linux-kernel - Re: [PATCH 3/3] sbitmap: Try each queue to wake up at least one waiter

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Date:   Wed, 16 Nov 2022 12:09:04 +0100
From:   Jan Kara <jack@...e.cz>
To:     Gabriel Krisman Bertazi <krisman@...e.de>
Cc:     axboe@...nel.dk, mingo@...hat.com, peterz@...radead.org,
        jack@...e.cz, linux-kernel@...r.kernel.org,
        linux-block@...r.kernel.org, liusong@...ux.alibaba.com,
        chaitanyak@...dia.com
Subject: Re: [PATCH 3/3] sbitmap: Try each queue to wake up at least one
 waiter

On Tue 15-11-22 17:45:53, Gabriel Krisman Bertazi wrote:
> Jan reported the new algorithm as merged might be problematic if the
> queue being awaken becomes empty between the waitqueue_active inside
> sbq_wake_ptr check and the wake up.  If that happens, wake_up_nr will
> not wake up any waiter and we loose too many wake ups.  In order to
> guarantee progress, we need to wake up at least one waiter here, if
> there are any.  This now requires trying to wake up from every queue.
> 
> Instead of walking through all the queues with sbq_wake_ptr, this call
> moves the wake up inside that function.  In a previous version of the
> patch, I found that updating wake_index several times when walking
> through queues had a measurable overhead.  This ensures we only update
> it once, at the end.
> 
> Fixes: 4f8126bb2308 ("sbitmap: Use single per-bitmap counting to wake up queued tags")
> Reported-by: Jan Kara <jack@...e.cz>
> Signed-off-by: Gabriel Krisman Bertazi <krisman@...e.de>

Elegant and looks good to me! Feel free to add:

Reviewed-by: Jan Kara <jack@...e.cz>

								Honza

> ---
>  lib/sbitmap.c | 28 ++++++++++++----------------
>  1 file changed, 12 insertions(+), 16 deletions(-)
> 
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index bea7984f7987..586deb333237 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -560,12 +560,12 @@ void sbitmap_queue_min_shallow_depth(struct sbitmap_queue *sbq,
>  }
>  EXPORT_SYMBOL_GPL(sbitmap_queue_min_shallow_depth);
>  
> -static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
> +static void __sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr)
>  {
>  	int i, wake_index;
>  
>  	if (!atomic_read(&sbq->ws_active))
> -		return NULL;
> +		return;
>  
>  	wake_index = atomic_read(&sbq->wake_index);
>  	for (i = 0; i < SBQ_WAIT_QUEUES; i++) {
> @@ -579,20 +579,22 @@ static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
>  		 */
>  		wake_index = sbq_index_inc(wake_index);
>  
> -		if (waitqueue_active(&ws->wait)) {
> -			if (wake_index != atomic_read(&sbq->wake_index))
> -				atomic_set(&sbq->wake_index, wake_index);
> -			return ws;
> -		}
> +		/*
> +		 * It is sufficient to wake up at least one waiter to
> +		 * guarantee forward progress.
> +		 */
> +		if (waitqueue_active(&ws->wait) &&
> +		    wake_up_nr(&ws->wait, nr))
> +			break;
>  	}
>  
> -	return NULL;
> +	if (wake_index != atomic_read(&sbq->wake_index))
> +		atomic_set(&sbq->wake_index, wake_index);
>  }
>  
>  void sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr)
>  {
>  	unsigned int wake_batch = READ_ONCE(sbq->wake_batch);
> -	struct sbq_wait_state *ws = NULL;
>  	unsigned int wakeups;
>  
>  	if (!atomic_read(&sbq->ws_active))
> @@ -604,16 +606,10 @@ void sbitmap_queue_wake_up(struct sbitmap_queue *sbq, int nr)
>  	do {
>  		if (atomic_read(&sbq->completion_cnt) - wakeups < wake_batch)
>  			return;
> -
> -		if (!ws) {
> -			ws = sbq_wake_ptr(sbq);
> -			if (!ws)
> -				return;
> -		}
>  	} while (!atomic_try_cmpxchg(&sbq->wakeup_cnt,
>  				     &wakeups, wakeups + wake_batch));
>  
> -	wake_up_nr(&ws->wait, wake_batch);
> +	__sbitmap_queue_wake_up(sbq, wake_batch);
>  }
>  EXPORT_SYMBOL_GPL(sbitmap_queue_wake_up);
>  
> -- 
> 2.35.3
> 
-- 
Jan Kara <jack@...e.com>
SUSE Labs, CR