linux-kernel - Re: [PATCH v2 03/20] perf/core: add PERF_PMU_CAP

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Date:   Mon, 26 Nov 2018 14:10:24 +0000
From:   Robin Murphy <robin.murphy@....com>
To:     Andrew Murray <andrew.murray@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Shawn Guo <shawnguo@...nel.org>,
        Sascha Hauer <s.hauer@...gutronix.de>,
        Will Deacon <will.deacon@....com>,
        Mark Rutland <mark.rutland@....com>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Borislav Petkov <bp@...en8.de>, x86@...nel.org,
        Ralf Baechle <ralf@...ux-mips.org>,
        Paul Burton <paul.burton@...s.com>,
        James Hogan <jhogan@...nel.org>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Heiko Carstens <heiko.carstens@...ibm.com>,
        "David S . Miller" <davem@...emloft.net>,
        sparclinux@...r.kernel.org, Michael Ellerman <mpe@...erman.id.au>
Cc:     linux-s390@...r.kernel.org, linux-mips@...ux-mips.org,
        linux-kernel@...r.kernel.org, linux-alpha@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH v2 03/20] perf/core: add PERF_PMU_CAP_EXCLUDE for
 exclusion capable PMUs

Hi Andrew,

On 26/11/2018 11:12, Andrew Murray wrote:
> Many PMU drivers do not have the capability to exclude counting events
> that occur in specific contexts such as idle, kernel, guest, etc. These
> drivers indicate this by returning an error in their event_init upon
> testing the events attribute flags. This approach is error prone and
> often inconsistent.
> 
> Let's instead allow PMU drivers to advertise their ability to exclude
> based on context via a new capability: PERF_PMU_CAP_EXCLUDE. This
> allows the perf core to reject requests for exclusion events where
> there is no support in the PMU.
> 
> Signed-off-by: Andrew Murray <andrew.murray@....com>
> ---
>   include/linux/perf_event.h | 1 +
>   kernel/events/core.c       | 9 +++++++++
>   2 files changed, 10 insertions(+)
> 
> diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
> index b2e806f..69b3d65 100644
> --- a/include/linux/perf_event.h
> +++ b/include/linux/perf_event.h
> @@ -244,6 +244,7 @@ struct perf_event;
>   #define PERF_PMU_CAP_EXCLUSIVE			0x10
>   #define PERF_PMU_CAP_ITRACE			0x20
>   #define PERF_PMU_CAP_HETEROGENEOUS_CPUS		0x40
> +#define PERF_PMU_CAP_EXCLUDE			0x80
>   
>   /**
>    * struct pmu - generic performance monitoring unit
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 5a97f34..9afb33c 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -9743,6 +9743,15 @@ static int perf_try_init_event(struct pmu *pmu, struct perf_event *event)
>   	if (ctx)
>   		perf_event_ctx_unlock(event->group_leader, ctx);
>   
> +	if (!ret) {
> +		if (!(pmu->capabilities & PERF_PMU_CAP_EXCLUDE) &&
> +				event_has_any_exclude_flag(event)) {

Technically this is a bisection-breaker, since no driver has this 
capability yet - ideally, this patch should come after all the ones 
introducing it to the relevant drivers (with the removal of the 
now-redundant code from the other drivers at the end).

Alternatively, since we already have several other negative 
capabilities, unless there's a strong feeling against adding any more 
then it might work out simpler to flip it to PERF_PMU_CAP_NO_EXCLUDE, 
such that we only need to introduce the core check then directly replace 
the open-coded event checks with the capability in the appropriate 
drivers, and need not touch the exclusion-supporting ones at all.

Robin.

> +			if (event->destroy)
> +				event->destroy(event);
> +			ret = -EINVAL;
> +		}
> +	}
> +
>   	if (ret)
>   		module_put(pmu->module);
>   
>