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

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Message-ID: <AANLkTikYsT=5qOTKTaQaf-OwdZDMkrfKWaSb=aDqgrrC@mail.gmail.com>
Date:	Fri, 26 Nov 2010 09:33:40 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Lin Ming <lin@...g.vg>
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

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.

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....


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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