Messages in this thread | | | Date | Mon, 22 Aug 2022 18:44:22 +0200 | From | Peter Zijlstra <> | Subject | Re: [RFC v2] perf: Rewrite core context handling |
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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.
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?
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