linux-kernel - Re: [RFC PROPOSAL] memcg: per-memcg user space reclaim interface

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Date:   Fri, 3 Jul 2020 08:50:21 -0700
From:   Roman Gushchin <guro@...com>
To:     Shakeel Butt <shakeelb@...gle.com>
CC:     Michal Hocko <mhocko@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Yang Shi <yang.shi@...ux.alibaba.com>,
        David Rientjes <rientjes@...gle.com>,
        Greg Thelen <gthelen@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linux MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Cgroups <cgroups@...r.kernel.org>
Subject: Re: [RFC PROPOSAL] memcg: per-memcg user space reclaim interface

On Fri, Jul 03, 2020 at 07:23:14AM -0700, Shakeel Butt wrote:
> On Thu, Jul 2, 2020 at 11:35 PM Michal Hocko <mhocko@...nel.org> wrote:
> >
> > On Thu 02-07-20 08:22:22, Shakeel Butt wrote:
> > [...]
> > > Interface options:
> > > ------------------
> > >
> > > 1) memcg interface e.g. 'echo 10M > memory.reclaim'
> > >
> > > + simple
> > > + can be extended to target specific type of memory (anon, file, kmem).
> > > - most probably restricted to cgroup v2.
> > >
> > > 2) fadvise(PAGEOUT) on cgroup_dir_fd
> > >
> > > + more general and applicable to other FSes (actually we are using
> > > something similar for tmpfs).
> > > + can be extended in future to just age the LRUs instead of reclaim or
> > > some new use cases.
> >
> > Could you explain why memory.high as an interface to trigger pro-active
> > memory reclaim is not sufficient. Also memory.low limit to protect
> > latency sensitve workloads?

I initially liked the proposal, but after some thoughts I've realized
that I don't know a good use case where memory.high is less useful.
Shakeel, what's the typical use case you thinking of?
Who and how will use the new interface?

> 
> Yes, we can use memory.high to trigger [proactive] reclaim in a memcg
> but note that it can also introduce stalls in the application running
> in that memcg. Let's suppose the memory.current of a memcg is 100MiB
> and we want to reclaim 20MiB from it, we can set the memory.high to
> 80MiB but any allocation attempt from the application running in that
> memcg can get stalled/throttled. I want the functionality of the
> reclaim without potential stalls.

But reclaiming some pagecache/swapping out anon pages can always
generate some stalls caused by pagefaults, no?

Thanks!