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

[Barry Song]

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.

>
> >> 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