linux-kernel - Re: [RESEND PATCH 2/2] perf/core: Allow reading package events from perf_event_read

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Date:   Tue, 12 Sep 2023 16:04:34 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tero Kristo <tero.kristo@...ux.intel.com>
Cc:     x86@...nel.org, tglx@...utronix.de, bp@...en8.de,
        dave.hansen@...ux.intel.com, irogers@...gle.com,
        mark.rutland@....com, linux-perf-users@...r.kernel.org,
        hpa@...or.com, mingo@...hat.com, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, acme@...nel.org,
        alexander.shishkin@...ux.intel.com, adrian.hunter@...el.com,
        namhyung@...nel.org, jolsa@...nel.org
Subject: Re: [RESEND PATCH 2/2] perf/core: Allow reading package events from
 perf_event_read_local

On Tue, Sep 12, 2023 at 03:44:32PM +0300, Tero Kristo wrote:
> Per-package perf events are typically registered with a single CPU only,
> however they can be read across all the CPUs within the package.
> Currently perf_event_read maps the event CPU according to the topology
> information to avoid an unnecessary SMP call, however
> perf_event_read_local deals with hard values and rejects a read with a
> failure if the CPU is not the one exactly registered. Allow similar
> mapping within the perf_event_read_local if the perf event in question
> can support this.
> 
> This allows users like BPF code to read the package perf events properly
> across different CPUs within a package.
> 
> Signed-off-by: Tero Kristo <tero.kristo@...ux.intel.com>
> ---
>  kernel/events/core.c | 10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 4c72a41f11af..780dde646e8a 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4528,6 +4528,7 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
>  {
>  	unsigned long flags;
>  	int ret = 0;
> +	int event_cpu;
>  
>  	/*
>  	 * Disabling interrupts avoids all counter scheduling (context
> @@ -4551,15 +4552,18 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
>  		goto out;
>  	}
>  
> +	event_cpu = READ_ONCE(event->oncpu);
> +	event_cpu = __perf_event_read_cpu(event, event_cpu);

What happens with __perf_event_read_cpu() when event_cpu == -1 ?

> +
>  	/* If this is a per-CPU event, it must be for this CPU */
>  	if (!(event->attach_state & PERF_ATTACH_TASK) &&
> -	    event->cpu != smp_processor_id()) {
> +	    event_cpu != smp_processor_id()) {
>  		ret = -EINVAL;
>  		goto out;
>  	}
>  
>  	/* If this is a pinned event it must be running on this CPU */
> -	if (event->attr.pinned && event->oncpu != smp_processor_id()) {
> +	if (event->attr.pinned && event_cpu != smp_processor_id()) {
>  		ret = -EBUSY;
>  		goto out;
>  	}
> @@ -4569,7 +4573,7 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
>  	 * or local to this CPU. Furthermore it means its ACTIVE (otherwise
>  	 * oncpu == -1).
>  	 */
> -	if (event->oncpu == smp_processor_id())
> +	if (event_cpu == smp_processor_id())
>  		event->pmu->read(event);
>  
>  	*value = local64_read(&event->count);
> -- 
> 2.40.1
>