linux-kernel - Re: [REGRESSION] 6.7.1: md: raid5 hang and unresponsive system; successfully bisected

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Date: Thu, 14 Mar 2024 15:36:38 -0700
From: Song Liu <song@...nel.org>
To: junxiao.bi@...cle.com
Cc: Yu Kuai <yukuai1@...weicloud.com>, 
	Linux regressions mailing list <regressions@...ts.linux.dev>, gregkh@...uxfoundation.org, 
	linux-kernel@...r.kernel.org, linux-raid@...r.kernel.org, 
	stable@...r.kernel.org, Dan Moulding <dan@...m.net>, "yukuai (C)" <yukuai3@...wei.com>
Subject: Re: [REGRESSION] 6.7.1: md: raid5 hang and unresponsive system;
 successfully bisected

On Thu, Mar 14, 2024 at 11:20 AM <junxiao.bi@...cle.com> wrote:
>
[...]
> >>
> >> This patch eliminated the benefit of blk_plug, i think it will not be
> >> good for IO performance perspective?
> >
> > There is only one daemon thread, so IO should not be handled here as
> > much as possible. The IO should be handled by the thread that is
> > submitting the IO, and let daemon to hanldle the case that IO failed or
> > can't be submitted at that time.

raid5 can have multiple threads calling handle_stripe(). See raid5_do_work().
Only chunk_aligned_read() can be handled in raid5_make_request.

>
> I am not sure how much it will impact regarding drop the blk_plug.
>
> Song, what's your take on this?

I think we need to evaluate the impact of (removing) blk_plug. We had
some performance regressions related to blk_plug a couple years ago.

Thanks,
Song