On 7/10/19 1:52 PM, Josef Bacik wrote:
> rq-qos sits in the io path so we want to take locks as sparingly as
> possible. To accomplish this we try not to take the waitqueue head lock
> unless we are sure we need to go to sleep, and we have an optimization
> to make sure that we don't starve out existing waiters. Since we check
> if there are existing waiters locklessly we need to be able to update
> our view of the waitqueue list after we've added ourselves to the
> waitqueue. Accomplish this by adding this helper to see if there are
> more than two waiters on the waitqueue.
>
> Suggested-by: Jens Axboe <axboe@xxxxxxxxx>
> Signed-off-by: Josef Bacik <josef@xxxxxxxxxxxxxx>
> ---
> include/linux/wait.h | 21 +++++++++++++++++++++
> 1 file changed, 21 insertions(+)
>
> diff --git a/include/linux/wait.h b/include/linux/wait.h
> index b6f77cf60dd7..89c41a7b3046 100644
> --- a/include/linux/wait.h
> +++ b/include/linux/wait.h
> @@ -126,6 +126,27 @@ static inline int waitqueue_active(struct wait_queue_head *wq_head)
> return !list_empty(&wq_head->head);
> }
>
> +/**
> + * wq_has_multiple_sleepers - check if there are multiple waiting prcesses
> + * @wq_head: wait queue head
> + *
> + * Returns true of wq_head has multiple waiting processes.
> + *
> + * Please refer to the comment for waitqueue_active.
> + */
> +static inline bool wq_has_multiple_sleepers(struct wait_queue_head *wq_head)
> +{
> + /*
> + * We need to be sure we are in sync with the
> + * add_wait_queue modifications to the wait queue.
> + *
> + * This memory barrier should be paired with one on the
> + * waiting side.
> + */
> + smp_mb();
> + return !list_is_singular(&wq_head->head);
> +}
> +
> /**
> * wq_has_sleeper - check if there are any waiting processes
> * @wq_head: wait queue head
This (and 2/2) looks good to me, better than v1 for sure. Peter/Ingo,
are you OK with adding this new helper? For reference, this (and the
next patch) replace the alternative, which is an open-coding of
prepare_to_wait():
https://lore.kernel.org/linux-block/20190710190514.86911-1-josef@xxxxxxxxxxxxxx/
--
Jens Axboe
