Re: [PATCH v4] mm: don't be stuck to rmap lock on reclaim path

[Andrew Morton <akpm@linux-foundation.org>]

On Tue, 10 May 2022 14:54:23 -0700 Minchan Kim <minchan@kernel.org> wrote:

> The rmap locks(i_mmap_rwsem and anon_vma->root->rwsem) could be
> contended under memory pressure if processes keep working on
> their vmas(e.g., fork, mmap, munmap). It makes reclaim path
> stuck. In our real workload traces, we see kswapd is waiting the
> lock for 300ms+(worst case, a sec) and it makes other processes
> entering direct reclaim, which were also stuck on the lock.
> 
> This patch makes lru aging path try_lock mode like shink_page_list
> so the reclaim context will keep working with next lru pages
> without being stuck. if it found the rmap lock contended, it rotates
> the page back to head of lru in both active/inactive lrus to make
> them consistent behavior, which is basic starting point rather than
> adding more heristic.
> 
> Since this patch introduces a new "contended" field as out-param
> along with try_lock in-param in rmap_walk_control, it's not
> immutable any longer if the try_lock is set so remove const
> keywords on rmap related functions. Since rmap walking is already
> expensive operation, I doubt the const would help sizable benefit(
> And we didn't have it until 5.17).
> 
> In a heavy app workload in Android, trace shows following statistics.
> It almost removes rmap lock contention from reclaim path.

What might be the worst-case failure modes using this approach?

Could we burn much CPU time pointlessly churning though the LRU?  Could
it mess up aging decisions enough to be performance-affecting in any
workload?

Something else?