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

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Message-ID: <20250826130806.GY4067720@noisy.programming.kicks-ass.net>
Date: Tue, 26 Aug 2025 15:08:06 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Robin Murphy <robin.murphy@....com>
Cc: mingo@...hat.com, will@...nel.org, mark.rutland@....com,
	acme@...nel.org, namhyung@...nel.org,
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Subject: Re: [PATCH 16/19] perf: Introduce positive capability for sampling

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?