linux-kernel - Re: 2.6.27.8 scheduler bug - threads not being scheduled for long periods

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Message-Id: <1231188958.11687.12.camel@twins>
Date:	Mon, 05 Jan 2009 21:55:58 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Dimitri Sivanich <sivanich@....com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...e.hu>,
	Mike Galbraith <efault@....de>,
	Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>,
	Gregory Haskins <ghaskins@...ell.com>,
	Greg KH <greg@...ah.com>, Nick Piggin <npiggin@...e.de>,
	Robin Holt <holt@....com>
Subject: Re: 2.6.27.8 scheduler bug - threads not being scheduled for long
 periods

On Mon, 2009-01-05 at 11:56 -0600, Dimitri Sivanich wrote:
> One place we've found this happens is in update_curr(), which calculates a
> delta_exec value as follows:
>         delta_exec = (unsigned long)(now - curr->exec_start);
> 
> Sometimes this value will be very large, as 'now' (the rq clock time) will
> be less than 'exec_start'.  When this happens, __update_curr() will
> calculate a delta_exec_weighted based on this large value and add it to the
> thread's vruntime:
>         curr->vruntime += delta_exec_weighted;

So you're saying your rq->clock = sched_clock_cpu(cpu) = sched_clock()
[on ia64] goes backwards?

If so, then that's an architecture bug, sched_clock() must never be seen
to go backwards!
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