Andi,
On Mon, Oct 7, 2013 at 11:45 PM, Andi Kleen <a...@firstfloor.org> wrote:
> Stephane Eranian <eran...@google.com> writes:
>>
>>>> + goto again;
>>>> +
>>>> + struct rapl_pmu *pmu = __get_cpu_var(rapl_pmu);
>>>> +
>>>> + if (WARN_ON_ONCE(!(event->hw.state & PERF_HES_STOPPED)))
>>>> + return;
>>>> +
>>>> + event->hw.state = 0;
>>>> +
>>>> + local64_set(&event->hw.prev_count, rapl_read_counter(event));
>>>> +
>>>> + pmu->n_active++;
>>>
>>> What lock protects this add?
>>>
>> None. I will add one. Bu then I am wondering about if it is really
>> necessary given
>> that RAPL event are system-wide and this pinned to a CPU. If the call came
>> from another CPU, then it IPI there, and that means that CPU is executing
>> that
>> code. Any other CPU will need IPI too, and that interrupt will be kept
>> pending.
>> Am I missing a test case here? Are IPI reentrant?
>
> they can be if interrupts are enabled (likely here)
>
So, I spent some time trying to figure this out via instrumentation and it seems
it is never the case that this function or in fact __perf_event_enable() for a
syswide event is called with interrupts enabled. Why?
Well, it has to do with cpu_function_call() which is ALWAYS called for a syswide
event on the perf_event_enable() code path.
If you are calling for an event on the same CPU, you end up executing:
smp_call_function_single()
if (cpu == this_cpu) {
local_irq_save(flags);
func(info);
local_irq_restore(flags);
If you are calling a remote CPU, then you end up in the APIC code to send
an IPI. On the receiving side, I could not find the local_irq_save() call, but
I verified that upon entry, __perf_event_enable() has interrupts disabled.
And that's either because I missed the interrupt masking call OR because
the HW does it automatically for us. I could not yet figure this out.
In any case, looks like both the start() and stop() routine are protected
from interrupts and thus preemption, so we may not need a lock to
protect n_active.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majord...@vger.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
