linux-kernel - Re: [PATCH v3] softlockup: remove hung_task_check

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Message-ID: <20090123092306.GB29820@elte.hu>
Date:	Fri, 23 Jan 2009 10:23:06 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Mandeep Baines <msb@...gle.com>
Cc:	fweisbec@...il.com, linux-kernel@...r.kernel.org,
	rientjes@...gle.com, mbligh@...gle.com, thockin@...gle.com
Subject: Re: [PATCH v3] softlockup: remove hung_task_check_count


* Mandeep Baines <msb@...gle.com> wrote:

> On Thu, Jan 22, 2009 at 11:55 AM, Mandeep Singh Baines <msb@...gle.com> wrote:
> >
> > The unlock and lock could be removed and only compiled in if PREEMPT.
> > If the number of tasks isn't bound, the lock might be held too long.
> >
> 
> This is incorrect. The adding the lock and unlock will not make the
> system more pre-emptive. To be more pre-emptive you'd want to check
> need_resched() as often as possible.
> 
> > It would be kinda funny if hung_task caused a softlockup.
> >
> 
> Again. This is incorrect. Rescheduling if need_resched() will prevent 
> softlockup.
> 
> Not sure what I was thinking this morning;)
> 
> However, I am happy with the patch. To give writers a chance, the lock 
> should held for bounded time. Holding the lock in khungtask (which is 
> running at low scheduler priority) could potentially be delaying 
> important work. The longer the lock is held, the bigger the priority 
> inversion problem.

not sure i like the whole idea of removing the max iterations check. In 
theory if there's a _ton_ of tasks, we could spend a lot of time looping 
there. So it always looked prudent to limit it somewhat.

	Ingo
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