Re: [PATCH 1/4] memcg, mm: introduce lowlimit reclaim

Michal Hocko
2014-05-02T14:15:20Z

On Fri 02-05-14 09:01:18, Johannes Weiner wrote:
> On Fri, May 02, 2014 at 02:07:15PM +0200, Michal Hocko wrote:
> > On Fri 02-05-14 11:36:28, Michal Hocko wrote:
> > > On Wed 30-04-14 18:55:50, Johannes Weiner wrote:
> > > > On Mon, Apr 28, 2014 at 02:26:42PM +0200, Michal Hocko wrote:
> > > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > > > index 19d620b3d69c..40e517630138 100644
> > > > > --- a/mm/memcontrol.c
> > > > > +++ b/mm/memcontrol.c
> > > > > @@ -2808,6 +2808,29 @@ static struct mem_cgroup *mem_cgroup_lookup(unsigned short id)
> > > > > return mem_cgroup_from_id(id);
> > > > > }
> > > > >
> > > > > +/**
> > > > > + * mem_cgroup_reclaim_eligible - checks whether given memcg is eligible for the
> > > > > + * reclaim
> > > > > + * @memcg: target memcg for the reclaim
> > > > > + * @root: root of the reclaim hierarchy (null for the global reclaim)
> > > > > + *
> > > > > + * The given group is reclaimable if it is above its low limit and the same
> > > > > + * applies for all parents up the hierarchy until root (including).
> > > > > + */
> > > > > +bool mem_cgroup_reclaim_eligible(struct mem_cgroup *memcg,
> > > > > + struct mem_cgroup *root)
> > > >
> > > > Could you please rename this to something that is more descriptive in
> > > > the reclaim callsite? How about mem_cgroup_within_low_limit()?
> > >
> > > I have intentionally used somethig that is not low_limit specific. The
> > > generic reclaim code does't have to care about the reason why a memcg is
> > > not reclaimable. I agree that having follow_low_limit paramter explicit
> > > and mem_cgroup_reclaim_eligible not is messy. So something should be
> > > renamed. I would probably go with s@follow_low_limit@check_reclaim_eligible@
> > > but I do not have a strong preference.
> >
> > What about this?
>
> I really don't like it.
>
> Yes, we should be hiding implementation details, but we should stop
> treating memcg like an alien in this code. The VM code obviously
> doesn't have to know HOW the guarantees are exactly implemented, but
> it's a perfectly fine *concept* that can be known outside of memcg:
>
> shrink_zone:
> for each memcg in system:
> if mem_cgroup_within_guarantee(memcg):
> continue
> reclaim(memcg-zone)
>
> is perfectly understandable and makes it easier to reason about the
> behavior of the reclaim code. If I just see !mem_cgroup_eligible(), I
> don't know if this affects the scenario I'm thinking about at all.
>
> It's obscuring useful information for absolutely no benefit. If you
> burden the reclaim code with a callback, you better explain what you
> are doing. You owe it to the reader.

OK fair enough, what about the following?
---
>From 4e0404fa2888d04de80f33fcb76712b0fbd44e1c Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@suse.cz>
Date: Fri, 2 May 2014 16:12:41 +0200
Subject: [PATCH] mmotm: memcg-mm-introduce-lowlimit-reclaim-fix.patch

mem_cgroup_reclaim_eligible -> mem_cgroup_within_guarantee as suggested
by Johannes.
---
include/linux/memcontrol.h | 6 +++---
mm/memcontrol.c | 15 ++++++++-------
mm/vmscan.c | 25 ++++++++++++++++---------
3 files changed, 27 insertions(+), 19 deletions(-)

diff --git a/include/linux/memcontrol.h b/include/linux/memcontrol.h
index 6c59056f4bc6..c00ccc5f70b9 100644
--- a/include/linux/memcontrol.h
+++ b/include/linux/memcontrol.h
@@ -92,7 +92,7 @@ bool __mem_cgroup_same_or_subtree(const struct mem_cgroup *root_memcg,
bool task_in_mem_cgroup(struct task_struct *task,
const struct mem_cgroup *memcg);

