linux-kernel - Re: [PATCH] mm: memcontrol: asynchronous reclaim for memory.high

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Date:   Wed, 19 Feb 2020 16:41:12 -0500
From:   Daniel Jordan <daniel.m.jordan@...cle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tejun Heo <tj@...nel.org>, Roman Gushchin <guro@...com>,
        linux-mm@...ck.org, cgroups@...r.kernel.org,
        linux-kernel@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH] mm: memcontrol: asynchronous reclaim for memory.high

On Wed, Feb 19, 2020 at 08:53:32PM +0100, Michal Hocko wrote:
> On Wed 19-02-20 14:16:18, Johannes Weiner wrote:
> > On Wed, Feb 19, 2020 at 07:37:31PM +0100, Michal Hocko wrote:
> > > On Wed 19-02-20 13:12:19, Johannes Weiner wrote:
> > > > This patch adds asynchronous reclaim to the memory.high cgroup limit
> > > > while keeping direct reclaim as a fallback. In our testing, this
> > > > eliminated all direct reclaim from the affected workload.
> > > 
> > > Who is accounted for all the work? Unless I am missing something this
> > > just gets hidden in the system activity and that might hurt the
> > > isolation. I do see how moving the work to a different context is
> > > desirable but this work has to be accounted properly when it is going to
> > > become a normal mode of operation (rather than a rare exception like the
> > > existing irq context handling).
> > 
> > Yes, the plan is to account it to the cgroup on whose behalf we're
> > doing the work.

How are you planning to do that?

I've been thinking about how to account a kernel thread's CPU usage to a cgroup
on and off while working on the parallelizing Michal mentions below.  A few
approaches are described here:

https://lore.kernel.org/linux-mm/20200212224731.kmss6o6agekkg3mw@ca-dmjordan1.us.oracle.com/

> shows that the amount of the work required for the high limit reclaim
> can be non-trivial. Somebody has to do that work and we cannot simply
> allow everybody else to pay for that.
> 
> > The problem is that we have a general lack of usable CPU control right
> > now - see Rik's work on this: https://lkml.org/lkml/2019/8/21/1208.
> > For workloads that are contended on CPU, we cannot enable the CPU
> > controller because the scheduling latencies are too high. And for
> > workloads that aren't CPU contended, well, it doesn't really matter
> > where the reclaim cycles are accounted to.
> > 
> > Once we have the CPU controller up to speed, we can add annotations
> > like these to account stretches of execution to specific
> > cgroups. There just isn't much point to do it before we can actually
> > enable CPU control on the real workloads where it would matter.

Which annotations do you mean?  I didn't see them when skimming through Rik's
work or in this patch.