On Thu 17-03-16 23:34:13, Tetsuo Handa wrote:
> Michal Hocko wrote:
> > On Thu 17-03-16 22:00:34, Tetsuo Handa wrote:
> > [...]
> > > If you worry about too much work for a single RCU, you can do like
> > > what kmallocwd does. kmallocwd adds a marker to task_struct so that
> > > kmallocwd can reliably resume reporting.
> >
> > It is you who is trying to add a different debugging output so you
> > should better make sure you won't swamp the user by something that might
> > be not helpful after all by _default_. I would care much less if this
> > was hidden by the debugging option like the current
> > debug_show_all_locks.
>
> Then, we can do something like this.
>
> ----------
> diff --git a/mm/oom_kill.c b/mm/oom_kill.c
> index affbb79..76b5c67 100644
> --- a/mm/oom_kill.c
> +++ b/mm/oom_kill.c
> @@ -502,26 +502,20 @@ static void oom_reap_vmas(struct mm_struct *mm)
> schedule_timeout_idle(HZ/10);
>
> if (attempts > MAX_OOM_REAP_RETRIES) {
> +#ifdef CONFIG_PROVE_LOCKING
> struct task_struct *p;
> struct task_struct *t;
> +#endif
>
> pr_info("oom_reaper: unable to reap memory\n");
> - rcu_read_lock();
> +#ifdef CONFIG_PROVE_LOCKING
> + read_lock(&tasklist_lock);
> for_each_process_thread(p, t) {
> - if (likely(t->mm != mm))
> - continue;
> - pr_info("oom_reaper: %s(%u) flags=0x%x%s%s%s%s\n",
> - t->comm, t->pid, t->flags,
> - (t->state & TASK_UNINTERRUPTIBLE) ?
> - " uninterruptible" : "",
> - (t->flags & PF_EXITING) ? " exiting" : "",
> - fatal_signal_pending(t) ? " dying" : "",
> - test_tsk_thread_flag(t, TIF_MEMDIE) ?
> - " victim" : "");
> - sched_show_task(t);
> - debug_show_held_locks(t);
> + if (t->mm == mm && t->state != TASK_RUNNING)
> + debug_show_held_locks(t);
> }
> - rcu_read_unlock();
> + read_unlock(&tasklist_lock);
> +#endif
> }
>
> /* Drop a reference taken by wake_oom_reaper */
> ----------
Please send a separate patch with a full description, ideally with the
example output and clarification why you think this is an improvement
over the current situation. I do not think the patch you are replying to
needs to be changed in any way. It seems correct and provides a useful
information already. If you believe you can provide something more
useful do it in an incremental change.
Making a lot of fuzz with something that doesn't point to a _real_ issue
in the patch to be merged is not particularly useful when we are in the
merge window already.
> Strictly speaking, neither debug_show_all_locks() nor debug_show_held_locks()
> are safe enough to guarantee that the system won't crash.
>
> commit 856848737bd944c1 "lockdep: fix debug_show_all_locks()"
> commit 82a1fcb90287052a "softlockup: automatically detect hung TASK_UNINTERRUPTIBLE tasks"
>
> They are convenient but we should avoid using them if we care about
> possibility of crash.
I really fail to see your point. debug_show_all_locks doesn't mention
any restriction of the risk nor it is restricted to a particular
context. Were there some bugs in that area? Probably yes, so what?
--
Michal Hocko
SUSE Labs
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