On Wed, Oct 2, 2024 at 10:02 AM Barry Song 21cnbao@gmail.com wrote:
On Wed, Oct 2, 2024 at 8:43 AM Huang, Ying ying.huang@intel.com wrote:
Barry Song 21cnbao@gmail.com writes:
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?
Hi Barry and Ying,
For the __read_swap_cache_async case, I'm not really against adding a similar workqueue, but if no one is really suffering from it, and if the workqueue do causes extra overhead, maybe we can ignore it for the __read_swap_cache_async case now, and I plan to resent the following patch: https://lore.kernel.org/linux-mm/20240326185032.72159-9-ryncsn@gmail.com/#r
It removed all schedule_timeout_uninterruptible workaround and other similar things, and the performance will go even higher.
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@worktop.programming.kick...
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.
yes. this can be implemented by adding another atomic variable.
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.
Yes, Kairui please run a test on your machine with lots of cores before and after adding a global atomic variable as suggested by Ying. I am sorry I don't have a server machine.
I just had a try with the build kernel test which I used for the allocator patch series, with -j64, 1G memcg on my local branch:
Without the patch: 2677.63user 9100.43system 3:33.15elapsed 5452%CPU (0avgtext+0avgdata 863284maxresident)k 2671.40user 8969.07system 3:33.67elapsed 5447%CPU (0avgtext+0avgdata 863316maxresident)k 2673.66user 8973.90system 3:33.18elapsed 5463%CPU (0avgtext+0avgdata 863284maxresident)k
With the patch: 2655.05user 9134.21system 3:35.63elapsed 5467%CPU (0avgtext+0avgdata 863288maxresident)k 2652.57user 9104.87system 3:35.07elapsed 5466%CPU (0avgtext+0avgdata 863272maxresident)k 2665.44user 9155.97system 3:35.92elapsed 5474%CPU (0avgtext+0avgdata 863316maxresident)k
Only three test runs, the main bottleneck for the test is still some other locks (list_lru lock, swap cgroup lock etc), but it does show the performance seems a bit lower. Could be considered a trivial amount of overhead so I think it's acceptable for the SYNC_IO path.