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

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Message-ID: <877caspv6u.fsf@yhuang6-desk2.ccr.corp.intel.com>
Date: Tue, 01 Oct 2024 07:40:09 +0800
From: "Huang, Ying" <ying.huang@...el.com>
To: Barry Song <21cnbao@...il.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

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.

>> 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.

--
Best Regards,
Huang, Ying