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