Re: [PATCH v6 1/7] mm/memcg: scanning_global_lru means mem_cgroup_disabled

Michal Hocko
2012-03-26T15:31:34Z

On Mon 26-03-12 17:18:15, Johannes Weiner wrote:
> On Mon, Mar 26, 2012 at 05:04:29PM +0200, Michal Hocko wrote:
> > [Adding Johannes to CC]
> >
> > On Fri 23-03-12 01:56:16, Konstantin Khlebnikov wrote:
> > > From: Hugh Dickins <hughd@google.com>
> > >
> > > Although one has to admire the skill with which it has been concealed,
> > > scanning_global_lru(mz) is actually just an interesting way to test
> > > mem_cgroup_disabled(). Too many developer hours have been wasted on
> > > confusing it with global_reclaim(): just use mem_cgroup_disabled().
> >
> > Is this really correct?
>
> Yes, if the memory controller is enabled, we never have a global LRU
> and always scan the per-memcg lists.
>
> > > Signed-off-by: Hugh Dickins <hughd@google.com>
> > > Signed-off-by: Konstantin Khlebnikov <khlebnikov@openvz.org>
> > > Acked-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
> > > Acked-by: Glauber Costa <glommer@parallels.com>
> > > ---
> > > mm/vmscan.c | 18 ++++--------------
> > > 1 files changed, 4 insertions(+), 14 deletions(-)
> > >
> > > diff --git a/mm/vmscan.c b/mm/vmscan.c
> > > index 49f15ef..c684f44 100644
> > > --- a/mm/vmscan.c
> > > +++ b/mm/vmscan.c
> > [...]
> > > @@ -1806,7 +1796,7 @@ static int inactive_anon_is_low(struct mem_cgroup_zone *mz)
> > > if (!total_swap_pages)
> > > return 0;
> > >
> > > - if (!scanning_global_lru(mz))
> > > + if (!mem_cgroup_disabled())
> > > return mem_cgroup_inactive_anon_is_low(mz->mem_cgroup,
> > > mz->zone);
> >
> > mem_cgroup_inactive_anon_is_low calculation is slightly different than
> > what we have for cgroup_disabled case. calculate_zone_inactive_ratio
> > considers _all_ present pages in the zone while memcg variant only
> > active+inactive.
>
> The memcg has nothing to go by but actual number of LRU pages; there
> is no 'present pages' equivalent.

Yes

> I don't think that it matters much in reality given the sqrt scale,
> but the difference is still unfortunate.

OK you are probably right that the scale is too small to be a problem.
I guess that a note about changed ratio calculation should be added to
the changelog.

> Konstantin was meaning to unify all this, though.
>
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--
Michal Hocko
SUSE Labs
SUSE LINUX s.r.o.
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Czech Republic