Re: [PATCH] mm: avoid unconditional one-tick sleep when swapcache_prepare fails

["Huang, Ying" <ying.huang@intel.com>]

Barry Song <21cnbao@gmail.com> writes:

> On Sun, Sep 29, 2024 at 3:43 PM Huang, Ying <ying.huang@intel.com> wrote:
>>
>> Hi, Barry,
>>
>> Barry Song <21cnbao@gmail.com> writes:
>>
>> > From: Barry Song <v-songbaohua@oppo.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