On Tue, Oct 1, 2024 at 7:43 AM Huang, Ying ying.huang@intel.com wrote:
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.
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