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 <[email protected]>
>> ---
>> 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
>>