Re: [PATCH V3] perf/x86/intel/ds: Flush the PEBS buffer in PEBS enable

["Liang, Kan" <kan.liang@linux.intel.com>]

On 2023-04-14 6:29 a.m., Peter Zijlstra wrote:
> On Mon, Apr 10, 2023 at 11:13:09AM -0700, kan.liang@linux.intel.com wrote:
> 
>>  arch/x86/events/intel/ds.c | 39 ++++++++++++++++++++++++++------------
>>  1 file changed, 27 insertions(+), 12 deletions(-)
>>
>> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
>> index 3a77f4336df7..4639d4c1e98d 100644
>> --- a/arch/x86/events/intel/ds.c
>> +++ b/arch/x86/events/intel/ds.c
>> @@ -1257,20 +1257,18 @@ pebs_update_state(bool needed_cb, struct cpu_hw_events *cpuc,
>>  	if (x86_pmu.intel_cap.pebs_baseline && add) {
>>  		u64 pebs_data_cfg;
>>  
>> -		/* Clear pebs_data_cfg and pebs_record_size for first PEBS. */
>> -		if (cpuc->n_pebs == 1) {
>> +		/* Clear pebs_data_cfg for first PEBS. */
>> +		if (cpuc->n_pebs == 1)
>>  			cpuc->pebs_data_cfg = 0;
>> -			cpuc->pebs_record_size = sizeof(struct pebs_basic);
>> -		}
>>  
>>  		pebs_data_cfg = pebs_update_adaptive_cfg(event);
>>  
>> -		/* Update pebs_record_size if new event requires more data. */
>> -		if (pebs_data_cfg & ~cpuc->pebs_data_cfg) {
>> +		/*
>> +		 * Only update the pebs_data_cfg here. The pebs_record_size
>> +		 * will be updated later when the new pebs_data_cfg takes effect.
>> +		 */
>> +		if (pebs_data_cfg & ~cpuc->pebs_data_cfg)
>>  			cpuc->pebs_data_cfg |= pebs_data_cfg;
>> -			adaptive_pebs_record_size_update();
>> -			update = true;
>> -		}
>>  	}
>>  
>>  	if (update)
> 		pebs_update_threshold(cpuc);
> 
> Now, pebs_update_threshold() will actually use
> ->pebs_record_size, but afaict the above now has a path through (for
> example for the first event) where update is true but ->pebs_record_size
> is unset/stale.
> 
> I think it all works out, but it is quite a mess and hard to follow.

With this patch, the pebs_update_threshold() will be delayed to
intel_pmu_pebs_enable() for the adaptive PEBS.

I think we may reuse the pebs_data_cfg method for the previous fixed
PEBS as well and delay the DS update to intel_pmu_pebs_enable() as well.
So everything will be consistent.

I will do more tests and probably send a clean up patch later separately.


> 
>> @@ -1331,6 +1329,13 @@ static void intel_pmu_pebs_via_pt_enable(struct perf_event *event)
>>  	wrmsrl(base + idx, value);
>>  }
>>  
>> +static inline void intel_pmu_drain_large_pebs(struct cpu_hw_events *cpuc)
>> +{
>> +	if (cpuc->n_pebs == cpuc->n_large_pebs &&
>> +	    cpuc->n_pebs != cpuc->n_pebs_via_pt)
>> +		intel_pmu_drain_pebs_buffer();
>> +}
> 
> Its been a minute since I looked at this code; but why only for large
> pebs? Surely flushing is quick when the DS is actually empty and that
> stops us having to worry if there's races where there might be a single
> entry in.

The AUTO_RELOAD is a separate feature. It should be always enabled when
a fixed period is set. That's not the case for the large PEBS, which
only supports partial sample type.

There should be some overhead for the AUTO_RELOAD + single PEBS case. We
have to update the event count.

> 
>>  void intel_pmu_pebs_enable(struct perf_event *event)
>>  {
>>  	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
>> @@ -1350,6 +1355,18 @@ void intel_pmu_pebs_enable(struct perf_event *event)
>>  	if (x86_pmu.intel_cap.pebs_baseline) {
>>  		hwc->config |= ICL_EVENTSEL_ADAPTIVE;
>>  		if (cpuc->pebs_data_cfg != cpuc->active_pebs_data_cfg) {
>> +			/*
>> +			 * A system-wide PEBS event with the large PEBS
>> +			 * config may still be enabled when switching the
>> +			 * context. Some PEBS records for the system-wide
>> +			 * PEBS may be generated while the old event has
>> +			 * been scheduled out but the new one hasn't been
>> +			 * scheduled in. It's not enough to only flush the
>> +			 * buffer when a PEBS event is disable.
>> +			 */
> 
> Perhaps just:
> 
> 			/*
> 			 * drain_pebs() assumes uniform record size;
> 			 * hence we need to drain when changing said
> 			 * size.
> 			 */
> 

Sure, I will update in V4.

Thanks,
Kan

> 
>> +			intel_pmu_drain_large_pebs(cpuc);
>> +			adaptive_pebs_record_size_update();
>> +			pebs_update_threshold(cpuc);
>>  			wrmsrl(MSR_PEBS_DATA_CFG, cpuc->pebs_data_cfg);
>>  			cpuc->active_pebs_data_cfg = cpuc->pebs_data_cfg;
>>  		}