linux-kernel - Re: [PATCH v6] block: move non sync requests complete flow to softirq

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Message-ID: <38a71a3f-b505-48a3-bbaf-2bdf60dfcd9d@kernel.dk>
Date: Sat, 7 Sep 2024 07:46:25 -0600
From: Jens Axboe <axboe@...nel.dk>
To: ZhangHui <zhanghui31@...omi.com>, bvanassche@....org,
 ming.lei@...hat.com, dlemoal@...nel.org
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6] block: move non sync requests complete flow to softirq

On 9/6/24 8:49 PM, 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 fix this issue by moving non sync requests completion to softirq
> context, and keeping sync requests completion in the IRQ top-half context.

You keep ignoring the feedback, and hence I too shall be ignoring this
patch going forward then.

The key issue here is that the completion takes so long, and adding a
heuristic that equates not-sync with latency-not-important is pretty
bogus and not a good way to attempt to work around it.

-- 
Jens Axboe