linux-kernel - Re: [PATCH 16/19] perf: Introduce positive capability for sampling

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Message-ID: <aK22izKE4r6wI_D9@J2N7QTR9R3>
Date: Tue, 26 Aug 2025 14:28:43 +0100
From: Mark Rutland <mark.rutland@....com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Robin Murphy <robin.murphy@....com>, mingo@...hat.com, will@...nel.org,
	acme@...nel.org, namhyung@...nel.org,
	alexander.shishkin@...ux.intel.com, jolsa@...nel.org,
	irogers@...gle.com, adrian.hunter@...el.com,
	kan.liang@...ux.intel.com, linux-perf-users@...r.kernel.org,
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Subject: Re: [PATCH 16/19] perf: Introduce positive capability for sampling

On Tue, Aug 26, 2025 at 03:08:06PM +0200, Peter Zijlstra wrote:
> On Wed, Aug 13, 2025 at 06:01:08PM +0100, Robin Murphy wrote:
> > Sampling is inherently a feature for CPU PMUs, given that the thing
> > to be sampled is a CPU context. These days, we have many more
> > uncore/system PMUs than CPU PMUs, so it no longer makes much sense to
> > assume sampling support by default and force the ever-growing majority
> > of drivers to opt out of it (or erroneously fail to). Instead, let's
> > introduce a positive opt-in capability that's more obvious and easier to
> > maintain.
> 
> > diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> > index 4d439c24c901..bf2cfbeabba2 100644
> > --- a/include/linux/perf_event.h
> > +++ b/include/linux/perf_event.h
> > @@ -294,7 +294,7 @@ struct perf_event_pmu_context;
> >  /**
> >   * pmu::capabilities flags
> >   */
> > -#define PERF_PMU_CAP_NO_INTERRUPT	0x0001
> > +#define PERF_PMU_CAP_SAMPLING		0x0001
> >  #define PERF_PMU_CAP_NO_NMI		0x0002
> >  #define PERF_PMU_CAP_AUX_NO_SG		0x0004
> >  #define PERF_PMU_CAP_EXTENDED_REGS	0x0008
> > @@ -305,6 +305,7 @@ struct perf_event_pmu_context;
> >  #define PERF_PMU_CAP_EXTENDED_HW_TYPE	0x0100
> >  #define PERF_PMU_CAP_AUX_PAUSE		0x0200
> >  #define PERF_PMU_CAP_AUX_PREFER_LARGE	0x0400
> > +#define PERF_PMU_CAP_NO_INTERRUPT	0x0800
> 
> So NO_INTERRUPT was supposed to be the negative of your new SAMPLING
> (and I agree with your reasoning).
> 
> What I'm confused/curious about is why we retain NO_INTERRUPT?

I see from your other reply that you spotted the next patch does that.

For the sake of other reviewers or anyone digging through the git
history it's probably worth adding a line to this commit message to say:

| A subsequent patch will remove PERF_PMU_CAP_NO_INTERRUPT as this
| requires some additional cleanup.

Mark.