linux-kernel - Re: [PATCH v2] mm: don't be stuck to rmap lock on reclaim path

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Message-Id: <20220509115449.f48559dd40a5e5ec95b8ead8@linux-foundation.org>
Date:   Mon, 9 May 2022 11:54:49 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Minchan Kim <minchan@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, linux-mm <linux-mm@...ck.org>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Michal Hocko <mhocko@...e.com>,
        John Dias <joaodias@...gle.com>,
        Tim Murray <timmurray@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>
Subject: Re: [PATCH v2] mm: don't be stuck to rmap lock on reclaim path

On Mon,  9 May 2022 08:47:10 -0700 Minchan Kim <minchan@...nel.org> wrote:

> The rmap locks(i_mmap_rwsem and anon_vma->root->rwsem) could be
> contented 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+(a sec as worst case) 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.
> 
> 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).

Some quantitative testing results would be helpful.  Demonstrate
the benefits of the patch?