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

[Huang, Ying]

Barry Song <21cnbao@xxxxxxxxx> writes:

> On Tue, Oct 1, 2024 at 7:43 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>>
>> Barry Song <21cnbao@xxxxxxxxx> writes:
>>
>> > On Sun, Sep 29, 2024 at 3:43 PM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>> >>
>> >> Hi, Barry,
>> >>
>> >> Barry Song <21cnbao@xxxxxxxxx> writes:
>> >>
>> >> > From: Barry Song <v-songbaohua@xxxxxxxx>
>> >> >
>> >> > 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.

Thanks!

>>
>> >> 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.

Unfortunately, qspinlock cannot eliminate cache ping-pong issue, as
discussed in the following thread.

https://lore.kernel.org/lkml/20220510192708.GQ76023@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/

> However, using a global atomic variable would still trigger cache broadcasts,
> correct?

We can only change the atomic variable to non-zero when
swapcache_prepare() returns non-zero, and call wake_up() when the atomic
variable is non-zero.  Because swapcache_prepare() returns 0 most times,
the atomic variable is 0 most times.  If we don't change the value of
atomic variable, cache ping-pong will not be triggered.

Hi, Kairui,

Do you have some test cases to test parallel zram swap-in?  If so, that
can be used to verify whether cache ping-pong is an issue and whether it
can be fixed via a global atomic variable.

--
Best Regards,
Huang, Ying