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Date: Tue, 17 Mar 2015 20:24:50 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: Aaron Tomlin <atomlin@redhat.com>
Cc: akpm@linux-foundation.org, rientjes@google.com, dwysocha@redhat.com,
        linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>
Subject: [PATCH 0/2] hung_task: improve rcu_lock_break() logic
Message-ID: <20150317192450.GA32579@redhat.com>
References: <1426601624-6703-1-git-send-email-atomlin@redhat.com> <1426601624-6703-2-git-send-email-atomlin@redhat.com> <20150317170920.GA21493@redhat.com>
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On 03/17, Oleg Nesterov wrote:
>
> On 03/17, Aaron Tomlin wrote:
> >
> > --- a/kernel/hung_task.c
> > +++ b/kernel/hung_task.c
> > @@ -169,7 +169,7 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
> >  		return;
> >
> >  	rcu_read_lock();
> > -	do_each_thread(g, t) {
> > +	for_each_process_thread(g, t) {
> >  		if (!max_count--)
> >  			goto unlock;
> >  		if (!--batch_count) {
> > @@ -180,7 +180,7 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
> >  		/* use "==" to skip the TASK_KILLABLE tasks waiting on NFS */
> >  		if (t->state == TASK_UNINTERRUPTIBLE)
> >  			check_hung_task(t, timeout);
> > -	} while_each_thread(g, t);
> > +	}
>
>
> Acked-by: Oleg Nesterov <oleg@redhat.com>
>
>
>
> Perhaps it also makes sense to improve this rcu_lock_break a bit...
> For example, if 't' is dead but 'g' is alive we can continue the
> "for_each_process" part of this double loop. And if 't' is still
> alive then we can find the new leader and continue...
>
> But I agree, lets start from this fix, then we will see.

Something like this. on top of Aaron's change.

But actually I am not sure this really makes a lot of sense. But if
yes, we can do more. We can save g->start_time before rcu_lock_break(),
and then "both dead" case can use for_each_process() to (try to) find
the first process whis has a ge start_time.

Oleg.