From: Shakeel Butt <shakeelb@google.com>
To: "Huang, Ying" <ying.huang@intel.com>
Cc: Tejun Heo <tj@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>,
Linux MM <linux-mm@kvack.org>,
LKML <linux-kernel@vger.kernel.org>,
Mel Gorman <mgorman@suse.de>,
Johannes Weiner <hannes@cmpxchg.org>,
Vladimir Davydov <vdavydov.dev@gmail.com>,
Michal Hocko <mhocko@suse.cz>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>
Subject: Re: [PATCH] vmscan: retry without cache trim mode if nothing scanned
Date: Sun, 14 Mar 2021 13:58:33 -0700 [thread overview]
Message-ID: <CALvZod6HC-oDQJgQxh=sJi91y0_SK2xuMMW35aac0YURjbPEOQ@mail.gmail.com> (raw)
In-Reply-To: <87v99yvzq8.fsf@yhuang-dev.intel.com>
On Thu, Mar 11, 2021 at 12:52 AM Huang, Ying <ying.huang@intel.com> wrote:
>
> Hi, Butt,
>
> Shakeel Butt <shakeelb@google.com> writes:
>
> > On Wed, Mar 10, 2021 at 4:47 PM Huang, Ying <ying.huang@intel.com> wrote:
> >>
> >> From: Huang Ying <ying.huang@intel.com>
> >>
> >> In shrink_node(), to determine whether to enable cache trim mode, the
> >> LRU size is gotten via lruvec_page_state(). That gets the value from
> >> a per-CPU counter (mem_cgroup_per_node->lruvec_stat[]). The error of
> >> the per-CPU counter from CPU local counting and the descendant memory
> >> cgroups may cause some issues. We run into this in 0-Day performance
> >> test.
> >>
> >> 0-Day uses the RAM file system as root file system, so the number of
> >> the reclaimable file pages is very small. In the swap testing, the
> >> inactive file LRU list will become almost empty soon. But the size of
> >> the inactive file LRU list gotten from the per-CPU counter may keep a
> >> much larger value (say, 33, 50, etc.). This will enable cache trim
> >> mode, but nothing can be scanned in fact. The following pattern
> >> repeats for long time in the test,
> >>
> >> priority inactive_file_size cache_trim_mode
> >> 12 33 0
> >> 11 33 0
> >> ...
> >> 6 33 0
> >> 5 33 1
> >> ...
> >> 1 33 1
> >>
> >> That is, the cache_trim_mode will be enabled wrongly when the scan
> >> priority decreases to 5. And the problem will not be recovered for
> >> long time.
> >>
> >> It's hard to get the more accurate size of the inactive file list
> >> without much more overhead. And it's hard to estimate the error of
> >> the per-CPU counter too, because there may be many descendant memory
> >> cgroups. But after the actual scanning, if nothing can be scanned
> >> with the cache trim mode, it should be wrong to enable the cache trim
> >> mode. So we can retry with the cache trim mode disabled. This patch
> >> implement this policy.
> >
> > Instead of playing with the already complicated heuristics, we should
> > improve the accuracy of the lruvec stats. Johannes already fixed the
> > memcg stats using rstat infrastructure and Tejun has suggestions on
> > how to use rstat infrastructure efficiently for lruvec stats at
> > https://lore.kernel.org/linux-mm/YCFgr300eRiEZwpL@slm.duckdns.org/.
>
> Thanks for your information! It should be better if we can improve the
> accuracy of lruvec stats without much overhead. But that may be not a
> easy task.
>
> If my understanding were correct, what Tejun suggested is to add a fast
> read interface to rstat to be used in hot path. And its accuracy is
> similar as that of traditional per-CPU counter. But if we can regularly
> update the lruvec rstat with something like vmstat_update(), that should
> be OK for the issue described in this patch.
>
This is also my understanding. Tejun, please correct us if we misunderstood you.
BTW Johannes was working on rstat-based lruvec stats patch. Johannes,
are you planning to work on the optimization Tejun has suggested.
next prev parent reply other threads:[~2021-03-14 20:58 UTC|newest]
Thread overview: 5+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-11 0:44 [PATCH] vmscan: retry without cache trim mode if nothing scanned Huang, Ying
2021-03-11 0:57 ` Shakeel Butt
2021-03-11 8:52 ` Huang, Ying
2021-03-14 20:58 ` Shakeel Butt [this message]
2021-03-14 22:51 ` Tejun Heo
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