Re: [PATCH v3] memcg: schedule high reclaim for remote memcgs on high_work

[Shakeel Butt <shakeelb@google.com>]

On Sun, Jan 13, 2019 at 10:34 AM Michal Hocko <mhocko@kernel.org> wrote:
>
> On Fri 11-01-19 14:54:32, Shakeel Butt wrote:
> > Hi Johannes,
> >
> > On Fri, Jan 11, 2019 at 12:59 PM Johannes Weiner <hannes@cmpxchg.org> wrote:
> > >
> > > Hi Shakeel,
> > >
> > > On Thu, Jan 10, 2019 at 09:44:32AM -0800, Shakeel Butt wrote:
> > > > If a memcg is over high limit, memory reclaim is scheduled to run on
> > > > return-to-userland.  However it is assumed that the memcg is the current
> > > > process's memcg.  With remote memcg charging for kmem or swapping in a
> > > > page charged to remote memcg, current process can trigger reclaim on
> > > > remote memcg.  So, schduling reclaim on return-to-userland for remote
> > > > memcgs will ignore the high reclaim altogether. So, record the memcg
> > > > needing high reclaim and trigger high reclaim for that memcg on
> > > > return-to-userland.  However if the memcg is already recorded for high
> > > > reclaim and the recorded memcg is not the descendant of the the memcg
> > > > needing high reclaim, punt the high reclaim to the work queue.
> > >
> > > The idea behind remote charging is that the thread allocating the
> > > memory is not responsible for that memory, but a different cgroup
> > > is. Why would the same thread then have to work off any high excess
> > > this could produce in that unrelated group?
> > >
> > > Say you have a inotify/dnotify listener that is restricted in its
> > > memory use - now everybody sending notification events from outside
> > > that listener's group would get throttled on a cgroup over which it
> > > has no control. That sounds like a recipe for priority inversions.
> > >
> > > It seems to me we should only do reclaim-on-return when current is in
> > > the ill-behaved cgroup, and punt everything else - interrupts and
> > > remote charges - to the workqueue.
> >
> > This is what v1 of this patch was doing but Michal suggested to do
> > what this version is doing. Michal's argument was that the current is
> > already charging and maybe reclaiming a remote memcg then why not do
> > the high excess reclaim as well.
>
> Johannes has a good point about the priority inversion problems which I
> haven't thought about.
>
> > Personally I don't have any strong opinion either way. What I actually
> > wanted was to punt this high reclaim to some process in that remote
> > memcg. However I didn't explore much on that direction thinking if
> > that complexity is worth it. Maybe I should at least explore it, so,
> > we can compare the solutions. What do you think?
>
> My question would be whether we really care all that much. Do we know of
> workloads which would generate a large high limit excess?
>

The current semantics of memory.high is that it can be breached under
extreme conditions. However any workload where memory.high is used and
a lot of remote memcg charging happens (inotify/dnotify example given
by Johannes or swapping in tmpfs file or shared memory region) the
memory.high breach will become common.

Shakeel