linux-kernel - Re: [PATCH v3 1/4] memcg: introduce per-memcg reclaim interface

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Date:   Fri, 8 Apr 2022 13:08:26 -0700
From:   Yosry Ahmed <yosryahmed@...gle.com>
To:     Dan Schatzberg <schatzberg.dan@...il.com>,
        Michal Hocko <mhocko@...e.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Shakeel Butt <shakeelb@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <roman.gushchin@...ux.dev>,
        David Rientjes <rientjes@...gle.com>,
        Tejun Heo <tj@...nel.org>, Zefan Li <lizefan.x@...edance.com>,
        Jonathan Corbet <corbet@....net>,
        Shuah Khan <shuah@...nel.org>, Yu Zhao <yuzhao@...gle.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Wei Xu <weixugc@...gle.com>, Greg Thelen <gthelen@...gle.com>,
        Chen Wandun <chenwandun@...wei.com>,
        Vaibhav Jain <vaibhav@...ux.ibm.com>,
        Michal Koutný <mkoutny@...e.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>, cgroups@...r.kernel.org,
        linux-doc@...r.kernel.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux-MM <linux-mm@...ck.org>, linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v3 1/4] memcg: introduce per-memcg reclaim interface

On Fri, Apr 8, 2022 at 7:55 AM Dan Schatzberg <schatzberg.dan@...il.com> wrote:
>
> On Fri, Apr 08, 2022 at 04:11:05PM +0200, Michal Hocko wrote:
> > Regarding "max" as a possible input. I am not really sure to be honest.
> > I can imagine that it could be legit to simply reclaim all the charges
> > (e.g. before removing the memcg) which should be achieveable by
> > reclaiming the reported consumption. Or what exactly should be the
> > semantic?
>
> Yeah, it just allows you to avoid reading memory.current to just
> reclaim everything if you can specify "max" - you're still protected
> by nretries to eventually bail out. Mostly, though I just feel like
> supporting "max" makes memory.reclaim semetric with a lot of the
> cgroup memory control files which tend to support "max".

One possible approach here is to have force_empty behavior when we
write "max" to memory.reclaim. From Google's perspective we don't have
a preference, but it seems to me like logical behavior. We can do this
either by directly calling mem_cgroup_force_empty() or just draining
stock and lrus in memory_reclaim().

This actually brings up another interesting point. Do you think we
should drain lrus if try_to_free_mem_cgroup_pages() fails to reclaim
the request amount? We can do this after the first call or before the
last one. It could introduce more evictable pages for
try_to_free_mem_cgroup_pages() to free.