Re: [patch] mm: memcontrol: remove hierarchy restrictions for swappiness and oom_control

[Johannes Weiner <hannes@cmpxchg.org>]

On Wed, Apr 16, 2014 at 02:34:25PM -0700, Andrew Morton wrote:
> On Wed, 16 Apr 2014 17:13:18 -0400 Johannes Weiner <hannes@cmpxchg.org> wrote:
> 
> > Per-memcg swappiness and oom killing can currently not be tweaked on a
> > memcg that is part of a hierarchy, but not the root of that hierarchy.
> > Users have complained that they can't configure this when they turned
> > on hierarchy mode.  In fact, with hierarchy mode becoming the default,
> > this restriction disables the tunables entirely.
> > 
> > But there is no good reason for this restriction.  The settings for
> > swappiness and OOM killing are taken from whatever memcg whose limit
> > triggered reclaim and OOM invocation, regardless of its position in
> > the hierarchy tree.
> > 
> > Allow setting swappiness on any group.  The knob on the root memcg
> > already reads the global VM swappiness, make it writable as well.
> > 
> > Allow disabling the OOM killer on any non-root memcg.
> 
> Documentation/cgroups/memory.txt needs updates?

Yes, that makes sense, thanks.  How about this?

---
Subject: [patch] mm: memcontrol: remove hierarchy restrictions for swappiness and oom_control fix

Update Documentation/cgroups/memory.txt

Signed-off-by: Johannes Weiner <hannes@cmpxchg.org>
---

diff --git a/Documentation/cgroups/memory.txt b/Documentation/cgroups/memory.txt
index 2622115276aa..1829c65f8371 100644
--- a/Documentation/cgroups/memory.txt
+++ b/Documentation/cgroups/memory.txt
@@ -535,17 +535,15 @@ Note:
 
 5.3 swappiness
 
-Similar to /proc/sys/vm/swappiness, but affecting a hierarchy of groups only.
+Similar to /proc/sys/vm/swappiness, but only affecting reclaim that is
+triggered by this cgroup's hard limit.  The tunable in the root cgroup
+corresponds to the global swappiness setting.
+
 Please note that unlike the global swappiness, memcg knob set to 0
 really prevents from any swapping even if there is a swap storage
 available. This might lead to memcg OOM killer if there are no file
 pages to reclaim.
 
-Following cgroups' swappiness can't be changed.
-- root cgroup (uses /proc/sys/vm/swappiness).
-- a cgroup which uses hierarchy and it has other cgroup(s) below it.
-- a cgroup which uses hierarchy and not the root of hierarchy.
-
 5.4 failcnt
 
 A memory cgroup provides memory.failcnt and memory.memsw.failcnt files.
@@ -754,7 +752,6 @@ You can disable the OOM-killer by writing "1" to memory.oom_control file, as:
 
 	#echo 1 > memory.oom_control
 
-This operation is only allowed to the top cgroup of a sub-hierarchy.
 If OOM-killer is disabled, tasks under cgroup will hang/sleep
 in memory cgroup's OOM-waitqueue when they request accountable memory.
 

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