Re: [External Mail]Re: [PATCH v6] block: move non sync requests complete flow to softirq

[章辉]

On 2024/9/7 21:46, Jens Axboe wrote:
> On 9/6/24 8:49 PM, ZhangHui wrote:
>> From: zhanghui <zhanghui31@xxxxxxxxxx>
>>
>> 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
>
hi Jens,

Sorry for not replying in time.

We have basically determined the plan for the f2fs side. The short-term
plan is to limit the size of a single bio, and the long-term plan is to
change f2fs from page to folio to reduce the pagecache traversal time.

However, I think it also makes sense to move less urgent work out of the
IRQ top-half.

Thanks
Zhang