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

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Date:	Fri, 26 Nov 2010 16:29:42 +0800
From:	Lin Ming <lin@...g.vg>
To:	Stephane Eranian <eranian@...gle.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 4:18 PM, 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?

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.

This is what my upcoming v3 patches doing.

>
>> 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?

Nehalem.

Thanks,
Lin Ming
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