linux-kernel - Re: [PATCH memcg 2/3] memcg: remove charge forcinig for dying tasks

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Date:   Wed, 20 Oct 2021 16:57:50 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Vasily Averin <vvs@...tuozzo.com>
Cc:     Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Roman Gushchin <guro@...com>,
        Uladzislau Rezki <urezki@...il.com>,
        Vlastimil Babka <vbabka@...e.cz>,
        Shakeel Butt <shakeelb@...gle.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        cgroups@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, kernel@...nvz.org
Subject: Re: [PATCH memcg 2/3] memcg: remove charge forcinig for dying tasks

On Wed 20-10-21 17:21:33, Vasily Averin wrote:
> On 20.10.2021 15:41, Michal Hocko wrote:
> > On Wed 20-10-21 15:13:46, Vasily Averin wrote:
> >> ToDo: should we keep task_is_dying() in mem_cgroup_out_of_memory() ?
> >>
> >> Signed-off-by: Vasily Averin <vvs@...tuozzo.com>
> >> ---
> >>  mm/memcontrol.c | 20 +++++++-------------
> >>  1 file changed, 7 insertions(+), 13 deletions(-)
> >>
> >> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> >> index 6da5020a8656..74a7379dbac1 100644
> >> --- a/mm/memcontrol.c
> >> +++ b/mm/memcontrol.c
> >> @@ -239,7 +239,7 @@ enum res_type {
> >>  	     iter != NULL;				\
> >>  	     iter = mem_cgroup_iter(NULL, iter, NULL))
> >>  
> >> -static inline bool should_force_charge(void)
> >> +static inline bool task_is_dying(void)
> >>  {
> >>  	return tsk_is_oom_victim(current) || fatal_signal_pending(current) ||
> >>  		(current->flags & PF_EXITING);
> >> @@ -1575,7 +1575,7 @@ static bool mem_cgroup_out_of_memory(struct mem_cgroup *memcg, gfp_t gfp_mask,
> >>  	 * A few threads which were not waiting at mutex_lock_killable() can
> >>  	 * fail to bail out. Therefore, check again after holding oom_lock.
> >>  	 */
> >> -	ret = should_force_charge() || out_of_memory(&oc);
> >> +	ret = task_is_dying() || out_of_memory(&oc);
> > 
> > Why are you keeping the task_is_dying check here? IIRC I have already
> > pointed out that out_of_memory already has some means to do a bypass
> > when needed.
> 
> It was a misunderstanding.

Sorry if I made you confused.

> I've been waiting for your final decision.
> 
> I have no good arguments "pro" or strong objection "contra". 
> However, I prefer to keep task_is_dying() so as not to touch other tasks unnecessarily.

One argument for removing it from here is the maintainability. Now you
have a memcg specific check which is not in sync with the oom. E.g.
out_of_memory does task_will_free_mem as the very first thing. You are
also automatically excluding oom killer for cases where that might make
a sense.
-- 
Michal Hocko
SUSE Labs