Re: [PATCH] perf/x86: read the FREEZE_WHILE_SMM bit during boot

[David Arcari <darcari@redhat.com>]

On 06/04/2018 10:12 AM, David Arcari wrote:
> On 06/04/2018 04:24 AM, Peter Zijlstra wrote:
>> On Sun, Jun 03, 2018 at 02:23:43PM -0400, David Arcari wrote:
>>> On some systems pressing the external NMI button is now failing to inject
>>> an NMI 5-10% of the time.  This causes confusion for a user that expects
>>> the NMI to dump the system.
>>>
>>> Commit 6089327f5424 ("perf/x86: Add sysfs entry to freeze counters on SMI")
>>> does not read the firmware setting of the FREEZE_WHILE_SMM bit and will
>>> always clear it when the PMU is initialized.  As a result the performance
>>> counters will always run and that greatly expands the race in which
>>> external NMI will not be processed if a local NMI is already being
>>> processed.
>>>
>>> One option is to change default_do_nmi().  The code snippet below shows the
>>> relevant portion of a patch that resolves the issue, but it is problematic
>>> from a performance perspective and was dismissed.
>>>
>>> -345,7 +345,17 @@ static void default_do_nmi(struct pt_regs *regs)
>>>  		 */
>>>  		if (handled > 1)
>>>  			__this_cpu_write(swallow_nmi, true);
>>> -		return;
>>> +
>>> +		/*
>>> +		 * Unfortunately, there is a race condition which can
>>> +		 * result in a missing an external NMI.  Typically, an
>>> +		 * external NMI is processed on cpu 0.  Therefore, on
>>> +		 * cpu 0 check for an external NMI before returning.
>>> +		 */
>>> +		if (smp_processor_id() ||
>>> +		    (x86_platform.get_nmi_reason() & NMI_REASON_MASK) == 0) {
>>> +			return;
>>> +		}
>>>  	}
>>>
>>> Ultimately, the issue can be resolved by storing the default firmware
>>> setting of FREEZE_WHILE_SMM before initializing the PMU.
>>
>> I'm sorry, I know it's Monday morning, but what?! I really don't
>> understand anything you write there.
>>
>> Maybe if you explain the race and how your proposed fix closes it things
>> will make sense. The above refers to too many things not here.
>>
> 
> Sorry.
> 
> default_do_nmi() will process both perf events (local interrupts) as well as
> external interrupts (such as the NMI button).  The handler is coded such that
> if a local interrupt occurs, no check is made for an external interrupt.
> Therefore, if the two interrupts occur simultaneously, the external interrupt
> is lost.
> 
> The code above, which was ultimately discounted, attempts to avoid this
> scenario with as little performance impact as possible by reading the register
> without the spinlock for cpu 0 only (currently only cpu 0 can handle an
> external NMI, I verified this on my system by testing the NMI button with
> cpu 0 offline).
> 
> The code above is problematic for a number of reasons not the least of which
> is performance.  Furthermore, I don't see a less intrusive solution wrt
> do_default_nmi().
> 
> Upstream 6089327f5424 ("perf/x86: Add sysfs entry to freeze counters on SMI")
> appears to have made it relatively easy to hit this race condition.  On some
> systems, this commit has resulted in a change to the default firmware setting
> of DEBUGCTLMSR_FREEZE_IN_SMM_BIT (it is now cleared by the OS by default).
> 
> With this bit cleared, the following situation occurs:
> 
> 1) external NMI - due to io check
> 2) long duration SMI (counters do not freeze)
> 3) NMI handler runs and misattributes interrupt to perf event
> 
> Ultimately, my solution was to restore the previous behavior by reading and
> storing the firmware setting of the bit rather than to always clear it.
> 

Hi Peter,

Have you had a chance to take a look at this?

Thanks,

-Dave