Date: Wed, 15 Aug 2012 14:39:31 +0200
From: Michal Hocko <mhocko@suse.cz>
To: Glauber Costa <glommer@parallels.com>
Cc: linux-kernel@vger.kernel.org, linux-mm@kvack.org, cgroups@vger.kernel.org,
        devel@openvz.org, Johannes Weiner <hannes@cmpxchg.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        kamezawa.hiroyu@jp.fujitsu.com, Christoph Lameter <cl@linux.com>,
        David Rientjes <rientjes@google.com>,
        Pekka Enberg <penberg@kernel.org>
Subject: Re: [PATCH v2 04/11] kmem accounting basic infrastructure
Message-ID: <20120815123931.GF23985@dhcp22.suse.cz>
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On Wed 15-08-12 13:33:55, Glauber Costa wrote:
[...]
> > This can
> > be quite confusing.  I am still not sure whether we should mix the two
> > things together. If somebody wants to limit the kernel memory he has to
> > touch the other limit anyway.  Do you have a strong reason to mix the
> > user and kernel counters?
> 
> This is funny, because the first opposition I found to this work was
> "Why would anyone want to limit it separately?" =p
> 
> It seems that a quite common use case is to have a container with a
> unified view of "memory" that it can use the way he likes, be it with
> kernel memory, or user memory. I believe those people would be happy to
> just silently account kernel memory to user memory, or at the most have
> a switch to enable it.
> 
> What gets clear from this back and forth, is that there are people
> interested in both use cases.

I am still not 100% sure myself. It is just clear that the reclaim would
need some work in order to do accounting like this.

> > My impression was that kernel allocation should simply fail while user
> > allocations might reclaim as well. Why should we reclaim just because of
> > the kernel allocation (which is unreclaimable from hard limit reclaim
> > point of view)?
> 
> That is not what the kernel does, in general. We assume that if he wants
> that memory and we can serve it, we should. Also, not all kernel memory
> is unreclaimable. We can shrink the slabs, for instance. Ying Han
> claims she has patches for that already...

Are those patches somewhere around?

[...]
> > This doesn't check for the hierachy so kmem_accounted might not be in 
> > sync with it's parents. mem_cgroup_create (below) needs to copy
> > kmem_accounted down from the parent and the above needs to check if this
> > is a similar dance like mem_cgroup_oom_control_write.
> > 
> 
> I don't see why we have to.
> 
> I believe in a A/B/C hierarchy, C should be perfectly able to set a
> different limit than its parents. Note that this is not a boolean.

Ohh, I wasn't clear enough. I am not against setting the _limit_ I just
meant that the kmem_accounted should be consistent within the hierarchy.

-- 
Michal Hocko
SUSE Labs
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