to record how many requests are in bfq, use it instead of busy_queues. Noted that bfq_has_work() can be called with 'bfqd->lock' held, thus the lock can't be held in bfq_has_work() to protect 'bfqd->queued'. Signed-off-by: Yu Kuai <yukuai3@huawei.com> --- block/bfq-iosched.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-)