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

[Barry Song <21cnbao@gmail.com>]

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?

> 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?

>
> [snip]
>
> --
> Best Regards,
> Huang, Ying

Thanks
Barry