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Date: Thu, 14 Jul 2011 18:30:14 +0900
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To: Michal Hocko <mhocko@...e.cz>
Cc: linux-mm@...ck.org, Balbir Singh <bsingharora@...il.com>,
Daisuke Nishimura <nishimura@....nes.nec.co.jp>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] memcg: make oom_lock 0 and 1 based rather than
coutner
On Thu, 14 Jul 2011 11:00:17 +0200
Michal Hocko <mhocko@...e.cz> wrote:
> On Thu 14-07-11 11:59:13, KAMEZAWA Hiroyuki wrote:
> > On Thu, 14 Jul 2011 10:02:59 +0900
> > KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:
> >
> > > On Wed, 13 Jul 2011 13:05:49 +0200
> > > Michal Hocko <mhocko@...e.cz> wrote:
> [...]
> > > > This patch replaces the counter by a simple {un}lock semantic. We are
> > > > using only 0 and 1 to distinguish those two states.
> > > > As mem_cgroup_oom_{un}lock works on the hierarchy we have to make sure
> > > > that we cannot race with somebody else which is already guaranteed
> > > > because we call both functions with the mutex held. All other consumers
> > > > just read the value atomically for a single group which is sufficient
> > > > because we set the value atomically.
> > > > The other thing is that only that process which locked the oom will
> > > > unlock it once the OOM is handled.
> > > >
> > > > Signed-off-by: Michal Hocko <mhocko@...e.cz>
> > > > ---
> > > > mm/memcontrol.c | 24 +++++++++++++++++-------
> > > > 1 files changed, 17 insertions(+), 7 deletions(-)
> > > >
> > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > > index e013b8e..f6c9ead 100644
> > > > --- a/mm/memcontrol.c
> > > > +++ b/mm/memcontrol.c
> > > > @@ -1803,22 +1803,31 @@ static int mem_cgroup_hierarchical_reclaim(struct mem_cgroup *root_mem,
> > > > /*
> > > > * Check OOM-Killer is already running under our hierarchy.
> > > > * If someone is running, return false.
> > > > + * Has to be called with memcg_oom_mutex
> > > > */
> > > > static bool mem_cgroup_oom_lock(struct mem_cgroup *mem)
> > > > {
> > > > - int x, lock_count = 0;
> > > > + int x, lock_count = -1;
> > > > struct mem_cgroup *iter;
> > > >
> > > > for_each_mem_cgroup_tree(iter, mem) {
> > > > - x = atomic_inc_return(&iter->oom_lock);
> > > > - lock_count = max(x, lock_count);
> > > > + x = !!atomic_add_unless(&iter->oom_lock, 1, 1);
> > > > + if (lock_count == -1)
> > > > + lock_count = x;
> > > > +
> > >
> > >
> > > Hmm...Assume following hierarchy.
> > >
> > > A
> > > B C
> > > D E
>
> IIUC, A, B, D, E are one hierarchy, right?
>
yes.
> > >
> > > The orignal code hanldes the situation
> > >
> > > 1. B-D-E is under OOM
> > > 2. A enters OOM after 1.
> > >
> > > In original code, A will not invoke OOM (because B-D-E oom will kill a process.)
> > > The new code invokes A will invoke new OOM....right ?
>
> Sorry, I do not understand what you mean by that.
This is your code.
==
for_each_mem_cgroup_tree(iter, mem) {
- x = atomic_inc_return(&iter->oom_lock);
- lock_count = max(x, lock_count);
+ x = !!atomic_add_unless(&iter->oom_lock, 1, 1);
+ if (lock_count == -1)
+ lock_count = x;
+
+ /* New child can be created but we shouldn't race with
+ * somebody else trying to oom because we are under
+ * memcg_oom_mutex
+ */
+ BUG_ON(lock_count != x);
}
==
When, B,D,E is under OOM,
A oom_lock = 0
B oom_lock = 1
C oom_lock = 0
D oom_lock = 1
E oom_lock = 1
Here, assume A enters OOM.
A oom_lock = 1 -- (*)
B oom_lock = 1
C oom_lock = 1
D oom_lock = 1
E oom_lock = 1
because of (*), mem_cgroup_oom_lock() will return lock_count=1, true.
Then, a new oom-killer will another oom-kiiler running in B-D-E.
> The original code and
> the new code do the same in that regards they lock the whole hierarchy.
> The only difference is that the original one increments the counter for
> all groups in the hierarchy while the new one just sets it to from 0->1
> BUG_ON just checks that we are not racing with somebody else.
>
In above situation, old code's result is
A oom_lock = 1
B oom_lock = 2
C oom_lock = 1
D oom_lock = 2
E oom_lock = 2
Then, max lockcount== 2 and return value is false.
> > >
> > > I wonder this kind of code
> > > ==
> > > bool success = true;
> > > ...
> > > for_each_mem_cgroup_tree(iter, mem) {
> > > success &= !!atomic_add_unless(&iter->oom_lock, 1, 1);
> > > /* "break" loop is not allowed because of css refcount....*/
> > > }
> > > return success.
> > > ==
> > > Then, one hierarchy can invoke one OOM kill within it.
> > > But this will not work because we can't do proper unlock.
>
> Why cannot we do a proper unlock?
>
After 1st lock by oom in B-D-E
A oom_lock = 0
B oom_lock = 1
C oom_lock = 0
D oom_lock = 1
E oom_lock = 1
returns true. and mem_cgroup_oom_unlock() will scan B-D-E and set oom_lock==0.
2nd lock by oom in A-B-C-D-E
A oom_lock = 1
B oom_lock = 1
C oom_lock = 1
D oom_lock = 1
E oom_lock = 1
returns false, then, mem_cgroup_oom_unlock() will not be called.
After unlock, we see
A oom_lock = 1
B oom_lock = 0
C oom_lock = 1
D oom_lock = 0
E oom_lock = 0
> > >
> > >
> > > Hm. how about this ? This has only one lock point and we'll not see the BUG.
> > > Not tested yet..
> > >
> > Here, tested patch + test program. this seems to work well.
>
> Will look at it later. At first glance it looks rather complicated. But
> maybe I am missing something. I have to confess I am not absolutely sure
> when it comes to hierarchies.
>
With my patch, oom_lock shows the lock owner memcg. oom status is divided to
the vairable, under_oom.
(1) At 1st vistor by oom in B-D-E
A oom_lock = 0 under_oom=0
B oom_lock = 1 under_oom=1
C oom_lock = 0 under_oom=0
D oom_lock = 0 under_oom=1
E oom_lock - 0 under_oom=0
returns true.
(2) At 2nd visrot by oom in A-B-C-D-E
A oom_lock = 1 under_oom=1
B oom_lock = 0 under_oom=1
C oom_lock = 0 under_oom=1
D oom_lock = 0 under_oom=1
E oom_lock = 0 under_oom=1
The lock moves to A and returns false.
(3) at unlock, the thread which did (1) will find group A and
unlock all hierarchy.
A oom_lock = 0 under_oom=0
B oom_lock = 0 under_oom=0
C oom_lock = 0 under_oom=0
D oom_lock = 0 under_oom=0
E oom_lock = 0 under_oom=0
Thanks,
-Kame
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