linux-kernel - Re: [PATCH] intel_pstate: set BYT MSR with wrmsrl_on

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Message-ID: <20150507232232.GA4313@jbkonno-beef.jf.intel.com>
Date:	Thu, 7 May 2015 16:22:32 -0700
From:	Joe Konno <joe.konno@...ux.intel.com>
To:	"Rafael J. Wysocki" <rjw@...ysocki.net>
Cc:	linux-pm@...r.kernel.org, viresh.kumar@...aro.org,
	linux-kernel@...r.kernel.org, kristen@...ux.intel.com
Subject: Re: [PATCH] intel_pstate: set BYT MSR with wrmsrl_on_cpu()

On Thu, May 07, 2015 at 10:58:11PM +0200, Rafael J. Wysocki wrote:
> On Thursday, May 07, 2015 09:59:39 AM Joe Konno wrote:
> > From: Joe Konno <joe.konno@...el.com>
> > 
> > In instances where the default cpufreq governor is Performance, reading
> 
> I'm not really sure what this is about.  You're talking about cpufreq governors
> and this is an intel_pstate patch.  What gives?

I'll reshuffle the paragraph to bring detail to the fix first, and the
"when/why" second.

In debug I have only seen the bug during boot when cpufreq calls
intel_pstate's init for each logical core-- often from one, sometimes
two logical cores.

The bug may occur after init as well, but not enough data to conclude
one way or the other. I personally have not seen it happen after init in
my local testing.

> 
> > from MSR 0x199 on an applicable multi-core Atom system saw boot-to-boot
> > variability in the P-State value set to each logical core.  Sometimes
> > only one logical core would be set properly, other times two or three.
> > There was an assumption in the code that only a thread on the intended
> > logical core would be calling the wrmsrl() function. That was disproven
> > during debug, as cpufreq, at init, was not always calling from the same
> > as the logical core it targeted. Thus, use wrmsrl_on_cpu() instead, as
> > done in the core_set_pstate() function.
> > 
> > For: LCK-1822
> 
> This tag is meaningless upstream.

Mimicked another subsystem's practice. I have no problem removing it.

> 
> > Fixes: 007bea098b86 ("intel_pstate: Add setting voltage value for
> >        baytrail P states.")
> 
> So, you're fixing a function introduced by the above commit, right?

Correct. That commit introduced the byt_set_pstate() function with the
wrmsrl() call.

> 
> > Signed-off-by: Joe Konno <joe.konno@...el.com>
> > ---
> >  drivers/cpufreq/intel_pstate.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/drivers/cpufreq/intel_pstate.c b/drivers/cpufreq/intel_pstate.c
> > index 6414661ac1c4..c45d274a75c8 100644
> > --- a/drivers/cpufreq/intel_pstate.c
> > +++ b/drivers/cpufreq/intel_pstate.c
> > @@ -535,7 +535,7 @@ static void byt_set_pstate(struct cpudata *cpudata, int pstate)
> >  
> >  	val |= vid;
> >  
> > -	wrmsrl(MSR_IA32_PERF_CTL, val);
> > +	wrmsrl_on_cpu(cpudata->cpu, MSR_IA32_PERF_CTL, val);
> 
> So the bug is that this may run on a CPU which is not cpudata->cpu in which
> case the write will not happen where it should.  Is that correct?

Yes-- I believe my first inline comment spoke to this.

> 
> >  }
> >  
> >  #define BYT_BCLK_FREQS 5
> > 
> 
> -- 
> I speak only for myself.
> Rafael J. Wysocki, Intel Open Source Technology Center.
> --
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