Re: [PATCH v9 12/14] x86: perf: intel_pt: Intel PT PMU driver

[Alexander Shishkin]

Alexander Shishkin <alexander.shishkin@xxxxxxxxxxxxxxx> writes:

> Peter Zijlstra <peterz@xxxxxxxxxxxxx> writes:
>
>> On Wed, Jan 14, 2015 at 02:18:21PM +0200, Alexander Shishkin wrote:
>>> +static __init int pt_init(void)
>>> +{
>>
>>> +	pt_pmu.pmu.attr_groups	= pt_attr_groups;
>>> +	pt_pmu.pmu.task_ctx_nr	= perf_hw_context;
>>
>> I just noticed this one, how can this ever work? We want the PT thing to
>> always get programmed, right? -- because we disallow creating more than
>> 1?
>>
>> Which reminds me; does that exclusive thing you did not allow you to
>> create one cpu wide and one per task (they're separate contexts) events?
>> At which point we're not schedulable at all.
>>
>> By sticking it on the HW context list it can end up not being programed
>> because its stuck after a bunch of hardware events that don't all fit on
>> the PMU.
>>
>> Would not the SW list be more appropriate; the SW list is a list of
>> events that's guaranteed to be schedulable.
>
> You're right, of course.
>
> As for the exclusive events, how about something like the code below (on
> top of the previous exclusive event patch)? The only remaining issue
> that I see is creating cpu-wide events in the presence of per-thread
> (event->cpu==-1) events. Both would still work, but only one of them
> will actually get scheduled at a time. I'm thinking about adding a
> counter for per-thread events to struct pmu for this purpose, so that if
> any are present, we can disallow creating cpu-wide events. Or, we can
> leave it as it is.
>
> What do you think?
>
> ---
>  kernel/events/core.c | 52 +++++++++++++++++++++++++++++++++++++++++++++++-----
>  1 file changed, 47 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index cf0bf99f53..e8c86530e2 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -7688,14 +7688,11 @@ static bool exclusive_event_match(struct perf_event *e1, struct perf_event *e2)
>  	return false;
>  }
>  
> -static bool exclusive_event_ok(struct perf_event *event,
> -			      struct perf_event_context *ctx)
> +static bool __exclusive_event_ok(struct perf_event *event,
> +				 struct perf_event_context *ctx)
>  {
>  	struct perf_event *iter_event;
>  
> -	if (!(event->pmu->capabilities & PERF_PMU_CAP_EXCLUSIVE))
> -		return true;
> -
>  	list_for_each_entry(iter_event, &ctx->event_list, event_entry) {
>  		if (exclusive_event_match(iter_event, event))
>  			return false;
> @@ -7704,6 +7701,51 @@ static bool exclusive_event_ok(struct perf_event *event,
>  	return true;
>  }
>  
> +static bool __exclusive_event_ok_on_cpu(struct perf_event *event, int cpu)
> +{
> +	struct perf_event_context *cpuctx;
> +	bool ret;
> +
> +	cpuctx = find_get_context(event->pmu, NULL, cpu);
> +	mutex_lock(&cpuctx->mutex);
> +	ret = __exclusive_event_ok(event, cpuctx);
> +	perf_unpin_context(cpuctx);
> +	put_ctx(cpuctx);
> +	mutex_unlock(&cpuctx->mutex);

Actually, find_get_context() is not needed here, the following should be
sufficient:

	cpuctx = &per_cpu_ptr(event->pmu->pmu_cpu_context, cpu)->ctx;

	mutex_lock(&cpuctx->mutex);
	ret = __exclusive_event_ok(event, cpuctx);
	mutex_unlock(&cpuctx->mutex);

Regards,
--
Alex
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