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 17:54:59 +0300
From:   Tero Kristo <tero.kristo@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
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 12/09/2023 17:04, Peter Zijlstra wrote:
> 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 ?

Good question. It looks like I need to add a check against that. Will 
update and send v2 out.

-Tero


>
>> +
>>   	/* 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
>>