linux-kernel - Re: [PATCH v6 1/8] perf/core: Allow multiple AUX PMU events with the same module

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Message-ID: <70a58dca-01dc-408b-a2f7-a854718a405b@intel.com>
Date: Tue, 3 Sep 2024 13:06:55 +0300
From: Adrian Hunter <adrian.hunter@...el.com>
To: Leo Yan <leo.yan@....com>, Peter Zijlstra <peterz@...radead.org>,
 Arnaldo Carvalho de Melo <acme@...nel.org>,
 Namhyung Kim <namhyung@...nel.org>, Mark Rutland <mark.rutland@....com>,
 Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
 "Liang, Kan" <kan.liang@...ux.intel.com>,
 Suzuki K Poulose <suzuki.poulose@....com>, Mike Leach
 <mike.leach@...aro.org>, James Clark <james.clark@...aro.org>,
 John Garry <john.g.garry@...cle.com>, Will Deacon <will@...nel.org>,
 Yicong Yang <yangyicong@...ilicon.com>,
 Jonathan Cameron <jonathan.cameron@...wei.com>,
 linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org,
 linux-arm-kernel@...ts.infradead.org, coresight@...ts.linaro.org
Subject: Re: [PATCH v6 1/8] perf/core: Allow multiple AUX PMU events with the
 same module

On 23/08/24 14:32, Leo Yan wrote:
> This commit changes the condition from checking the same PMU instance to
> checking the same .setup_aux() callback pointer. If PMU events have the
> same callback pointer, it means they share the same PMU driver module.
> This allows support for multiple PMU events with the same driver module.
> 
> As a result, more than one AUX event (e.g. arm_spe_0 and arm_spe_1)
> can record trace into the AUX ring buffer.
> 
> Signed-off-by: Leo Yan <leo.yan@....com>
> ---
>  kernel/events/core.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index c973e3c11e03..883c457911a3 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -12345,9 +12345,16 @@ perf_event_set_output(struct perf_event *event, struct perf_event *output_event)
>  
>  	/*
>  	 * If both events generate aux data, they must be on the same PMU
> +	 * module but can be with different PMU instances.
> +	 *
> +	 * For a built-in PMU module, the 'pmu->module' pointer is NULL,
> +	 * thus it is not feasible to compare the module pointers when
> +	 * AUX PMU drivers are built into the kernel image. Instead,
> +	 * comparing the .setup_aux() callback pointer can determine if
> +	 * the two PMU events come from the same PMU driver.
>  	 */
>  	if (has_aux(event) && has_aux(output_event) &&
> -	    event->pmu != output_event->pmu)
> +	    event->pmu->setup_aux != output_event->pmu->setup_aux)

It is not very flexible and risks someone adding aux PMUs that
do not want that rule but accidentally support it.  Another
option is to add a PMU callback, but really you need to Peter's
feedback.