linux-kernel - Re: [PATCH] mm: avoid unconditional one-tick sleep when swapcache

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Message-ID: <CAGsJ_4w2PjN+4DKWM6qvaEUAX=FQW0rp+6Wjx1Qrq=jaAz7wsw@mail.gmail.com>
Date: Tue, 1 Oct 2024 02:18:13 +1300
From: Barry Song <21cnbao@...il.com>
To: "Huang, Ying" <ying.huang@...el.com>
Cc: akpm@...ux-foundation.org, linux-mm@...ck.org, 
	linux-kernel@...r.kernel.org, Barry Song <v-songbaohua@...o.com>, 
	Kairui Song <kasong@...cent.com>, Yu Zhao <yuzhao@...gle.com>, 
	David Hildenbrand <david@...hat.com>, Chris Li <chrisl@...nel.org>, Hugh Dickins <hughd@...gle.com>, 
	Johannes Weiner <hannes@...xchg.org>, Matthew Wilcox <willy@...radead.org>, Michal Hocko <mhocko@...e.com>, 
	Minchan Kim <minchan@...nel.org>, Yosry Ahmed <yosryahmed@...gle.com>, 
	SeongJae Park <sj@...nel.org>, Kalesh Singh <kaleshsingh@...gle.com>, 
	Suren Baghdasaryan <surenb@...gle.com>, stable@...r.kernel.org, 
	Oven Liyang <liyangouwen1@...o.com>
Subject: Re: [PATCH] mm: avoid unconditional one-tick sleep when
 swapcache_prepare fails

On Sun, Sep 29, 2024 at 3:43 PM Huang, Ying <ying.huang@...el.com> wrote:
>
> Hi, Barry,
>
> Barry Song <21cnbao@...il.com> writes:
>
> > From: Barry Song <v-songbaohua@...o.com>
> >
> > Commit 13ddaf26be32 ("mm/swap: fix race when skipping swapcache")
> > introduced an unconditional one-tick sleep when `swapcache_prepare()`
> > fails, which has led to reports of UI stuttering on latency-sensitive
> > Android devices. To address this, we can use a waitqueue to wake up
> > tasks that fail `swapcache_prepare()` sooner, instead of always
> > sleeping for a full tick. While tasks may occasionally be woken by an
> > unrelated `do_swap_page()`, this method is preferable to two scenarios:
> > rapid re-entry into page faults, which can cause livelocks, and
> > multiple millisecond sleeps, which visibly degrade user experience.
>
> In general, I think that this works.  Why not extend the solution to
> cover schedule_timeout_uninterruptible() in __read_swap_cache_async()
> too?  We can call wake_up() when we clear SWAP_HAS_CACHE.  To avoid

Hi Ying,
Thanks for your comments.
I feel extending the solution to __read_swap_cache_async() should be done
in a separate patch. On phones, I've never encountered any issues reported
on that path, so it might be better suited for an optimization rather than a
hotfix?

> overhead to call wake_up() when there's no task waiting, we can use an
> atomic to count waiting tasks.

I'm not sure it's worth adding the complexity, as wake_up() on an empty
waitqueue should have a very low cost on its own?

>
> [snip]
>
> --
> Best Regards,
> Huang, Ying

Thanks
Barry