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

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Message-ID: <20090105215429.GA20319@sgi.com>
Date:	Mon, 5 Jan 2009 15:54:29 -0600
From:	Dimitri Sivanich <sivanich@....com>
To:	Peter Zijlstra <peterz@...radead.org>
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, Jan 05, 2009 at 09:55:58PM +0100, Peter Zijlstra wrote:
> 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!

Actually, sched_clock() should not go backwards on any one cpu, but the readings will be different between cpus.

Also, we noticed the following code is being used for sched_clock_cpu():
u64 sched_clock_cpu(int cpu)
{
        if (unlikely(!sched_clock_running))
                return 0;
        return sched_clock();
}

and is called when smp_processor_id() != cpu.
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