linux-kernel - Re: [PATCH v2] softlockup: decouple hung tasks check from softlockup detection

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Date:	Sat, 17 Jan 2009 15:07:32 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Mandeep Singh Baines <msb@...gle.com>
Cc:	mingo@...e.hu, linux-kernel@...r.kernel.org, rientjes@...gle.com,
	mbligh@...gle.com, thockin@...gle.com
Subject: Re: [PATCH v2] softlockup: decouple hung tasks check from
	softlockup detection

On Fri, Jan 16, 2009 at 08:13:30PM -0800, Mandeep Singh Baines wrote:
> Hi Frédéric,
> 
> Frédéric Weisbecker (fweisbec@...il.com) wrote:
> > > -       read_lock(&tasklist_lock);
> > > -       do_each_thread(g, t) {
> > > -               if (!--max_count)
> > > -                       goto unlock;
> > 
> > 
> > Instead of having this arbitrary limit of tasks, why not just
> > lurk the need_resched() and then schedule if it needs too.
> > 
> > I know that sounds a bit racy, because you will have to release the
> > tasklist_lock and
> > a lot of things can happen in the task list until you become resched.
> > But you can do a get_task_struct() on g and t before your thread is
> > going to sleep and then put them
> > when it is awaken.
> > Perhaps some tasks will disappear or be appended in the list before g
> > and t, but that doesn't really matter:
> > if they disappear, they didn't lockup, and if they were appended, they
> > are not enough cold to be analyzed :-)
> > 
> > This way you can drop the arbitrary limit of task number given by the user....
> > 
> > Frederic.
> > 
> 
> Would be nice to remove the limit. But I don't think get_task_struct()
> can be used to prevent a task from being unlinked from the task list. It
> only prevents the task_struct from being freed. So hung_task could end up
> holding a reference to an unlinked task after it returns from schedule().
> 
> That doesn't mean what you are suggesting can't be implemented. Just means
> that the case of the held task being unlinked needs to be handled.
> 
> Regards,
> Mandeep

Hmm, you're right.
Why not testing 1024 tasks, then check need_resched and if you sleep
and the task becomes unlinked (there are few chances) so... that's not
a big deal actually, you will have better chances on the next check :-)

I think that's a bit important since you are more likely to see a
soft-lockup if you have a lot of tasks.

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