linux-kernel - Re: [PATCH] mm: memcontrol: avoid workload stalls when lowering memory.high

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Date:   Tue, 14 Jul 2020 10:41:23 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Shakeel Butt <shakeelb@...gle.com>
Cc:     Roman Gushchin <guro@...com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Linux MM <linux-mm@...ck.org>,
        Kernel Team <kernel-team@...com>,
        LKML <linux-kernel@...r.kernel.org>,
        Domas Mituzas <domas@...com>, Tejun Heo <tj@...nel.org>,
        Chris Down <chris@...isdown.name>
Subject: Re: [PATCH] mm: memcontrol: avoid workload stalls when lowering
 memory.high

On Fri 10-07-20 12:19:37, Shakeel Butt wrote:
> On Fri, Jul 10, 2020 at 11:42 AM Roman Gushchin <guro@...com> wrote:
> >
> > On Fri, Jul 10, 2020 at 07:12:22AM -0700, Shakeel Butt wrote:
> > > On Fri, Jul 10, 2020 at 5:29 AM Michal Hocko <mhocko@...nel.org> wrote:
> > > >
> > > > On Thu 09-07-20 12:47:18, Roman Gushchin wrote:
> > > > > Memory.high limit is implemented in a way such that the kernel
> > > > > penalizes all threads which are allocating a memory over the limit.
> > > > > Forcing all threads into the synchronous reclaim and adding some
> > > > > artificial delays allows to slow down the memory consumption and
> > > > > potentially give some time for userspace oom handlers/resource control
> > > > > agents to react.
> > > > >
> > > > > It works nicely if the memory usage is hitting the limit from below,
> > > > > however it works sub-optimal if a user adjusts memory.high to a value
> > > > > way below the current memory usage. It basically forces all workload
> > > > > threads (doing any memory allocations) into the synchronous reclaim
> > > > > and sleep. This makes the workload completely unresponsive for
> > > > > a long period of time and can also lead to a system-wide contention on
> > > > > lru locks. It can happen even if the workload is not actually tight on
> > > > > memory and has, for example, a ton of cold pagecache.
> > > > >
> > > > > In the current implementation writing to memory.high causes an atomic
> > > > > update of page counter's high value followed by an attempt to reclaim
> > > > > enough memory to fit into the new limit. To fix the problem described
> > > > > above, all we need is to change the order of execution: try to push
> > > > > the memory usage under the limit first, and only then set the new
> > > > > high limit.
> > > >
> > > > Shakeel would this help with your pro-active reclaim usecase? It would
> > > > require to reset the high limit right after the reclaim returns which is
> > > > quite ugly but it would at least not require a completely new interface.
> > > > You would simply do
> > > >         high = current - to_reclaim
> > > >         echo $high > memory.high
> > > >         echo infinity > memory.high # To prevent direct reclaim
> > > >                                     # allocation stalls
> > > >
> > >
> > > This will reduce the chance of stalls but the interface is still
> > > non-delegatable i.e. applications can not change their own memory.high
> > > for the use-cases like application controlled proactive reclaim and
> > > uswapd.
> >
> > Can you, please, elaborate a bit more on this? I didn't understand
> > why.
> >
> 
> Sure. Do we want memory.high a CFTYPE_NS_DELEGATABLE type file? I
> don't think so otherwise any job on a system can change their
> memory.high and can adversely impact the isolation and memory
> scheduling of the system.

Is this really the case? There should always be a parent cgroup that
overrides the setting. Also you can always set the hard limit if you do
not want to add another layer of cgroup in the hierarchy before
delegation. Or am I missing something?

-- 
Michal Hocko
SUSE Labs