linux-kernel - Re: [PATCH v2 3/4] mm/vmscan: Don't change pgdat state on base of a single LRU list state.

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Message-Id: <20180405151751.c07ee14496f9d5b691b49c64@linux-foundation.org>
Date:   Thu, 5 Apr 2018 15:17:51 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Andrey Ryabinin <aryabinin@...tuozzo.com>
Cc:     Mel Gorman <mgorman@...hsingularity.net>,
        Tejun Heo <tj@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, cgroups@...r.kernel.org
Subject: Re: [PATCH v2 3/4] mm/vmscan: Don't change pgdat state on base of a
 single LRU list state.

On Fri, 23 Mar 2018 18:20:28 +0300 Andrey Ryabinin <aryabinin@...tuozzo.com> wrote:

> We have separate LRU list for each memory cgroup. Memory reclaim iterates
> over cgroups and calls shrink_inactive_list() every inactive LRU list.
> Based on the state of a single LRU shrink_inactive_list() may flag
> the whole node as dirty,congested or under writeback. This is obviously
> wrong and hurtful. It's especially hurtful when we have possibly
> small congested cgroup in system. Than *all* direct reclaims waste time
> by sleeping in wait_iff_congested(). And the more memcgs in the system
> we have the longer memory allocation stall is, because
> wait_iff_congested() called on each lru-list scan.
> 
> Sum reclaim stats across all visited LRUs on node and flag node as dirty,
> congested or under writeback based on that sum. Also call
> congestion_wait(), wait_iff_congested() once per pgdat scan, instead of
> once per lru-list scan.
> 
> This only fixes the problem for global reclaim case. Per-cgroup reclaim
> may alter global pgdat flags too, which is wrong. But that is separate
> issue and will be addressed in the next patch.
> 
> This change will not have any effect on a systems with all workload
> concentrated in a single cgroup.
> 

Could we please get this reviewed?