linux-kernel - Re: [PATCH 0/2] oom, memcg: do not report racy no-eligible OOM

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Date:   Fri, 11 Jan 2019 17:45:36 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Johannes Weiner <hannes@...xchg.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 0/2] oom, memcg: do not report racy no-eligible OOM

On Sat 12-01-19 00:37:05, Tetsuo Handa wrote:
> On 2019/01/12 0:07, Michal Hocko wrote:
> > On Fri 11-01-19 23:31:18, Tetsuo Handa wrote:
> >> The OOM killer invoked by [ T9694] called printk() but didn't kill anything.
> >> Instead, SIGINT from Ctrl-C killed all thread groups sharing current->mm.
> > 
> > I still do not get it. Those other processes are not sharing signals.
> > Or is it due to injecting the signal too all of them with the proper
> > timing?
> 
> Pressing Ctrl-C between after task_will_free_mem(p) in oom_kill_process() and
> before __oom_kill_process() (e.g. dump_header()) made fatal_signal_pending() = T
> for all of them.
> 
> > Anyway, could you update your patch and abstract 
> > 	if (unlikely(tsk_is_oom_victim(current) ||
> > 		     fatal_signal_pending(current) ||
> > 		     current->flags & PF_EXITING))
> > 
> > in try_charge and reuse it in mem_cgroup_out_of_memory under the
> > oom_lock with an explanation please?
> 
> I don't think doing so makes sense, for
> 
>   tsk_is_oom_victim(current) = T && fatal_signal_pending(current) == F
> 
> can't happen for mem_cgroup_out_of_memory() under the oom_lock, and
> current->flags cannot get PF_EXITING when current is inside
> mem_cgroup_out_of_memory(). fatal_signal_pending(current) alone is
> appropriate for mem_cgroup_out_of_memory() under the oom_lock because
> 
>   tsk_is_oom_victim(current) = F && fatal_signal_pending(current) == T
> 
> can happen there.

I meant to use the same check consistently. If we can bypass the charge
under a list of conditions in the charge path we should be surely be
able to the the same for the oom path. I will not insist but unless
there is a strong reason I would prefer that.

> Also, doing so might become wrong in future, for mem_cgroup_out_of_memory()
> is also called from memory_max_write() which does not bail out upon
> PF_EXITING. I don't think we can call memory_max_write() after current
> thread got PF_EXITING, but nobody knows what change will happen in future.

No, this makes no sense what so ever.
-- 
Michal Hocko
SUSE Labs