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

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Message-ID: <8d6ac059-fc8f-4a5d-b49e-d02777c01cfb@arm.com>
Date: Tue, 26 Aug 2025 17:35:15 +0100
From: Robin Murphy <robin.murphy@....com>
To: Mark Rutland <mark.rutland@....com>, Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...hat.com, will@...nel.org, acme@...nel.org, namhyung@...nel.org,
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Subject: Re: [PATCH 16/19] perf: Introduce positive capability for sampling

On 2025-08-26 2:28 pm, Mark Rutland wrote:
> 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.

Yup, the main reason is the set of drivers getting the new cap is 
smaller than the set of drivers currently not rejecting sampling events, 
so I wanted it to be clearly visible in the patch. Indeed I shall 
clarify the relationship to NO_INTERRUPT in the commit message.

Thanks,
Robin.