linux-kernel - Re: perf hw in kexeced kernel broken in tip

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Date:	Wed, 08 Dec 2010 15:20:05 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Don Zickus <dzickus@...hat.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Vivek Goyal <vgoyal@...hat.com>,
	Yinghai Lu <yinghai@...nel.org>, Ingo Molnar <mingo@...e.hu>,
	Jason Wessel <jason.wessel@...driver.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Haren Myneni <hbabu@...ibm.com>
Subject: Re: perf hw  in kexeced kernel broken in tip

On Wed, 2010-12-08 at 09:01 -0500, Don Zickus wrote:
> On Wed, Dec 08, 2010 at 12:30:20AM +0100, Peter Zijlstra wrote:
> > On Thu, 2010-12-02 at 11:15 -0500, Don Zickus wrote:
> > 
> > > Vivek suggested to me this morning that I should just blantantly disable the
> > > perf counter during init when running my test. 
> > 
> > Nah, we should actively scan for that during the bring-up and kill
> > hw-perf when we find an enable bit set, some BIOSes actively use the
> > PMU, this is something that should be discouraged.
> 
> Ok, the reboot notifier addresses the kexec problem but doesn't fix it
> though (I have to test to confirm that, comments below).  


> The bios check
> should catch those situations (ironically I stumbled upon a machine with
> this problem, so I will test your patch with it, though it only uses perf
> counter 0). 

Right, they usually only steal one or two counters, but the fact that
they're using them at all is insane and should be punished.

>  The kdump problem will still exist, not sure if we care and
> perhaps we should document in the changelog that we know kdump is still
> broken (unless we do care).

You mean even if we cure the kexec reboot notifier patch thing kdump is
still borken?


> > ---
> >  arch/x86/kernel/cpu/perf_event.c |   30 +++++++++++++++++++++++++++---
> >  1 files changed, 27 insertions(+), 3 deletions(-)
> > 
> > diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> > index 817d2b1..7f92833 100644
> > --- a/arch/x86/kernel/cpu/perf_event.c
> > +++ b/arch/x86/kernel/cpu/perf_event.c
> > @@ -375,15 +375,40 @@ static void release_pmc_hardware(void) {}
> >  static bool check_hw_exists(void)
> >  {
> >  	u64 val, val_new = 0;
> > -	int ret = 0;
> > +	int i, reg, ret = 0;
> >  
> >  	val = 0xabcdUL;
> >  	ret |= checking_wrmsrl(x86_pmu.perfctr, val);
> >  	ret |= rdmsrl_safe(x86_pmu.perfctr, &val_new);
> > -	if (ret || val != val_new)
> > +	if (ret || val != val_new) {
> > +		printk(KERN_CONT "Broken PMU hardware detected, software events only.\n");
> >  		return false;
> > +	}
> > +
> > +	/*
> > +	 * Check to see if the BIOS enabled any of the counters, if so
> > +	 * complain and bail.
> > +	 */
> > +	for (i = 0; i < x86_pmu.num_counters; i++) {
> > +		reg = x86_pmu.eventsel + i;
> > +		rdmsrl(reg, val);
> > +		if (val & ARCH_PERFMON_EVENTSEL_ENABLE)
> > +			goto bios_fail;
> > +	}
> > +
> > +	for (i = 0; i < x86_pmu.num_counters_fixed; i++) {
> > +		reg = MSR_ARCH_PERFMON_FIXED_CTR_CTRL;
> > +		rdmsrl(reg, val);
> > +		if (val & (0x03 << i*4))
> > +			goto bios_fail;
> > +	}
> 
> I wonder if you should reverse these checks.  If the bios has the perf
> counter enabled, there might be a high chance that it fails the first
> check and never gets to the actually bios checks.

Ah, good point.

> >  
> >  	return true;
> > +
> > +bios_fail:
> > +	printk(KERN_CONT "Broken BIOS detected, software events only.\n");
> > +	printk(KERN_ERR FW_BUG "invalid MSR %x=%Lx\n", reg, val);
> > +	return false;
> >  }
> >  
> >  static void reserve_ds_buffers(void);
> > @@ -1379,7 +1404,6 @@ int __init init_hw_perf_events(void)
> >  
> >  	/* sanity check that the hardware exists or is emulated */
> >  	if (!check_hw_exists()) {
> > -		pr_cont("Broken PMU hardware detected, software events only.\n");
> >  		return 0;
> >  	}
> 
> nitpick - you can probably remove the curly braces, no?

Quite so.


> > @@ -6383,6 +6384,25 @@ static void perf_event_exit_cpu(int cpu)
> >  static inline void perf_event_exit_cpu(int cpu) { }
> >  #endif
> >  
> > +static int 
> > +perf_reboot(struct notifier_block *notifier, unsigned long val, void *v)
> > +{
> > +	/*
> > +	 * XXX this relies on hotplug, does kexec do too?
> > +	 */
> > +	perf_event_exit_cpu(0);
> > +	return NOTIFY_OK;
> 
> Ok, so this shuts down the perf counters on cpu0, but the other cpus are
> still running and will fail your new bios check, no?
> 
> Privately, I used the above wrapped with for_each_online_cpu(cpu) and it
> worked fine for me.

Oh, so reboot doesn't actually stop the non-boot cpus? I was unsure of
that (see my XXX there), so yeah, if it doesn't then I guess the
for_each_possible_cpu() thing is the way out.



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