Re: [tip:perf/urgent] perf, x86: Catch spurious interrupts after disabling counters

[Stephane Eranian <eranian@google.com>]

On Wed, Sep 29, 2010 at 3:39 PM, Robert Richter <robert.richter@amd.com> wrote:
> On 29.09.10 09:13:30, Stephane Eranian wrote:
>
>> for_each_set_bit(bit, (unsigned long *)&status, X86_PMC_IDX_MAX) {
>>                 struct perf_event *event = cpuc->events[bit];
>>
>>                 handled++;
>>
>>                 if (!test_bit(bit, cpuc->active_mask))
>
>                        /* spurious interrupt here */
>
>>                     continue;
>> }
>>
>> I think the logic is similar. What makes the difference, it seems, is that
>> handled is incremented unconditionally if the ovfl_mask says it has
>> an overflow, i.e., before active_mask is checked.
>
> Note that we can use here for_each_set_bit() since we have the status
> mask. So we may increment handled always.
>
> On AMD we use for_each_counter(), but only check active counters to
> avoid unnecessary rdmsrl()s for unused counters. But here, we only can
> increment handled if we detect an overflow or if we know a counter was
> disabled.
>
>> On Westmere, we've seen situations where the overflow mask and active
>> mask did not agree.
>
> It's the 'spurious interrupt' branch above.
>
>> On counter disable, the overflow mask bit is not cleared, thus one may iterate
>> in the loop and fail the active_mask. But handled would be incremented in that
>> case, so that would behave like in your patch.
>
> Right, spurious interrupts are counted and a 'handled' is returned.
>
Ok, I think we agree on this. It is handled in the Intel case, though it
is not clearly explained with a comment.
The P4 case needs to be fixed.

Here is another difference I noticed in x86_handle_irq() vs.
intel_pmu_handle_irq().
For Intel, handled is incremented even if there is no 64-bit overflow.

With generic X86, it is incremented only when you have a 64-bit
overflow. I think that's wrong. You don't hit that condition very often
on AMD because counters are 47 bits wide, but this is generic code
and on P6 you definitively will. I believe you need to hoist handled++
just after the check on active_mask.


What do you think?

> -Robert
>
> --
> Advanced Micro Devices, Inc.
> Operating System Research Center
>
>