linux-kernel - Re: [PATCH] memcg: handle panic_on

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Date:	Wed, 17 Feb 2010 17:51:03 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"balbir@...ux.vnet.ibm.com" <balbir@...ux.vnet.ibm.com>,
	"nishimura@....nes.nec.co.jp" <nishimura@....nes.nec.co.jp>,
	rientjes@...gle.com,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>
Subject: Re: [PATCH] memcg: handle panic_on_oom=always case

On Wed, 17 Feb 2010 19:45:26 +1100
Nick Piggin <npiggin@...e.de> wrote:

> On Wed, Feb 17, 2010 at 03:04:45PM +0900, KAMEZAWA Hiroyuki wrote:
> > tested on mmotm-Feb11.
> > 
> > Balbir-san, Nishimura-san, I want review from both of you.
> > 
> > ==
> > 
> > From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > 
> > Now, if panic_on_oom=2, the whole system panics even if the oom happend
> > in some special situation (as cpuset, mempolicy....).
> > Then, panic_on_oom=2 means painc_on_oom_always.
> > 
> > Now, memcg doesn't check panic_on_oom flag. This patch adds a check.
> > 
> > Maybe someone doubts how it's useful. kdump+panic_on_oom=2 is the
> > last tool to investigate what happens in oom-ed system. If a task is killed,
> > the sysytem recovers and used memory were freed, there will be few hint
> > to know what happnes. In mission critical system, oom should never happen.
> > Then, investigation after OOM is very important.
> > Then, panic_on_oom=2+kdump is useful to avoid next OOM by knowing
> > precise information via snapshot.
> 
> No I don't doubt it is useful, and I think this probably is the simplest
> and most useful semantic. So thanks for doing this.
> 
Thank you for review.


> I hate to pick nits in a trivial patch but I will anyway:
> 
> 
> > TODO:
> >  - For memcg, it's for isolate system's memory usage, oom-notiifer and
> >    freeze_at_oom (or rest_at_oom) should be implemented. Then, management
> >    daemon can do similar jobs (as kdump) in safer way or taking snapshot
> >    per cgroup.
> > 
> > CC: Balbir Singh <balbir@...ux.vnet.ibm.com>
> > CC: Daisuke Nishimura <nishimura@....nes.nec.co.jp>
> > CC: David Rientjes <rientjes@...gle.com>
> > Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
> > ---
> >  Documentation/cgroups/memory.txt |    2 ++
> >  Documentation/sysctl/vm.txt      |    5 ++++-
> >  mm/oom_kill.c                    |    2 ++
> >  3 files changed, 8 insertions(+), 1 deletion(-)
> > 
> > Index: mmotm-2.6.33-Feb11/Documentation/cgroups/memory.txt
> > ===================================================================
> > --- mmotm-2.6.33-Feb11.orig/Documentation/cgroups/memory.txt
> > +++ mmotm-2.6.33-Feb11/Documentation/cgroups/memory.txt
> > @@ -182,6 +182,8 @@ list.
> >  NOTE: 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.
> > +
> 
> Maybe:
> 
> NOTE2: When panic_on_oom is set to "2", the whole system will panic in
> case of an oom event in any cgroup.
> 

ok.


> >  2. Locking
> >  
> >  The memory controller uses the following hierarchy
> > Index: mmotm-2.6.33-Feb11/Documentation/sysctl/vm.txt
> > ===================================================================
> > --- mmotm-2.6.33-Feb11.orig/Documentation/sysctl/vm.txt
> > +++ mmotm-2.6.33-Feb11/Documentation/sysctl/vm.txt
> > @@ -573,11 +573,14 @@ Because other nodes' memory may be free.
> >  may be not fatal yet.
> >  
> >  If this is set to 2, the kernel panics compulsorily even on the
> > -above-mentioned.
> > +above-mentioned. Even oom happens under memoyr cgroup, the whole
> > +system panics.
>                                            memory
> 
> >  
> >  The default value is 0.
> >  1 and 2 are for failover of clustering. Please select either
> >  according to your policy of failover.
> > +2 seems too strong but panic_on_oom=2+kdump gives you very strong
> > +tool to investigate a system which should never cause OOM.
> 
> I don't think you need say 2 seems too strong because as you rightfully
> say, it has real uses. The hint about using it to investigate OOM
> conditions is good though.
> 

ok. I'll update this patch.

Thanks,
-Kame

> >  
> >  =============================================================
> >  
> > Index: mmotm-2.6.33-Feb11/mm/oom_kill.c
> > ===================================================================
> > --- mmotm-2.6.33-Feb11.orig/mm/oom_kill.c
> > +++ mmotm-2.6.33-Feb11/mm/oom_kill.c
> > @@ -471,6 +471,8 @@ void mem_cgroup_out_of_memory(struct mem
> >  	unsigned long points = 0;
> >  	struct task_struct *p;
> >  
> > +	if (sysctl_panic_on_oom == 2)
> > +		panic("out of memory(memcg). panic_on_oom is selected.\n");
> >  	read_lock(&tasklist_lock);
> >  retry:
> >  	p = select_bad_process(&points, mem);
> 

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