linux-kernel - Re: [PATCHv2 2/2] perf/core: Allow reading package events from perf_event_read

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Date:   Tue, 3 Oct 2023 13:00:18 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Tero Kristo <tero.kristo@...ux.intel.com>
Cc:     x86@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com,
        tglx@...utronix.de, hpa@...or.com, irogers@...gle.com,
        jolsa@...nel.org, namhyung@...nel.org, adrian.hunter@...el.com,
        acme@...nel.org, mingo@...hat.com, bpf@...r.kernel.org,
        linux-kernel@...r.kernel.org, alexander.shishkin@...ux.intel.com,
        linux-perf-users@...r.kernel.org, mark.rutland@....com
Subject: Re: [PATCHv2 2/2] perf/core: Allow reading package events from
 perf_event_read_local

On Wed, Sep 13, 2023 at 03:59:56PM +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>
> ---
> v2:
>   * prevent illegal array access in case event->oncpu == -1
>   * split the event->cpu / event->oncpu handling to their own variables
> 
>  kernel/events/core.c | 18 +++++++++++++++---
>  1 file changed, 15 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/events/core.c b/kernel/events/core.c
> index 4c72a41f11af..6b343bac0a71 100644
> --- a/kernel/events/core.c
> +++ b/kernel/events/core.c
> @@ -4425,6 +4425,9 @@ static int __perf_event_read_cpu(struct perf_event *event, int event_cpu)
>  {
>  	u16 local_pkg, event_pkg;
>  
> +	if (event_cpu < 0 || event_cpu >= nr_cpu_ids)
> +		return event_cpu;

	if ((unsigned)event_cpu >= nr_cpu_ids)
		return event_cpu;

As you could also find at the current __perf_event_read_cpu() callsite.

> +
>  	if (event->group_caps & PERF_EV_CAP_READ_ACTIVE_PKG) {
>  		int local_cpu = smp_processor_id();
>  
> @@ -4528,6 +4531,8 @@ int perf_event_read_local(struct perf_event *event, u64 *value,
>  {
>  	unsigned long flags;
>  	int ret = 0;
> +	int event_cpu;
> +	int event_oncpu;

You wrecked the x-mas tree :-)

I'll fix both up.

Thanks!