linux-kernel - Re: [PATCH] mm, vmscan: Don't turn on cache_trim

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Date: Wed, 21 Feb 2024 14:30:13 -0800
From: Andrew Morton <akpm@...ux-foundation.org>
To: Yu Zhao <yuzhao@...gle.com>
Cc: Byungchul Park <byungchul@...com>, linux-kernel@...r.kernel.org,
 linux-mm@...ck.org, kernel_team@...ynix.com
Subject: Re: [PATCH] mm, vmscan: Don't turn on cache_trim_mode at the
 highest scan priority

On Sat, 17 Feb 2024 00:11:25 -0500 Yu Zhao <yuzhao@...gle.com> wrote:

> On Fri, Feb 16, 2024 at 2:24 AM Byungchul Park <byungchul@...com> wrote:
> >
> > On Fri, Feb 16, 2024 at 12:55:17AM -0500, Yu Zhao wrote:
> > > On Thu, Feb 8, 2024 at 1:18 AM Byungchul Park <byungchul@...com> wrote:
> > > >
> > > > With cache_trim_mode on, reclaim logic doesn't bother reclaiming anon
> > > > pages. However, it should be more careful to turn on the mode because
> > > > it's going to prevent anon pages from reclaimed even if there are huge
> > > > ammount of anon pages that are very cold so should be reclaimed. Even
> > > > worse, that can lead kswapd_failures to be MAX_RECLAIM_RETRIES and stop
> > > > until direct reclaim eventually works to resume kswapd.
> > >
> > > Is a theory or something observed in the real world? If it's the
> > > former, would this change risk breaking existing use cases? It's the
> >
> > I faced the latter case.
> >
> > > latter, where are the performance numbers to show what it looks like
> > > before and after this patch?
> 
> Let me ask again: where are the performance numbers to show what it
> looks like before and after this patch?
> 
> > Before:
> >
> > Whenever the system meets the condition to turn on cache_trim_mode but
> > few cache pages to trim, kswapd fails without scanning anon pages that
> > are plenty and cold for sure and it retries 8 times and looks *stopped
> > for ever*.

Does "stopped for ever" mean that kswapd simply stops functioning?

If so, that's a pretty serious issue.  Please fully describe all of
this in the changelog.  Please also address Yu Zhao's review comments
and send us a v2 patch?  Thanks.