linux-kernel - Re: [RESEND PATCH V2 3/4] perf/x86/intel: drain PEBS buffer in event read

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Message-ID: <662a138a-ba53-246f-9b6f-60c7dcbb3f5c@linux.intel.com>
Date:   Tue, 16 Jan 2018 13:49:13 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Jiri Olsa <jolsa@...hat.com>
Cc:     peterz@...radead.org, mingo@...hat.com, acme@...nel.org,
        linux-kernel@...r.kernel.org, tglx@...utronix.de,
        eranian@...gle.com, ak@...ux.intel.com
Subject: Re: [RESEND PATCH V2 3/4] perf/x86/intel: drain PEBS buffer in event
 read



On 1/11/2018 10:45 AM, Jiri Olsa wrote:
> On Thu, Jan 11, 2018 at 10:21:25AM -0500, Liang, Kan wrote:
> 
> SNIP
> 
>>>
>>> hum, but the PEBS drain is specific just for
>>> PERF_X86_EVENT_AUTO_RELOAD events, right?
>>
>> Accurately, PEBS drain is specific for PERF_X86_EVENT_FREERUNNING here.
>> PERF_X86_EVENT_FREERUNNING event must be _AUTO_RELOAD event.
>> But in some cases, _AUTO_RELOAD event cannot be _FREERUNNING event.
>>
>> Only the event which is both _FREERUNNING and _AUTO_RELOAD need to do PEBS
>> drain in _read().
>>
>> So it does the check in intel_pmu_pebs_read()
>> +	if (pebs_needs_sched_cb(cpuc))
>> +		return intel_pmu_drain_pebs_buffer();
>>
>>>
>>> wrt readability maybe you could add function like:
>>
>> The existing function pebs_needs_sched_cb() can do the check.
>> We just need to expose it, and also the intel_pmu_drain_pebs_buffer().
>>
>> But to be honest, I still cannot see a reason for that.
>> It could save a call to intel_pmu_pebs_read(), but _read() is not critical
>> path. It doesn't save much.
> 
> hum, pmu->read is also called for PERF_SAMPLE_READ for sample,
> check perf_output_read
> 
> for non sampling event you shouldn't be able to create PEBS
> event, there's check in x86_pmu_hw_config
> 
> I agree it does not save much, it just confused me while
> I was reading the code, like why is this needed for all
> events with precise_ip
>


Sorry for the late response.

How about the patch as below?
The patch will be split into two patches in V3. One is to introduce 
intel_pmu_large_pebs_read, the other is to introduce intel_pmu_read_event.

Thanks,
Kan

diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
index 731153a..1610a9d 100644
--- a/arch/x86/events/intel/core.c
+++ b/arch/x86/events/intel/core.c
@@ -2060,6 +2060,14 @@ static void intel_pmu_del_event(struct perf_event 
*event)
  		intel_pmu_pebs_del(event);
  }

+static void intel_pmu_read_event(struct perf_event *event)
+{
+	if (intel_pmu_large_pebs_read(event))
+		return;
+
+	x86_perf_event_update(event);
+}
+
  static void intel_pmu_enable_fixed(struct hw_perf_event *hwc)
  {
  	int idx = hwc->idx - INTEL_PMC_IDX_FIXED;
@@ -3495,6 +3503,7 @@ static __initconst const struct x86_pmu intel_pmu = {
  	.disable		= intel_pmu_disable_event,
  	.add			= intel_pmu_add_event,
  	.del			= intel_pmu_del_event,
+	.read			= intel_pmu_read_event,
  	.hw_config		= intel_pmu_hw_config,
  	.schedule_events	= x86_schedule_events,
  	.eventsel		= MSR_ARCH_PERFMON_EVENTSEL0,
diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
index a1edd91..3f11ede 100644
--- a/arch/x86/events/intel/ds.c
+++ b/arch/x86/events/intel/ds.c
@@ -978,6 +978,19 @@ void intel_pmu_pebs_del(struct perf_event *event)
  	pebs_update_state(needed_cb, cpuc, event->ctx->pmu);
  }

+int intel_pmu_large_pebs_read(struct perf_event *event)
+{
+	struct cpu_hw_events *cpuc = this_cpu_ptr(&cpu_hw_events);
+
+	/* Check if the event has large pebs */
+	if (!pebs_needs_sched_cb(cpuc))
+		return 0
+
+	intel_pmu_drain_pebs_buffer();
+
+	return 1;
+}
+
  void intel_pmu_pebs_disable(struct perf_event *event)
  {
  	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 805400b..7d3cd32 100644
--- a/arch/x86/events/perf_event.h
+++ b/arch/x86/events/perf_event.h
@@ -923,6 +923,8 @@ void intel_pmu_pebs_disable_all(void);

  void intel_pmu_pebs_sched_task(struct perf_event_context *ctx, bool 
sched_in);

+int intel_pmu_large_pebs_read(struct perf_event *event);
+
  void intel_ds_init(void);

  void intel_pmu_lbr_sched_task(struct perf_event_context *ctx, bool 
sched_in);