linux-kernel - Re: [PATCH 3/4] memcg, doc: clarify global vs. limit reclaims

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Date:	Fri, 2 May 2014 11:43:51 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Johannes Weiner <hannes@...xchg.org>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	KOSAKI Motohiro <kosaki.motohiro@...fujitsu.com>,
	Greg Thelen <gthelen@...gle.com>,
	Michel Lespinasse <walken@...gle.com>,
	Tejun Heo <tj@...nel.org>, Hugh Dickins <hughd@...gle.com>,
	Roman Gushchin <klamm@...dex-team.ru>,
	LKML <linux-kernel@...r.kernel.org>, linux-mm@...ck.org
Subject: Re: [PATCH 3/4] memcg, doc: clarify global vs. limit reclaims

On Wed 30-04-14 19:03:50, Johannes Weiner wrote:
> On Mon, Apr 28, 2014 at 02:26:44PM +0200, Michal Hocko wrote:
> > Be explicit about global and hard limit reclaims in our documentation.
> > 
> > Signed-off-by: Michal Hocko <mhocko@...e.cz>
> > ---
> >  Documentation/cgroups/memory.txt | 31 +++++++++++++++++--------------
> >  1 file changed, 17 insertions(+), 14 deletions(-)
> > 
> > diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
> > index 4937e6fff9b4..add1be001416 100644
> > --- a/Documentation/cgroups/memory.txt
> > +++ b/Documentation/cgroups/memory.txt
> > @@ -236,23 +236,26 @@ it by cgroup.
> >  2.5 Reclaim
> >  
> >  Each cgroup maintains a per cgroup LRU which has the same structure as
> > -global VM. When a cgroup goes over its limit, we first try
> > -to reclaim memory from the cgroup so as to make space for the new
> > -pages that the cgroup has touched. If the reclaim is unsuccessful,
> > -an OOM routine is invoked to select and kill the bulkiest task in the
> > -cgroup. (See 10. OOM Control below.)
> > -
> > -The reclaim algorithm has not been modified for cgroups, except that
> > -pages that are selected for reclaiming come from the per-cgroup LRU
> > -list.
> > -
> > -NOTE: Reclaim does not work for the root cgroup, since we cannot set any
> > -limits on the root cgroup.
> > +global VM. Cgroups can get reclaimed basically under two conditions
> > + - under global memory pressure when all cgroups are reclaimed
> > +   proportionally wrt. their LRU size in a round robin fashion
> > + - when a cgroup or its hierarchical parent (see 6. Hierarchical support)
> > +   hits hard limit. If the reclaim is unsuccessful, an OOM routine is invoked
> > +   to select and kill the bulkiest task in the cgroup. (See 10. OOM Control
> > +   below.)
> 
> In the whole hierarchy, not just that cgroup.

Right. Fixed
 
> > +Global and hard-limit reclaims share the same code the only difference
> > +is the objective of the reclaim. The global reclaim aims at balancing
> > +zones' watermarks while the limit reclaim frees some memory to allow new
> > +charges.
> 
> This is a kswapd vs. direct reclaim issue, not global vs. memcg.
> Memcg reclaim just happens to be direct reclaim.  Either way, I'd
> rather not have such implementation details in the user documentation.

OK, removed
 
> > +NOTE: Hard limit reclaim does not work for the root cgroup, since we cannot set
> > +any limits on the root cgroup.
> 
> Not sure it's necessary to include this...

removed as well.

Incremental patch on top:
---
>From 30b9505169e574cdb553226e1a361cc527ed492b Mon Sep 17 00:00:00 2001
From: Michal Hocko <mhocko@...e.cz>
Date: Fri, 2 May 2014 11:42:35 +0200
Subject: [PATCH] mmotm: memcg-doc-clarify-global-vs-limit-reclaims-fix.patch

update doc as per Johannes

Signed-off-by: Michal Hocko <mhocko@...e.cz>
---
 Documentation/cgroups/memory.txt | 10 +---------
 1 file changed, 1 insertion(+), 9 deletions(-)

diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
index add1be001416..2cde96787ceb 100644
--- a/Documentation/cgroups/memory.txt
+++ b/Documentation/cgroups/memory.txt
@@ -241,17 +241,9 @@ global VM. Cgroups can get reclaimed basically under two conditions
    proportionally wrt. their LRU size in a round robin fashion
  - when a cgroup or its hierarchical parent (see 6. Hierarchical support)
    hits hard limit. If the reclaim is unsuccessful, an OOM routine is invoked
-   to select and kill the bulkiest task in the cgroup. (See 10. OOM Control
+   to select and kill the bulkiest task in the hiearchy. (See 10. OOM Control
    below.)
 
-Global and hard-limit reclaims share the same code the only difference
-is the objective of the reclaim. The global reclaim aims at balancing
-zones' watermarks while the limit reclaim frees some memory to allow new
-charges.
-
-NOTE: Hard limit reclaim does not work for the root cgroup, since we cannot set
-any limits on the root cgroup.
-
 Note2: When panic_on_oom is set to "2", the whole system will panic.
 
 When oom event notifier is registered, event will be delivered to the root
-- 
2.0.0.rc0

-- 
Michal Hocko
SUSE Labs
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