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Date:   Wed, 11 May 2022 15:33:49 -0700
From:   Andrew Morton <akpm@linux-foundation.org>
To:     Minchan Kim <minchan@kernel.org>
Cc:     LKML <linux-kernel@vger.kernel.org>, linux-mm <linux-mm@kvack.org>,
        Suren Baghdasaryan <surenb@google.com>,
        Michal Hocko <mhocko@suse.com>,
        John Dias <joaodias@google.com>,
        Tim Murray <timmurray@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Vladimir Davydov <vdavydov.dev@gmail.com>,
        Martin Liu <liumartin@google.com>,
        Johannes Weiner <hannes@cmpxchg.org>
Subject: Re: [PATCH v4] mm: don't be stuck to rmap lock on reclaim path
Message-Id: <20220511153349.045ab3865f25920dce11ca16@linux-foundation.org>
In-Reply-To: <20220510215423.164547-1-minchan@kernel.org>
References: <20220510215423.164547-1-minchan@kernel.org>
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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?