Re: perf hw in kexeced kernel broken in tip

[Yinghai Lu <yinghai@kernel.org>]

On 12/07/2010 01:16 PM, Don Zickus wrote:
> On Thu, Dec 02, 2010 at 08:34:30AM +0100, Peter Zijlstra wrote:
>>>  void __init lockup_detector_init(void)
>>>  {
>>>  	void *cpu = (void *)(long)smp_processor_id();
>>> @@ -563,6 +576,7 @@ void __init lockup_detector_init(void)
>>>  
>>>  	cpu_callback(&cpu_nfb, CPU_ONLINE, cpu);
>>>  	register_cpu_notifier(&cpu_nfb);
>>> +	register_reboot_notifier(&reboot_nfb);
>>>  
>>>  	return;
>>>  }
>>
>> We'd really want a perf_event.c callback there to do as the hot-unplug
>> code does and detach all running counters from the cpu.
> 
> Ok, here is a simpler patch for now.
> 
> --------------------------------8<--------
> From: Don Zickus <dzickus@redhat.com>
> Date: Tue, 7 Dec 2010 16:06:59 -0500
> Subject: [PATCH] perf:  Use event select bits for hardware check
> 
> The counter registers can continue to increment if left enabled
> across a kexec or a kdump.  The makes the perf hardware check
> accidentally return false when the hardware really does exist.
> 
> Change the check to use the first bits of event selection.  Those
> bits should be safe as they are used to program the type of events
> to use.  And more importantly, they won't increment across kexec/kdump.
> 
> Signed-off-by: Don Zickus <dzickus@redhat.com>
> ---
>  arch/x86/kernel/cpu/perf_event.c |    8 ++++----
>  1 files changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kernel/cpu/perf_event.c b/arch/x86/kernel/cpu/perf_event.c
> index 7b91396..7d869c0 100644
> --- a/arch/x86/kernel/cpu/perf_event.c
> +++ b/arch/x86/kernel/cpu/perf_event.c
> @@ -377,10 +377,10 @@ static bool check_hw_exists(void)
>  	u64 val, val_new = 0;
>  	int ret = 0;
>  
> -	val = 0xabcdUL;
> -	ret |= checking_wrmsrl(x86_pmu.perfctr, val);
> -	ret |= rdmsrl_safe(x86_pmu.perfctr, &val_new);
> -	if (ret || val != val_new)
> +	val = 0xabUL;
> +	ret |= checking_wrmsrl(x86_pmu.eventsel, val);
> +	ret |= rdmsrl_safe(x86_pmu.eventsel, &val_new);
> +	if (ret || val != (val_new & 0xFF))
>  		return false;
>  
>  	return true;

Thanks. it fixes the problem.

Yinghai