linux-kernel - Re: [PATCH v4] block: move non sync requests complete flow to softirq

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Message-ID: <4ce112ca-9c2d-4091-9e98-f2d08e5db299@kernel.org>
Date: Fri, 6 Sep 2024 15:18:29 +0900
From: Damien Le Moal <dlemoal@...nel.org>
To: ZhangHui <zhanghui31@...omi.com>, axboe@...nel.dk, bvanassche@....org,
 ming.lei@...hat.com
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4] block: move non sync requests complete flow to softirq

On 9/6/24 12:14, ZhangHui wrote:
> From: zhanghui <zhanghui31@...omi.com>
> 
> Currently, for a controller that supports multiple queues, like UFS4.0,
> the mq_ops->complete is executed in the interrupt top-half. Therefore, 
> the file system's end io is executed during the request completion process,
> such as f2fs_write_end_io on smartphone.
> 
> However, we found that the execution time of the file system end io
> is strongly related to the size of the bio and the processing speed
> of the CPU. Because the file system's end io will traverse every page
> in bio, this is a very time-consuming operation.
> 
> We measured that the 80M bio write operation on the little CPU will
> cause the execution time of the top-half to be greater than 100ms,
> which will undoubtedly affect interrupt response latency.
> 
> Let's fixed this issue by moved non sync request completion flow to
> softirq, and keep the sync request completion in the top-half.

Typos/grammar:

Let's fix this issue by moving non sync requests completion to softirq context,
and keeping sync requests completion in the irq top-half context.

> 
> Signed-off-by: zhanghui <zhanghui31@...omi.com>
> ---
> Changes in v4:
> - fix commit log from "scheduling efficiency" to "interrupt response latency"
> 
> Changes in v3:
> - modify op_is_sync to rq_is_sync
> 
> Changes in v2:
> - fix build warning
> ---
>  block/blk-mq.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index e3c3c0c21b55..06b232edff11 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1193,6 +1193,8 @@ static void blk_mq_raise_softirq(struct request *rq)
>  
>  bool blk_mq_complete_request_remote(struct request *rq)
>  {
> +	const bool is_sync = rq_is_sync(rq);
> +
>  	WRITE_ONCE(rq->state, MQ_RQ_COMPLETE);
>  
>  	/*
> @@ -1210,7 +1212,7 @@ bool blk_mq_complete_request_remote(struct request *rq)
>  		return true;
>  	}
>  
> -	if (rq->q->nr_hw_queues == 1) {
> +	if ((rq->q->nr_hw_queues == 1) || !is_sync) {

I do not see the point of the is_sync variable, and you do not need the inner
parenthesis either. Why not:

	if (rq->q->nr_hw_queues == 1 || !rq_is_sync(rq)) {

And a comment before the "if" explaining why soft-irq is raised for non-sync
requests would be nice too.

>  		blk_mq_raise_softirq(rq);
>  		return true;
>  	}

-- 
Damien Le Moal
Western Digital Research