linux-kernel - Re: [RFC PATCH 2/3 v2] perf: Implement Nehalem uncore pmu

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Date:	Fri, 26 Nov 2010 17:00:09 +0800
From:	Lin Ming <lin@...g.vg>
To:	Stephane Eranian <eranian@...gle.com>,
	Don Zickus <dzickus@...hat.com>
Cc:	Lin Ming <ming.m.lin@...el.com>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	lkml <linux-kernel@...r.kernel.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arjan van de Ven <arjan@...radead.org>
Subject: Re: [RFC PATCH 2/3 v2] perf: Implement Nehalem uncore pmu

On Fri, Nov 26, 2010 at 4:33 PM, Stephane Eranian <eranian@...gle.com> wrote:
> Lin,
>
> Looked at the perfmon code, and it seems the mask is actual
> cores, not threads:
>                rdmsrl(MSR_NHM_UNC_GLOBAL_CTRL, val);
>                val |= 1ULL << (48 + cpu_data(smp_processor_id()).cpu_core_id);
>                wrmsrl(MSR_NHM_UNC_GLOBAL_CTRL, val);
>
> That seems to imply both threads will get the interrupt.
>
> In the the overflowed event was programmed from on of the two threads, that
> means one will process the overflow, the other will get spurious.
>
> On the cores where no uncore was programmed, then both threads will have
> a spurious interrupt.

But in my test, if HT is on, only the 2 theads in one of the four cores
will receive the interrupt. Even worse, we don't know which core will
receive the interrupt
when overflow happens.

I'll do more tests to verify this.

>
> That brings up back to the 'spurious interrupt' issue and the 'NMI
> Dazed' message
> that Don tried to eliminate. Now we have a new situation where we will
> get interrupt
> with no work to do, so the perf_event will pass the interrupt onto the
> next subsystem
> and eventually we will get the 'dazed' message. I am just guessing here....

Add Don.

Thanks,
Lin Ming

>
>
> On Fri, Nov 26, 2010 at 9:18 AM, Stephane Eranian <eranian@...gle.com> wrote:
>> On Fri, Nov 26, 2010 at 6:15 AM, Lin Ming <lin@...g.vg> wrote:
>>> On Tue, Nov 23, 2010 at 6:17 PM, Stephane Eranian <eranian@...gle.com> wrote:
>>>> Lin,
>>>>
>>>> On Sun, Nov 21, 2010 at 1:01 PM, Lin Ming <ming.m.lin@...el.com> wrote:
>>>>> +static void uncore_pmu_enable_all(void)
>>>>> +{
>>>>> +       u64 ctrl;
>>>>> +
>>>>> +       /*
>>>>> +        * (0xFULL << 48): 1 of the 4 cores can receive NMI each time
>>>>> +        * but we don't know which core will receive the NMI when overflow happens
>>>>> +        */
>>>>
>>>> That does not sound right. If you set bit 48-51 to 1, then all 4 cores
>>>> will receive EVERY
>>>> interrupt, i.e., it's a broadcast. That seems to contradict your
>>>> comment: 1 of the 4. Unless
>>>> you meant, they all get the interrupt and one will handle it, the
>>>> other will find nothing to
>>>> process. But I don't see the atomic op that would make this true in
>>>> uncore_handle_irq().
>>>
>>> Stephane,
>>>
>>> The interrupt model is strange, it behaves differently when HT on/off.
>>>
>>> If HT is off, all 4 cores will receive every interrupt, i.e., it's a broadcast.
>>>
>> That's if yo set the mask to 0xf, right?
>>
>> In the perf_event model, given that any one of the 4 cores can be used
>> to program uncore events, you have no choice but to broadcast to all
>> 4 cores. Each has to demultiplex and figure out which of its counters
>> have overflowed.
>>
>>> If HT is on, only 1 of the 4 cores will receive the interrupt(both
>>> Threads in that core receive the interrupt),
>>> and it can't be determined which core will receive the interrupt.
>>>
>>> Did you ever observe this?
>>>
>> No because I never set more than one bit in the mask.
>>
>>> I tried to set the mask 0xff when HT is on, but kernel panics, because
>>> the reserve bits are set.
>>
>> Let me check on this. It would seem to imply that in HT mode, both threads
>> necessarily receive the interrupts.
>>
>> Was that on Nehalem or Westmere?
>>
>
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