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

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Message-ID: <CAGsJ_4wfjo2-dnGwybx5YR_o+FEzoVG+V=O1mxQ801FdHPSGiA@mail.gmail.com>
Date: Tue, 1 Oct 2024 22:16:40 +0800
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 Tue, Oct 1, 2024 at 7:43 AM Huang, Ying <ying.huang@...el.com> wrote:
>
> Barry Song <21cnbao@...il.com> writes:
>
> > 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?
>
> Yes.  It's fine to do that in another patch as optimization.

Ok. I'll prepare a separate patch for optimizing that path.

>
> >> 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?
>
> wake_up() needs to call spin_lock_irqsave() unconditionally on a global
> shared lock.  On systems with many CPUs (such servers), this may cause
> severe lock contention.  Even the cache ping-pong may hurt performance
> much.

I understand that cache synchronization was a significant issue before
qspinlock, but it seems to be less of a concern after its implementation.
However, using a global atomic variable would still trigger cache broadcasts,
correct?

>
> --
> Best Regards,
> Huang, Ying

Thanks
Barry