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

[Stephane Eranian <eranian@google.com>]

On Wed, Sep 29, 2010 at 5:45 PM, Cyrill Gorcunov <gorcunov@gmail.com> wrote:
> On Wed, Sep 29, 2010 at 05:33:07PM +0200, Stephane Eranian wrote:
>> Robert,
>>
>> There is something else bothering me with cpuc->running.
>>
>> It is not reset outside of the interrupt handler. So what if
>> event scheduling shuffles things around and an event is
>> moved somewhere else. Don't you need to clear the
>> cpuc->running[idx] for the old counter index?
>>
>>
>
> Both bitmasks are set and test with same index though it might
> be a bit obscure scheme (we could be clearing this bit in
> x86_pmu_stop but it just a wasting cycles).
>
But you cannot clear it in x86_pmu_stop() because otherwise it
turns into active_mask[]. My understanding is that you need
to remember this counter has been active at some point in the
past.

My point is that you cannot keep this around forever. After a
"while" it becomes stale and you have to remove it otherwise
you may wrongly increment handled.

Here is a scenario:

event A -> counter 0, cpuc->running = 0x1 active_mask = 0x1
move A
event A -> counter 1, cpuc->running = 0x3, active_mask = 0x2

No interrupt, we are just counting for a short period.
Then, you get an NMI interrupt, suppose it is not generated
by the PMU, it is destined for another handler.

For i=0, you have (active_mask & 0x1) == 0, but (running & 0x1) == 1,
you mark the interrupt as handled, i.e., you swallow it, the actual
handler never gets it.


> Btw, since x86 architectural and p4 are using same tests for
> running I presume better to have some helper rather then
> open coded pile?
>
> Cyrill
>