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

[Kirill Tkhai <ktkhai@virtuozzo.com>]

On 28.12.2018 14:00, Tetsuo Handa wrote:
> 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.

There is signal_pending_state() primitive, and this is the check,
which should be used instead of fatal_signal_pending() in mutex
code.

Let's ask Peter :) Peter, what you think about the approach overall?
I.e., changing __mutex_lock_common() by adding one more check of
signal_pending_state() after schedule_preempt_disabled() (with respect
to other mutex code, e.g., waking next waiter etc)?

Kirill