Re: [PATCH] memcg: killed threads should not invoke memcg OOM killer

[Tetsuo Handa <penguin-kernel@i-love.sakura.ne.jp>]

On 2018/12/28 19:22, Kirill Tkhai wrote:
>> @@ -1389,8 +1389,13 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
>>  	};
>>  	bool ret;
>>  
>> -	mutex_lock(&oom_lock);
>> -	ret = out_of_memory(&oc);
>> +	if (mutex_lock_killable(&oom_lock))
>> +		return true;
>> +	/*
>> +	 * A few threads which were not waiting at mutex_lock_killable() can
>> +	 * fail to bail out. Therefore, check again after holding oom_lock.
>> +	 */
>> +	ret = fatal_signal_pending(current) || out_of_memory(&oc);
> 
> This fatal_signal_pending() check has a sense because of
> it's possible, a killed task is waking up slowly, and it
> returns from schedule(), when there are no more waiters
> for a lock.

Thanks. Michal thinks that mutex_lock_killable() would be sufficient
( https://lkml.kernel.org/r/20181107100810.GA27423@dhcp22.suse.cz ) but
I can confirm that mutex_lock_killable() is not sufficient when I test
using a VM with 8 CPUs. Thus, I'd like to keep this fatal_signal_pending()
check.

> 
> Why not make this approach generic, and add a check into
> __mutex_lock_common() after schedule_preempt_disabled()
> instead of this? This will handle all the places like
> that at once.
> 
> (The only adding a check is not enough for __mutex_lock_common(),
>  since mutex code will require to wake next waiter also. So,
>  you will need a couple of changes in mutex code).

I think that we should not assume that everybody is ready for making
mutex_lock_killable() to return -EINTR if fatal_signal_pending() is
true, and that adding below version would be a safer choice.

int __sched mutex_lock_unless_killed(struct mutex *lock)
{
	const int ret = mutex_lock_killable(lock);

	if (ret)
		return ret;
	if (fatal_signale_pending(current)) {
		mutex_unlock(lock);
		return -EINTR;
	}
	return 0;
}