Re: [PATCH] memcg: consider per-cpu stock reserves when returning RES_USAGE for _MEM

[Daisuke Nishimura <nishimura@mxp.nes.nec.co.jp>]

On Tue, 22 Mar 2011 08:31:50 +0100
Michal Hocko <mhocko@suse.cz> wrote:

> On Tue 22-03-11 10:47:23, Daisuke Nishimura wrote:
> > On Tue, 22 Mar 2011 09:10:14 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com> wrote:
> > 
> > > On Mon, 21 Mar 2011 11:24:20 +0100
> > > Michal Hocko <mhocko@suse.cz> wrote:
> > > 
> > > > [Sorry for reposting but I forgot to fully refresh the patch before
> > > > posting...]
> > > > 
> > > > On Mon 21-03-11 10:34:19, Michal Hocko wrote:
> > > > > On Fri 18-03-11 16:25:32, Michal Hocko wrote:
> > > > > [...]
> > > > > > According to our documention this is a reasonable test case:
> > > > > > Documentation/cgroups/memory.txt:
> > > > > > memory.usage_in_bytes           # show current memory(RSS+Cache) usage.
> > > > > > 
> > > > > > This however doesn't work after your commit:
> > > > > > cdec2e4265d (memcg: coalesce charging via percpu storage)
> > > > > > 
> > > > > > because since then we are charging in bulks so we can end up with
> > > > > > rss+cache <= usage_in_bytes.
> > > > > [...]
> > > > > > I think we have several options here
> > > > > > 	1) document that the value is actually >= rss+cache and it shows
> > > > > > 	   the guaranteed charges for the group
> > > > > > 	2) use rss+cache rather then res->count
> > > > > > 	3) remove the file
> > > > > > 	4) call drain_all_stock_sync before asking for the value in
> > > > > > 	   mem_cgroup_read
> > > > > > 	5) collect the current amount of stock charges and subtract it
> > > > > > 	   from the current res->count value
> > > > > > 
> > > > > > 1) and 2) would suggest that the file is actually not very much useful.
> > > > > > 3) is basically the interface change as well
> > > > > > 4) sounds little bit invasive as we basically lose the advantage of the
> > > > > > pool whenever somebody reads the file. Btw. for who is this file
> > > > > > intended?
> > > > > > 5) sounds like a compromise
> > > > > 
> > > > > I guess that 4) is really too invasive - for no good reason so here we
> > > > > go with the 5) solution.
> > > 
> > > I think the test in LTP is bad...(it should be fuzzy.) because we cannot
> > > avoid races...
> > I agree.
> 
> I think that as well. In fact, I quite do not understand what it is
> testing actually (that we account charges correctly? If yes then what if
> both values are wrong?). The other point is, though, we have exported this
> interface and documented its semantic. This is the reason I have asked
> for the initial purpose of the file in the first place. Is this
> something for debugging only? Can we make use of the value somehow
> (other than a shortcut for rss+cache)?
> 
> If there is realy no strong reason for the file existence I would rather
> vote for its removing than having a unusable semantic.
> 
Considering more, without these files, we cannot know the actual usage of
a res_counter, although we set a limit to a res_counter. So, I want to keep
these files.

If no-one have any objections, I'll prepare a patch to update the documentation.

Thanks,
Daisuke Nishimura.

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