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Date:   Tue, 7 Sep 2021 09:26:18 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Zhaoyang Huang <huangzhaoyang@gmail.com>
Cc:     Vlastimil Babka <vbabka@suse.cz>,
        Andrew Morton <akpm@linux-foundation.org>,
        Minchan Kim <minchan@kernel.org>,
        Zhaoyang Huang <zhaoyang.huang@unisoc.com>,
        "open list:MEMORY MANAGEMENT" <linux-mm@kvack.org>,
        LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH] mm : bail out from psi memstall after submit_bio in
 swap_readpage
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 <CAGWkznHY-iZ0ERbJdX-j4S5qOsUAzV0+x67maprz3W+xbq4hTQ@mail.gmail.com>
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On Tue, Sep 07, 2021 at 08:15:30PM +0800, Zhaoyang Huang wrote:
> On Tue, Sep 7, 2021 at 8:03 PM Vlastimil Babka <vbabka@suse.cz> wrote:
> >
> > On 9/7/21 13:59, Huangzhaoyang wrote:
> > > From: Zhaoyang Huang <zhaoyang.huang@unisoc.com>
> > >
> > > It doesn't make sense to count IO time into psi memstall. Bail out after
> > > bio submitted.
> >
> > Isn't that the point if psi, to observe real stalls, which include IO?

Yes, correct.

> IO stalls could be observed within blk_io_schedule. The time cost of
> the data from block device to RAM is counted here.

Yes, that is on purpose. The time a thread waits for swap read IO is
time in which the thread is not productive due to a lack of memory.

For async-submitted IO, this happens in lock_page() called from
do_swap_page(). If the submitting thread directly waits after the
submit_bio(), then that should be accounted too.

This patch doesn't make sense to me.