Re: [PATCH v16 00/22] per memcg lru_lock

Alexander Duyck
2020-07-16T14:11:12Z

On Fri, Jul 10, 2020 at 5:59 PM Alex Shi <alex.shi-KPsoFbNs7GizrGE5bRqYAgC/G2K4zDHf@public.gmane.org> wrote:
>
> The new version which bases on v5.8-rc4. Add 2 more patchs:
> 'mm/thp: remove code path which never got into'
> 'mm/thp: add tail pages into lru anyway in split_huge_page()'
> and modified 'mm/mlock: reorder isolation sequence during munlock'
>
> Current lru_lock is one for each of node, pgdat->lru_lock, that guard for
> lru lists, but now we had moved the lru lists into memcg for long time. Still
> using per node lru_lock is clearly unscalable, pages on each of memcgs have
> to compete each others for a whole lru_lock. This patchset try to use per
> lruvec/memcg lru_lock to repleace per node lru lock to guard lru lists, make
> it scalable for memcgs and get performance gain.
>
> Currently lru_lock still guards both lru list and page's lru bit, that's ok.
> but if we want to use specific lruvec lock on the page, we need to pin down
> the page's lruvec/memcg during locking. Just taking lruvec lock first may be
> undermined by the page's memcg charge/migration. To fix this problem, we could
> take out the page's lru bit clear and use it as pin down action to block the
> memcg changes. That's the reason for new atomic func TestClearPageLRU.
> So now isolating a page need both actions: TestClearPageLRU and hold the
> lru_lock.
>
> The typical usage of this is isolate_migratepages_block() in compaction.c
> we have to take lru bit before lru lock, that serialized the page isolation
> in memcg page charge/migration which will change page's lruvec and new
> lru_lock in it.
>
> The above solution suggested by Johannes Weiner, and based on his new memcg
> charge path, then have this patchset. (Hugh Dickins tested and contributed much
> code from compaction fix to general code polish, thanks a lot!).
>
> The patchset includes 3 parts:
> 1, some code cleanup and minimum optimization as a preparation.
> 2, use TestCleanPageLRU as page isolation's precondition
> 3, replace per node lru_lock with per memcg per node lru_lock
>
> Following Daniel Jordan's suggestion, I have run 208 'dd' with on 104
> containers on a 2s * 26cores * HT box with a modefied case:
> https://git.kernel.org/pub/scm/linux/kernel/git/wfg/vm-scalability.git/tree/case-lru-file-readtwice
> With this patchset, the readtwice performance increased about 80%
> in concurrent containers.
>
> Thanks Hugh Dickins and Konstantin Khlebnikov, they both brought this
> idea 8 years ago, and others who give comments as well: Daniel Jordan,
> Mel Gorman, Shakeel Butt, Matthew Wilcox etc.
>
> Thanks for Testing support from Intel 0day and Rong Chen, Fengguang Wu,
> and Yun Wang. Hugh Dickins also shared his kbuild-swap case. Thanks!

Hi Alex,

I think I am seeing a regression with this patch set when I run the
will-it-scale/page_fault3 test. Specifically the processes result is
dropping from 56371083 to 43127382 when I apply these patches.

I haven't had a chance to bisect and figure out what is causing it,
and wanted to let you know in case you are aware of anything specific
that may be causing this.

Thanks.

- Alex