Re: [PATCH 1/2] wait: add wq_has_multiple_sleepers helper

[Jens Axboe <axboe@kernel.dk>]

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@kernel.dk>
> Signed-off-by: Josef Bacik <josef@toxicpanda.com>
> ---
>   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@toxicpanda.com/

-- 
Jens Axboe