Re: [PATCH] mm, oom: remove 'prefer children over parent' heuristic

[Shakeel Butt]

On Mon, Jan 21, 2019 at 1:19 AM Michal Hocko <mhocko@xxxxxxxxxx> wrote:
>
> On Sun 20-01-19 13:50:59, Shakeel Butt wrote:
> > >From the start of the git history of Linux, the kernel after selecting
> > the worst process to be oom-killed, prefer to kill its child (if the
> > child does not share mm with the parent). Later it was changed to prefer
> > to kill a child who is worst. If the parent is still the worst then the
> > parent will be killed.
> >
> > This heuristic assumes that the children did less work than their parent
> > and by killing one of them, the work lost will be less. However this is
> > very workload dependent. If there is a workload which can benefit from
> > this heuristic, can use oom_score_adj to prefer children to be killed
> > before the parent.
> >
> > The select_bad_process() has already selected the worst process in the
> > system/memcg. There is no need to recheck the badness of its children
> > and hoping to find a worse candidate. That's a lot of unneeded racy
> > work. So, let's remove this whole heuristic.
>
> Yes, I agree with this direction. Let's try it and see whether there is
> anything really depending on the heuristic. I hope that is not the case
> but at least we will hear about it and the reasoning behind.
>
> I think the changelog should also mension that the heuristic is
> dangerous because it make fork bomb like workloads to recover much later
> because we constantly pick and kill processes which are not memory hogs.
>
> > Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
>
> Appart from the nit in the printk output
> Acked-by: Michal Hocko <mhocko@xxxxxxxx>
>
> Also I would prefer s@p@victim@ because it makes the code more readable
>
> I pressume you are going to send this along with the fix for the
> use-after-free in one series.
>
> Thanks.

Yes, I will resend the series after incorporating the feedback.

>
> > ---
> >  mm/oom_kill.c | 49 ++++---------------------------------------------
> >  1 file changed, 4 insertions(+), 45 deletions(-)
> >
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index 1a007dae1e8f..6cee185dc147 100644
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -944,12 +944,7 @@ static int oom_kill_memcg_member(struct task_struct *task, void *unused)
> >  static void oom_kill_process(struct oom_control *oc, const char *message)
> >  {
> >       struct task_struct *p = oc->chosen;
> > -     unsigned int points = oc->chosen_points;
> > -     struct task_struct *victim = p;
> > -     struct task_struct *child;
> > -     struct task_struct *t;
> >       struct mem_cgroup *oom_group;
> > -     unsigned int victim_points = 0;
> >       static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
> >                                             DEFAULT_RATELIMIT_BURST);
> >
> > @@ -971,53 +966,17 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
> >       if (__ratelimit(&oom_rs))
> >               dump_header(oc, p);
> >
> > -     pr_err("%s: Kill process %d (%s) score %u or sacrifice child\n",
> > -             message, task_pid_nr(p), p->comm, points);
> > -
> > -     /*
> > -      * If any of p's children has a different mm and is eligible for kill,
> > -      * the one with the highest oom_badness() score is sacrificed for its
> > -      * parent.  This attempts to lose the minimal amount of work done while
> > -      * still freeing memory.
> > -      */
> > -     read_lock(&tasklist_lock);
> > -
> > -     /*
> > -      * The task 'p' might have already exited before reaching here. The
> > -      * put_task_struct() will free task_struct 'p' while the loop still try
> > -      * to access the field of 'p', so, get an extra reference.
> > -      */
> > -     get_task_struct(p);
> > -     for_each_thread(p, t) {
> > -             list_for_each_entry(child, &t->children, sibling) {
> > -                     unsigned int child_points;
> > -
> > -                     if (process_shares_mm(child, p->mm))
> > -                             continue;
> > -                     /*
> > -                      * oom_badness() returns 0 if the thread is unkillable
> > -                      */
> > -                     child_points = oom_badness(child,
> > -                             oc->memcg, oc->nodemask, oc->totalpages);
> > -                     if (child_points > victim_points) {
> > -                             put_task_struct(victim);
> > -                             victim = child;
> > -                             victim_points = child_points;
> > -                             get_task_struct(victim);
> > -                     }
> > -             }
> > -     }
> > -     put_task_struct(p);
> > -     read_unlock(&tasklist_lock);
> > +     pr_err("%s: Kill process %d (%s) score %lu or sacrifice child\n",
> > +             message, task_pid_nr(p), p->comm, oc->chosen_points);
> >
> >       /*
> >        * Do we need to kill the entire memory cgroup?
> >        * Or even one of the ancestor memory cgroups?
> >        * Check this out before killing the victim task.
> >        */
> > -     oom_group = mem_cgroup_get_oom_group(victim, oc->memcg);
> > +     oom_group = mem_cgroup_get_oom_group(p, oc->memcg);
> >
> > -     __oom_kill_process(victim);
> > +     __oom_kill_process(p);
> >
> >       /*
> >        * If necessary, kill all tasks in the selected memory cgroup.
> > --
> > 2.20.1.321.g9e740568ce-goog
>
> --
> Michal Hocko
> SUSE Labs