On Tue 25-07-17 13:31:13, Roman Gushchin wrote: > On Tue, Jul 25, 2017 at 03:05:37PM +0300, Vladimir Davydov wrote: > > On Tue, Jul 25, 2017 at 12:40:47PM +0100, Roman Gushchin wrote: > > > A removed memory cgroup with a defined low limit and some belonging > > > pagecache has very low chances to be freed. > > > > > > If a cgroup has been removed, there is likely no memory pressure inside > > > the cgroup, and the pagecache is protected from the external pressure > > > by the defined low limit. The cgroup will be freed only after > > > the reclaim of all belonging pages. And it will not happen until > > > there are any reclaimable memory in the system. That means, > > > there is a good chance, that a cold pagecache will reside > > > in the memory for an undefined amount of time, wasting > > > system resources. > > > > > > Fix this issue by zeroing memcg->low during memcg offlining. > > > > > > Signed-off-by: Roman Gushchin <guro@xxxxxx> > > > Cc: Tejun Heo <tj@xxxxxxxxxx> > > > Cc: Johannes Weiner <hannes@xxxxxxxxxxx> > > > Cc: Michal Hocko <mhocko@xxxxxxxxxx> > > > Cc: Vladimir Davydov <vdavydov.dev@xxxxxxxxx> > > > Cc: kernel-team@xxxxxx > > > Cc: cgroups@xxxxxxxxxxxxxxx > > > Cc: linux-mm@xxxxxxxxx > > > Cc: linux-kernel@xxxxxxxxxxxxxxx > > > --- > > > mm/memcontrol.c | 2 ++ > > > 1 file changed, 2 insertions(+) > > > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c > > > index aed11b2d0251..2aa204b8f9fd 100644 > > > --- a/mm/memcontrol.c > > > +++ b/mm/memcontrol.c > > > @@ -4300,6 +4300,8 @@ static void mem_cgroup_css_offline(struct cgroup_subsys_state *css) > > > } > > > spin_unlock(&memcg->event_list_lock); > > > > > > + memcg->low = 0; > > > + > > > memcg_offline_kmem(memcg); > > > wb_memcg_offline(memcg); > > > > > > > We already have that - see mem_cgroup_css_reset(). > > Hm, I see... > > But are you sure, that calling mem_cgroup_css_reset() from offlining path > is always a good idea? Well, originally I wanted to suggest the same but then I asked the very same question and couldn't answer it myself. memcg_offline_kmem feels much more generic. > As I understand, css_reset() callback is intended to _completely_ disable all > limits, as if there were no cgroup at all. And it's main purpose to be called > when controllers are detached from the hierarhy. yes, that is my understanding as well. > Offlining is different: some limits make perfect sence after offlining > (e.g. we want to limit the writeback speed), and other might be tweaked > (e.g. we can set soft limit to prioritize reclaiming of abandoned cgroups). and the writeback path was exactly the one that triggered my suspicious... -- Michal Hocko SUSE Labs -- To unsubscribe from this list: send the line "unsubscribe cgroups" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
