linux-kernel - Re: [PATCH] sbitmap: fix permanent io blocking caused by insufficient wakeup times

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Date:   Sat, 17 Sep 2022 18:20:30 +0800
From:   Yu Kuai <yukuai1@...weicloud.com>
To:     Liu Song <liusong@...ux.alibaba.com>, axboe@...nel.dk
Cc:     linux-kernel@...r.kernel.org, linux-block@...r.kernel.org,
        "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [PATCH] sbitmap: fix permanent io blocking caused by insufficient
 wakeup times

Hi,

在 2022/09/17 10:33, Liu Song 写道:
> From: Liu Song <liusong@...ux.alibaba.com>
> 
> In "sbitmap_queue_clear_batch", a batch of completed requests may be
> processed at once, but "wait_cnt" is only reduced once by
> "sbitmap_queue_wake_up".
> In our environment, if "/sys/block/nvme0n1/nr_requests" is adjusted to
> 32, it is easily because no tag and then enter the waiting situation,
> if continue change "nr_requests" to 1023 at this time, it will basically
> fall into the situation of permanent blocking. Because there will be
> "blk_freeze_queue" in "blk_mq_update_nr_requests", which will prevent
> any new requests, but due to insufficient wake-up, there are tasks
> waiting for wake-up, but no new wake-up opportunities will be generated
> at this time, so this situation needs to be repaired.
> 

It seems Keith already fixed this recently. Can you have a check at his
patch?

Thanks,
Kuai
> Signed-off-by: Liu Song <liusong@...ux.alibaba.com>
> ---
>   include/linux/sbitmap.h |  8 ++++++++
>   lib/sbitmap.c           | 22 ++++++++++++++++++----
>   2 files changed, 26 insertions(+), 4 deletions(-)
> 
> diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
> index 8f5a86e..153382e 100644
> --- a/include/linux/sbitmap.h
> +++ b/include/linux/sbitmap.h
> @@ -579,6 +579,14 @@ static inline struct sbq_wait_state *sbq_wait_ptr(struct sbitmap_queue *sbq,
>   void sbitmap_queue_wake_up(struct sbitmap_queue *sbq);
>   
>   /**
> + * sbitmap_queue_wake_up_batch() - Attempt to wake up waiters in batches
> + * on a &struct sbitmap_queue.
> + * @sbq: Bitmap queue to wake up.
> + * @nr: The number of attempts to wake the waiter.
> + */
> +void sbitmap_queue_wake_up_batch(struct sbitmap_queue *sbq, int nr);
> +
> +/**
>    * sbitmap_queue_show() - Dump &struct sbitmap_queue information to a &struct
>    * seq_file.
>    * @sbq: Bitmap queue to show.
> diff --git a/lib/sbitmap.c b/lib/sbitmap.c
> index 29eb048..f2aa1da 100644
> --- a/lib/sbitmap.c
> +++ b/lib/sbitmap.c
> @@ -600,7 +600,7 @@ static struct sbq_wait_state *sbq_wake_ptr(struct sbitmap_queue *sbq)
>   	return NULL;
>   }
>   
> -static bool __sbq_wake_up(struct sbitmap_queue *sbq)
> +static bool __sbq_wake_up(struct sbitmap_queue *sbq, int *nr)
>   {
>   	struct sbq_wait_state *ws;
>   	unsigned int wake_batch;
> @@ -610,6 +610,7 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
>   	if (!ws)
>   		return false;
>   
> +again:
>   	wait_cnt = atomic_dec_return(&ws->wait_cnt);
>   	if (wait_cnt <= 0) {
>   		int ret;
> @@ -632,10 +633,14 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
>   		if (ret == wait_cnt) {
>   			sbq_index_atomic_inc(&sbq->wake_index);
>   			wake_up_nr(&ws->wait, wake_batch);
> -			return false;
> +			if (!nr || *nr <= 0)
> +				return false;
>   		}
>   
>   		return true;
> +	} else if (nr && *nr) {
> +		(*nr)--;
> +		goto again;
>   	}
>   
>   	return false;
> @@ -643,11 +648,20 @@ static bool __sbq_wake_up(struct sbitmap_queue *sbq)
>   
>   void sbitmap_queue_wake_up(struct sbitmap_queue *sbq)
>   {
> -	while (__sbq_wake_up(sbq))
> +	while (__sbq_wake_up(sbq, NULL))
>   		;
>   }
>   EXPORT_SYMBOL_GPL(sbitmap_queue_wake_up);
>   
> +void sbitmap_queue_wake_up_batch(struct sbitmap_queue *sbq, int nr)
> +{
> +	int i = SBQ_WAIT_QUEUES;
> +
> +	while (__sbq_wake_up(sbq, &nr) && --i)
> +		;
> +}
> +EXPORT_SYMBOL_GPL(sbitmap_queue_wake_up_batch);
> +
>   static inline void sbitmap_update_cpu_hint(struct sbitmap *sb, int cpu, int tag)
>   {
>   	if (likely(!sb->round_robin && tag < sb->depth))
> @@ -683,7 +697,7 @@ void sbitmap_queue_clear_batch(struct sbitmap_queue *sbq, int offset,
>   		atomic_long_andnot(mask, (atomic_long_t *) addr);
>   
>   	smp_mb__after_atomic();
> -	sbitmap_queue_wake_up(sbq);
> +	sbitmap_queue_wake_up_batch(sbq, nr_tags);
>   	sbitmap_update_cpu_hint(&sbq->sb, raw_smp_processor_id(),
>   					tags[nr_tags - 1] - offset);
>   }
>