Re: [patch] Revert "memcg: add memory.vmscan_stat"

Johannes Weiner
2011-08-30T07:05:34Z

On Tue, Aug 30, 2011 at 10:12:33AM +0900, KAMEZAWA Hiroyuki wrote:
> On Mon, 29 Aug 2011 17:51:13 +0200
> Johannes Weiner <jweiner@redhat.com> wrote:
>
> > On Tue, Aug 09, 2011 at 08:33:45AM +0900, KAMEZAWA Hiroyuki wrote:
> > > On Mon, 8 Aug 2011 14:43:33 +0200
> > > Johannes Weiner <jweiner@redhat.com> wrote:
> > >
> > > > On Fri, Jul 22, 2011 at 05:15:40PM +0900, KAMEZAWA Hiroyuki wrote:
> > > > > +When under_hierarchy is added in the tail, the number indicates the
> > > > > +total memcg scan of its children and itself.
> > > >
> > > > In your implementation, statistics are only accounted to the memcg
> > > > triggering the limit and the respectively scanned memcgs.
> > > >
> > > > Consider the following setup:
> > > >
> > > > A
> > > > / \
> > > > B C
> > > > /
> > > > D
> > > >
> > > > If D tries to charge but hits the limit of A, then B's hierarchy
> > > > counters do not reflect the reclaim activity resulting in D.
> > > >
> > > yes, as I expected.
> >
> > Andrew,
> >
> > with a flawed design, the author unwilling to fix it, and two NAKs,
> > can we please revert this before the release?
>
> How about this ?

> @@ -1710,11 +1711,18 @@ static void mem_cgroup_record_scanstat(s
> spin_lock(&memcg->scanstat.lock);
> __mem_cgroup_record_scanstat(memcg->scanstat.stats[context], rec);
> spin_unlock(&memcg->scanstat.lock);
> -
> - memcg = rec->root;
> - spin_lock(&memcg->scanstat.lock);
> - __mem_cgroup_record_scanstat(memcg->scanstat.rootstats[context], rec);
> - spin_unlock(&memcg->scanstat.lock);
> + cgroup = memcg->css.cgroup;
> + do {
> + spin_lock(&memcg->scanstat.lock);
> + __mem_cgroup_record_scanstat(
> + memcg->scanstat.hierarchy_stats[context], rec);
> + spin_unlock(&memcg->scanstat.lock);
> + if (!cgroup->parent)
> + break;
> + cgroup = cgroup->parent;
> + memcg = mem_cgroup_from_cont(cgroup);
> + } while (memcg->use_hierarchy && memcg != rec->root);

Okay, so this looks correct, but it sums up all parents after each
memcg scanned, which could have a performance impact. Usually,
hierarchy statistics are only summed up when a user reads them.

I don't get why this has to be done completely different from the way
we usually do things, without any justification, whatsoever.

Why do you want to pass a recording structure down the reclaim stack?
Why not make it per-cpu counters that are only summed up, together
with the hierarchy values, when someone is actually interested in
them? With an interface like mem_cgroup_count_vm_event(), or maybe
even an extension of that function?