linux-kernel - Re: intel_pstate divide error with v3.13-rc4-256-gb7000ad

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Date:	Fri, 27 Dec 2013 15:35:29 +0100
From:	"Rafael J. Wysocki" <rjw@...ysocki.net>
To:	Viresh Kumar <viresh.kumar@...aro.org>,
	Josh Boyer <jwboyer@...oraproject.org>
Cc:	Dirk Brandewie <dirk.brandewie@...il.com>,
	"cpufreq@...r.kernel.org" <cpufreq@...r.kernel.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	"Linux-Kernel@...r. Kernel. Org" <linux-kernel@...r.kernel.org>
Subject: Re: intel_pstate divide error with v3.13-rc4-256-gb7000ad

On Tuesday, December 24, 2013 09:36:01 PM Viresh Kumar wrote:
> Adding Dirk..
> 
> On 24 December 2013 20:06, Josh Boyer <jwboyer@...oraproject.org> wrote:
> > Hi All,
> >
> > We've had a report [1] that the pstate driver causes KVM guests to
> > fail to boot because of a divide error.  See the backtrace below.
> >
> >     4.839784] Intel P-state driver initializing.
> > [    4.859972] Intel pstate controlling: cpu 0
> > [    4.867653] cpufreq: __cpufreq_add_dev: ->get() failed
> 
> After a call to ->init(), ->get() is supposed to work.
> @Dirk: Any idea why it failed?

Well, it looks like sample->freq is 0 in intel_pstate_get().

> And then I don't know what made this divide by zero to happen :)

>From code inspection it looks like that is caused by the
intel_pstate_get_scaled_busy() called from intel_pstate_adjust_busy_pstate(),
so it appears that cpu->pstate.current_state is 0 at that point.

I'm wondering if something like the (untested) patch below helps, then?

Rafael


---
 drivers/cpufreq/intel_pstate.c |    5 +++++
 1 file changed, 5 insertions(+)

Index: linux-pm/drivers/cpufreq/intel_pstate.c
===================================================================
--- linux-pm.orig/drivers/cpufreq/intel_pstate.c
+++ linux-pm/drivers/cpufreq/intel_pstate.c
@@ -652,6 +652,11 @@ static int intel_pstate_init_cpu(unsigne
 	cpu = all_cpu_data[cpunum];
 
 	intel_pstate_get_cpu_pstates(cpu);
+	if (!cpu->pstate.current_state) {
+		all_cpu_data[cpunum] = NULL;
+		kfree(cpu);
+		return -ENODATA;
+	}
 
 	cpu->cpu = cpunum;
 

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