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>>>> } >>>> >>>> + // 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