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Date: Tue, 22 Jan 2019 09:52:44 +0100
From: Michal Hocko <mhocko@...nel.org>
To: Shakeel Butt <shakeelb@...gle.com>
Cc: Johannes Weiner <hannes@...xchg.org>,
David Rientjes <rientjes@...gle.com>,
Andrew Morton <akpm@...ux-foundation.org>,
Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
Roman Gushchin <guro@...com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
Linux MM <linux-mm@...ck.org>,
LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 2/2] mm, oom: remove 'prefer children over parent'
heuristic
On Mon 21-01-19 18:41:28, Shakeel Butt wrote:
> On Mon, Jan 21, 2019 at 1:59 PM Shakeel Butt <shakeelb@...gle.com> 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@...gle.com>
> > Acked-by: Michal Hocko <mhocko@...e.com>
>
> 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@...com>
> > Cc: Andrew Morton <akpm@...ux-foundation.org>
> > Cc: David Rientjes <rientjes@...gle.com>
> > Cc: Johannes Weiner <hannes@...xchg.org>
> > Cc: Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
> > Cc: linux-mm@...ck.org
> > Cc: linux-kernel@...r.kernel.org
> >
> > ---
> > 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
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