linux-kernel - Re: [PATCH v3] block: move non sync requests complete flow to softirq

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Message-ID: <ZtgT4HhEsyRJMoQH@fedora>
Date: Wed, 4 Sep 2024 16:01:36 +0800
From: Ming Lei <ming.lei@...hat.com>
To: ZhangHui <zhanghui31@...omi.com>
Cc: axboe@...nel.dk, bvanassche@....org, linux-block@...r.kernel.org,
	linux-kernel@...r.kernel.org, ming.lei@...hat.com
Subject: Re: [PATCH v3] block: move non sync requests complete flow to softirq

On Tue, Sep 03, 2024 at 07:54:37PM +0800, 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

What is 80M bio?

It is one known issue that soft lockup may be triggered in case of N:M
blk-mq mapping, but not sure if that is the case.

What is nr_hw_queues(blk_mq) and nr_cpus in your system?

> cause the execution time of the top-half to be greater than 100ms.
> The CPU tick on a smartphone is only 4ms, which will undoubtedly affect
> scheduling efficiency.

schedule is off too in softirq(bottom-half).

> 
> Let's fixed this issue by moved non sync request completion flow to
> softirq, and keep the sync request completion in the top-half.

If you do care interrupt-off or schedule-off latency, you may have to move
the IO handling into thread context in the driver.

BTW, threaded irq can't help you too.


Thanks, 
Ming