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Date:   Tue, 25 Oct 2022 16:54:02 -0400
From:   Johannes Weiner <hannes@cmpxchg.org>
To:     Yang Shi <shy828301@gmail.com>
Cc:     Andrew Morton <akpm@linux-foundation.org>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org, Eric Bergen <ebergen@meta.com>
Subject: Re: [PATCH] mm: vmscan: split khugepaged stats from direct reclaim
 stats
Message-ID: <Y1hM6sMRpBGUPRyo@cmpxchg.org>
References: <20221025170519.314511-1-hannes@cmpxchg.org>
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On Tue, Oct 25, 2022 at 12:40:15PM -0700, Yang Shi wrote:
> On Tue, Oct 25, 2022 at 10:05 AM Johannes Weiner <hannes@cmpxchg.org> wrote:
> >
> > Direct reclaim stats are useful for identifying a potential source for
> > application latency, as well as spotting issues with kswapd. However,
> > khugepaged currently distorts the picture: as a kernel thread it
> > doesn't impose allocation latencies on userspace, and it explicitly
> > opts out of kswapd reclaim. Its activity showing up in the direct
> > reclaim stats is misleading. Counting it as kswapd reclaim could also
> > cause confusion when trying to understand actual kswapd behavior.
> >
> > Break out khugepaged from the direct reclaim counters into new
> > pgsteal_khugepaged, pgdemote_khugepaged, pgscan_khugepaged counters.
> >
> > Test with a huge executable (CONFIG_READ_ONLY_THP_FOR_FS):
> >
> > pgsteal_kswapd 1342185
> > pgsteal_direct 0
> > pgsteal_khugepaged 3623
> > pgscan_kswapd 1345025
> > pgscan_direct 0
> > pgscan_khugepaged 3623
> 
> There are other kernel threads or works may allocate memory then
> trigger memory reclaim, there may be similar problems for them and
> someone may try to add a new stat. So how's about we make the stats
> more general, for example, call it "pg{steal|scan}_kthread"?

I'm not convinved that's a good idea.

Can you generally say that userspace isn't indirectly waiting for one
of those allocating threads? With khugepaged, we know.

And those other allocations are usually ___GFP_KSWAPD_RECLAIM, so if
they do direct reclaim, we'd probably want to know that kswapd is
failing to keep up (doubly so if userspace is waiting). In a shared
kthread counter, khugepaged would again muddy the waters.