Re: [PATCH v3 2/2] mm, oom: remove 'prefer children over parent' heuristic

[Michal Hocko]

On Mon 21-01-19 18:41:28, Shakeel Butt wrote:
> On Mon, Jan 21, 2019 at 1:59 PM Shakeel Butt <shakeelb@xxxxxxxxxx> 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. Also 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. So, let's remove this whole
> > heuristic.
> >
> > Signed-off-by: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > Acked-by: Michal Hocko <mhocko@xxxxxxxx>
> 
> Michal, though I have kept your Acked-by but I have made a couple of
> changes in the code. Please let me know if you are ok with the
> changes.

So the only change I can see is that we no longer print the score of the
selected oom victim and that each killed task gets the oom scope prefix.
I cannot think of anybody relying on the former and the later makes
sense to me. So yeah, I am still OK with the resulting code.

> > Cc: Roman Gushchin <guro@xxxxxx>
> > Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> > Cc: David Rientjes <rientjes@xxxxxxxxxx>
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
> > Cc: linux-mm@xxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> >
> > ---
> > Changelog since v2:
> > - Propagate the message to __oom_kill_process().
> >
> > Changelog since v1:
> > - Improved commit message based on mhocko's comment.
> > - Replaced 'p' with 'victim'.
> > - Removed extra pr_err message.
> >
> > ---
> >  mm/oom_kill.c | 78 ++++++++++++---------------------------------------
> >  1 file changed, 18 insertions(+), 60 deletions(-)
> >
> > diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> > index 1a007dae1e8f..c90184fd48a3 100644
> > --- a/mm/oom_kill.c
> > +++ b/mm/oom_kill.c
> > @@ -843,7 +843,7 @@ static bool task_will_free_mem(struct task_struct *task)
> >         return ret;
> >  }
> >
> > -static void __oom_kill_process(struct task_struct *victim)
> > +static void __oom_kill_process(struct task_struct *victim, const char *message)
> >  {
> >         struct task_struct *p;
> >         struct mm_struct *mm;
> > @@ -874,8 +874,9 @@ static void __oom_kill_process(struct task_struct *victim)
> >          */
> >         do_send_sig_info(SIGKILL, SEND_SIG_PRIV, victim, PIDTYPE_TGID);
> >         mark_oom_victim(victim);
> > -       pr_err("Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n",
> > -               task_pid_nr(victim), victim->comm, K(victim->mm->total_vm),
> > +       pr_err("%s: Killed process %d (%s) total-vm:%lukB, anon-rss:%lukB, file-rss:%lukB, shmem-rss:%lukB\n",
> > +               message, task_pid_nr(victim), victim->comm,
> > +               K(victim->mm->total_vm),
> >                 K(get_mm_counter(victim->mm, MM_ANONPAGES)),
> >                 K(get_mm_counter(victim->mm, MM_FILEPAGES)),
> >                 K(get_mm_counter(victim->mm, MM_SHMEMPAGES)));
> > @@ -932,24 +933,19 @@ static void __oom_kill_process(struct task_struct *victim)
> >   * Kill provided task unless it's secured by setting
> >   * oom_score_adj to OOM_SCORE_ADJ_MIN.
> >   */
> > -static int oom_kill_memcg_member(struct task_struct *task, void *unused)
> > +static int oom_kill_memcg_member(struct task_struct *task, void *message)
> >  {
> >         if (task->signal->oom_score_adj != OOM_SCORE_ADJ_MIN) {
> >                 get_task_struct(task);
> > -               __oom_kill_process(task);
> > +               __oom_kill_process(task, message);
> >         }
> >         return 0;
> >  }
> >
> >  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 task_struct *victim = oc->chosen;
> >         struct mem_cgroup *oom_group;
> > -       unsigned int victim_points = 0;
> >         static DEFINE_RATELIMIT_STATE(oom_rs, DEFAULT_RATELIMIT_INTERVAL,
> >                                               DEFAULT_RATELIMIT_BURST);
> >
> > @@ -958,57 +954,18 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
> >          * its children or threads, just give it access to memory reserves
> >          * so it can die quickly
> >          */
> > -       task_lock(p);
> > -       if (task_will_free_mem(p)) {
> > -               mark_oom_victim(p);
> > -               wake_oom_reaper(p);
> > -               task_unlock(p);
> > -               put_task_struct(p);
> > +       task_lock(victim);
> > +       if (task_will_free_mem(victim)) {
> > +               mark_oom_victim(victim);
> > +               wake_oom_reaper(victim);
> > +               task_unlock(victim);
> > +               put_task_struct(victim);
> >                 return;
> >         }
> > -       task_unlock(p);
> > +       task_unlock(victim);
> >
> >         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);
> > +               dump_header(oc, victim);
> >
> >         /*
> >          * Do we need to kill the entire memory cgroup?
> > @@ -1017,14 +974,15 @@ static void oom_kill_process(struct oom_control *oc, const char *message)
> >          */
> >         oom_group = mem_cgroup_get_oom_group(victim, oc->memcg);
> >
> > -       __oom_kill_process(victim);
> > +       __oom_kill_process(victim, message);
> >
> >         /*
> >          * If necessary, kill all tasks in the selected memory cgroup.
> >          */
> >         if (oom_group) {
> >                 mem_cgroup_print_oom_group(oom_group);
> > -               mem_cgroup_scan_tasks(oom_group, oom_kill_memcg_member, NULL);
> > +               mem_cgroup_scan_tasks(oom_group, oom_kill_memcg_member,
> > +                                     (void*) message);
> >                 mem_cgroup_put(oom_group);
> >         }
> >  }
> > --
> > 2.20.1.321.g9e740568ce-goog
> >

-- 
Michal Hocko
SUSE Labs