Re: [RFC v2] perf: Rewrite core context handling

[Ravi Bangoria]

On 22-Aug-22 10:14 PM, Peter Zijlstra wrote:
> On Tue, Aug 02, 2022 at 11:40:34AM +0530, Ravi Bangoria wrote:
>> On 13-Jun-22 8:25 PM, Peter Zijlstra wrote:
>>> On Mon, Jun 13, 2022 at 04:35:11PM +0200, Peter Zijlstra wrote:
>>>> @@ -12125,6 +12232,8 @@ SYSCALL_DEFINE5(perf_event_open,
>>>>  		goto err_task;
>>>>  	}
>>>>  
>>>> +	// XXX premature; what if this is allowed, but we get moved to a PMU
>>>> +	// that doesn't have this.
>>>>  	if (is_sampling_event(event)) {
>>>>  		if (event->pmu->capabilities & PERF_PMU_CAP_NO_INTERRUPT) {
>>>>  			err = -EOPNOTSUPP;
>>>
>>> No; this really should be against the event's native PMU. If the event
>>> can't natively sample, it can't sample when placed in another group
>>> either.
>>
>> Right. But IIUC, the question was, would there be any issue if we allow
>> grouping of perf_sw_context sampling event as group leader and
>> perf_{hw|invalid}_context counting event as group member. I think no. It
>> should just work fine. And, there could be real usecases of it as you
>> described in one old thread[1].
> 
> Like you I need to bend my brain around this again, but I'm not seeing a
> contradiction. The use-case from [1] is a software sampler with a bunch
> of non-sampling uncore events.
> 
> The uncore events aren't sampling, the are simply read by the software
> event (SAMPLE_READ). And moving the sampling software event to the
> non-sample capable uncore PMU shouldn't matter.

Ok.

> That is; the code as it stands here seems right, we should check
> is_sampling_event() against an event's native pmu->capabilities.
> 
> Or am I misunderstanding things?

No, that's correct. We must use event's native pmu to check capabilities.
I'll remove this comment from code.

Thanks,
Ravi