-extern bool mem_cgroup_reclaim_eligible(struct mem_cgroup *memcg,
+extern bool mem_cgroup_within_guarantee(struct mem_cgroup *memcg,
struct mem_cgroup *root);

extern struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page);
@@ -291,10 +291,10 @@ static inline struct lruvec *mem_cgroup_page_lruvec(struct page *page,
return &zone->lruvec;
}

-static inline bool mem_cgroup_reclaim_eligible(struct mem_cgroup *memcg,
+static inline bool mem_cgroup_within_guarantee(struct mem_cgroup *memcg,
struct mem_cgroup *root)
{
- return true;
+ return false;
}

static inline struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page)
diff --git a/mm/memcontrol.c b/mm/memcontrol.c
index 7a276c0d141e..58982d18f6ea 100644
--- a/mm/memcontrol.c
+++ b/mm/memcontrol.c
@@ -2810,26 +2810,27 @@ static struct mem_cgroup *mem_cgroup_lookup(unsigned short id)
}

/**
- * mem_cgroup_reclaim_eligible - checks whether given memcg is eligible for the
- * reclaim
+ * mem_cgroup_within_guarantee - checks whether given memcg is within its
+ * memory guarantee
* @memcg: target memcg for the reclaim
* @root: root of the reclaim hierarchy (null for the global reclaim)
*
- * The given group is reclaimable if it is above its low limit and the same
- * applies for all parents up the hierarchy until root (including).
+ * The given group is within its reclaim gurantee if it is below its low limit
+ * or the same applies for any parent up the hierarchy until root (including).
+ * Such a group might be excluded from the reclaim.
*/
-bool mem_cgroup_reclaim_eligible(struct mem_cgroup *memcg,
+bool mem_cgroup_within_guarantee(struct mem_cgroup *memcg,
struct mem_cgroup *root)
{
do {
if (!res_counter_low_limit_excess(&memcg->res))
- return false;
+ return true;
if (memcg == root)
break;

} while ((memcg = parent_mem_cgroup(memcg)));

- return true;
+ return false;
}

struct mem_cgroup *try_get_mem_cgroup_from_page(struct page *page)
diff --git a/mm/vmscan.c b/mm/vmscan.c
index 0f428158254e..20ca95fbaebb 100644
--- a/mm/vmscan.c
+++ b/mm/vmscan.c
@@ -2215,8 +2215,18 @@ static inline bool should_continue_reclaim(struct zone *zone,
}
}

+/**
+ * __shrink_zone - shrinks a given zone
+ *
+ * @zone: zone to shrink
+ * @sc: scan control with additional reclaim parameters
+ * @force_memcg_guarantee: do not reclaim memcgs which are within their memory
+ * guarantee
+ *
+ * Returns the number of reclaimed memcgs.
+ */
static unsigned __shrink_zone(struct zone *zone, struct scan_control *sc,
- bool follow_low_limit)
+ bool force_memcg_guarantee)
{
unsigned long nr_reclaimed, nr_scanned;
unsigned nr_scanned_groups = 0;
@@ -2236,12 +2246,9 @@ static unsigned __shrink_zone(struct zone *zone, struct scan_control *sc,
do {
struct lruvec *lruvec;

- /*
- * Memcg might be under its low limit so we have to
- * skip it during the first reclaim round
- */
- if (follow_low_limit &&
- !mem_cgroup_reclaim_eligible(memcg, root)) {
+ /* Memcg might be protected from the reclaim */
+ if (force_memcg_guarantee &&
+ mem_cgroup_within_guarantee(memcg, root)) {
/*
* It would be more optimal to skip the memcg
* subtree now but we do not have a memcg iter
@@ -2289,8 +2296,8 @@ static void shrink_zone(struct zone *zone, struct scan_control *sc)
if (!__shrink_zone(zone, sc, true)) {
/*
* First round of reclaim didn't find anything to reclaim
- * because of low limit protection so try again and ignore
- * the low limit this time.
+ * because of the memory guantees for all memcgs in the
+ * reclaim target so try again and ignore guarantees this time.
*/
__shrink_zone(zone, sc, false);
}
--
2.0.0.rc0

--
Michal Hocko
SUSE Labs