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    SubjectRe: [PATCH -next 2/2] block, bfq: make bfq_has_work() more accurate
    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
    >
    --
    Jan Kara <jack@suse.com>
    SUSE Labs, CR

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