Re: [patch] memcg: give current access to memory reserves if it's trying to die

[KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>]

On Thu, 17 Mar 2011 16:53:19 -0700
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Wed, 9 Mar 2011 13:27:50 -0800 (PST)
> David Rientjes <rientjes@google.com> wrote:
> 
> > When a memcg is oom and current has already received a SIGKILL, then give
> > it access to memory reserves with a higher scheduling priority so that it
> > may quickly exit and free its memory.
> > 
> > This is identical to the global oom killer and is done even before
> > checking for panic_on_oom: a pending SIGKILL here while panic_on_oom is
> > selected is guaranteed to have come from userspace; the thread only needs
> > access to memory reserves to exit and thus we don't unnecessarily panic
> > the machine until the kernel has no last resort to free memory.
> > 
> > Signed-off-by: David Rientjes <rientjes@google.com>
> > ---
> >  mm/oom_kill.c |   11 +++++++++++
> >  1 files changed, 11 insertions(+), 0 deletions(-)
> > 
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -537,6 +537,17 @@ void mem_cgroup_out_of_memory(struct mem_cgroup *mem, gfp_t gfp_mask)
> >  	unsigned int points = 0;
> >  	struct task_struct *p;
> >  
> > +	/*
> > +	 * If current has a pending SIGKILL, then automatically select it.  The
> > +	 * goal is to allow it to allocate so that it may quickly exit and free
> > +	 * its memory.
> > +	 */
> > +	if (fatal_signal_pending(current)) {
> > +		set_thread_flag(TIF_MEMDIE);
> > +		boost_dying_task_prio(current, NULL);
> > +		return;
> > +	}
> > +
> >  	check_panic_on_oom(CONSTRAINT_MEMCG, gfp_mask, 0, NULL);
> >  	limit = mem_cgroup_get_limit(mem) >> PAGE_SHIFT;
> >  	read_lock(&tasklist_lock);
> 
> The code duplication seems a bit gratuitous.
> 
> 
> 
> Was it deliberate that mem_cgroup_out_of_memory() ignores the oom
> notifier callbacks?
> 

I'm not sure for what purpose notifier chain for oom exists.
At a loock, it's for s390/powerpc Collaborative Memory Manager.. ?

About memcg, notifier to userland already exists and I though I don't
need to call CMM callbacks (for now, there is no user with memcg, I guess.)

Thanks,
-kame

> (Why does that notifier list exist at all?  Wouldn't it be better to do
> this via a vmscan shrinker?  Perhaps altered to be passed the scanning
> priority?)
> 
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