Re: [PATCH v2 3/4] perf/x86/intel: implement LBR callstacks context synchronization

[Alexey Budankov <alexey.budankov@linux.intel.com>]

On 16.10.2019 15:20, Liang, Kan wrote:
> 
> 
> On 10/16/2019 5:50 AM, Alexey Budankov wrote:
>>
>> Implement intel_pmu_lbr_sync_task_ctx() method that updates counter
>> of the events that requested LBR callstack data on a sample.
>>
>> The counter can be zero for the case when task context belongs to
>> a thread that has just come from a block on a futex and the context
>> contains saved (lbr_stack_state == LBR_VALID) LBR register values.
>>
>> For the values to be restored at LBR registers on the next thread's
>> switch-in event it copies the counter value that is expected to be
>> non zero from the previous equivalent task perf event context.
>>
>> Signed-off-by: Alexey Budankov <alexey.budankov@linux.intel.com>
>> ---
>>   arch/x86/events/intel/lbr.c  | 9 +++++++++
>>   arch/x86/events/perf_event.h | 3 +++
>>   2 files changed, 12 insertions(+)
>>
>> diff --git a/arch/x86/events/intel/lbr.c b/arch/x86/events/intel/lbr.c
>> index ea54634eabf3..152a3f8b516a 100644
>> --- a/arch/x86/events/intel/lbr.c
>> +++ b/arch/x86/events/intel/lbr.c
>> @@ -417,6 +417,15 @@ static void __intel_pmu_lbr_save(struct x86_perf_task_context *task_ctx)
>>       cpuc->last_log_id = ++task_ctx->log_id;
>>   }
>>   +void intel_pmu_lbr_sync_task_ctx(struct x86_perf_task_context *one,
>> +                 struct x86_perf_task_context *another)
>> +{
>> +    if (!one || !another)
>> +        return;
>> +
>> +    one->lbr_callstack_users = another->lbr_callstack_users;
> 
> We may want to swap here?

Well, in this particular case lbr_callstack_users has to stay consistent 
with the amount of events in task perf event context that requested 
LBR callstack.

~Alexey

> 
> Thanks,
> Kan
> 
>> +}
>> +
>>   void intel_pmu_lbr_sched_task(struct perf_event_context *ctx, bool sched_in)
>>   {
>>       struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>> diff --git a/arch/x86/events/perf_event.h b/arch/x86/events/perf_event.h
>> index a25e6d7eb87b..3e0087c06fc9 100644
>> --- a/arch/x86/events/perf_event.h
>> +++ b/arch/x86/events/perf_event.h
>> @@ -1024,6 +1024,9 @@ void intel_pmu_store_pebs_lbrs(struct pebs_lbr *lbr);
>>     void intel_ds_init(void);
>>   +void intel_pmu_lbr_sync_task_ctx(struct x86_perf_task_context *one,
>> +                 struct x86_perf_task_context *another);
>> +
>>   void intel_pmu_lbr_sched_task(struct perf_event_context *ctx, bool sched_in);
>>     u64 lbr_from_signext_quirk_wr(u64 val);
>>
>