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

[Johannes Weiner <hannes@cmpxchg.org>]

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.