Re: [PATCH] mm: memcontrol: reclaim when shrinking memory.high below usage

[Michal Hocko]

On Fri 11-03-16 14:49:34, Vladimir Davydov wrote:
> On Fri, Mar 11, 2016 at 10:53:09AM +0100, Michal Hocko wrote:
> > > OTOH memory.low and memory.high are perfect to be changed dynamically,
> > > basing on containers' memory demand/pressure. A load manager might want
> > > to reconfigure these knobs say every 5 seconds. Spawning a thread per
> > > each container that often would look unnecessarily overcomplicated IMO.
> > 
> > The question however is whether we want to hide a potentially costly
> > operation and have it unaccounted and hidden in the kworker context.
> 
> There's already mem_cgroup->high_work doing reclaim in an unaccounted
> context quite often if tcp accounting is enabled.

I suspect this is done because the charging context cannot do much
better.

> And there's kswapd.
> memory.high knob is for the root only so it can't be abused by an
> unprivileged user. Regarding a privileged user, e.g. load manager, it
> can screw things up anyway, e.g. by configuring sum of memory.low to be
> greater than total RAM on the host and hence driving kswapd mad.

I am not worried about abuse. It is just weird to move something which
can be perfectly sync to an async mode.
 
> > I mean fork() + write() doesn't sound terribly complicated to me to have
> > a rather subtle behavior in the kernel.
> 
> It'd be just a dubious API IMHO. With memory.max everything's clear: it
> tries to reclaim memory hard, may stall for several seconds, may invoke
> OOM, but if it finishes successfully we have memory.current less than
> memory.max. With this patch memory.high knob behaves rather strangely:
> it might stall, but there's no guarantee you'll have memory.current less
> than memory.high; moreover, according to the documentation it's OK to
> have memory.current greater than memory.high, so what's the point in
> calling synchronous reclaim blocking the caller?

Even if the reclaim is best effort it doesn't mean we should hide it
into an async context. There is simply no reason to do so. We do the
some for other knobs which are performing a potentially expensive
operation and do not guarantee the result.

-- 
Michal Hocko
SUSE Labs