Re: [PATCH v3] mm,oom: speed up select_bad_process() loop.

[Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>]

Michal Hocko wrote:
> On Wed 18-05-16 22:30:14, Tetsuo Handa wrote:
> > Even if you call p->signal->oom_score_adj == OOM_SCORE_ADJ_MIN case a bug,
> > (p->flags & PF_KTHREAD) || is_global_init(p) case is still possible.
> 
> I couldn't care less about such a case to be honest, and that is not a
> reason the cripple the code for such an insanity. There simply doesn't
> make any sense to share init's mm with a different task.

The global init called vfork(), and the child tried to call execve() with
large argv/envp array, and the child got OOM-killed is possible.

> OK, this looks correct. Strictly speaking the patch is missing any note
> on _why_ this is needed or an improvement. I would add something like
> the following:
> "
> Although the original code was correct it was quite inefficient because
> each thread group was scanned num_threads times which can be a lot
> especially with processes with many threads. Even though the OOM is
> extremely cold path it is always good to be as effective as possible
> when we are inside rcu_read_lock() - aka unpreemptible context.
> "

rcu_read_lock() is not always unpreemptible context. rcu_read_lock() says:

  In non-preemptible RCU implementations (TREE_RCU and TINY_RCU),
  it is illegal to block while in an RCU read-side critical section.
  In preemptible RCU implementations (PREEMPT_RCU) in CONFIG_PREEMPT
  kernel builds, RCU read-side critical sections may be preempted,
  but explicit blocking is illegal.  Finally, in preemptible RCU
  implementations in real-time (with -rt patchset) kernel builds, RCU
  read-side critical sections may be preempted and they may also block, but
  only when acquiring spinlocks that are subject to priority inheritance.

We will need preempt_disable() if we want to make out_of_memory() return
as fast as possible.

> 
> > Signed-off-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
> > Cc: David Rientjes <rientjes@google.com>
> > Cc: Oleg Nesterov <oleg@redhat.com>
> 
> Acked-by: Michal Hocko <mhocko@suse.com>

Thank you.

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