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

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Date:   Thu, 11 Jan 2018 10:21:25 -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 6:10 AM, Jiri Olsa wrote:
> On Wed, Jan 10, 2018 at 09:31:56AM -0500, Liang, Kan wrote:
>>
>>
>> On 1/10/2018 5:39 AM, Jiri Olsa wrote:
>>> On Mon, Jan 08, 2018 at 07:15:15AM -0800, kan.liang@...el.com wrote:
>>>> From: Kan Liang <kan.liang@...ux.intel.com>
>>>>
>>>> When the PEBS interrupt threshold is larger than one, there is no way to
>>>> get exact auto-reload times and value needed for event update unless
>>>> flush the PEBS buffer.
>>>>
>>>> Drain the PEBS buffer in event read when large PEBS is enabled.
>>>>
>>>> For the threshold is one, even auto-reload is enabled, it doesn't need
>>>> to be specially handled. Because auto-reload is only effect when event
>>>> overflow. There is no overflow in event read.
>>>>
>>>> Signed-off-by: Kan Liang <kan.liang@...ux.intel.com>
>>>> ---
>>>>    arch/x86/events/intel/core.c |  9 +++++++++
>>>>    arch/x86/events/intel/ds.c   | 10 ++++++++++
>>>>    arch/x86/events/perf_event.h |  2 ++
>>>>    3 files changed, 21 insertions(+)
>>>>
>>>> diff --git a/arch/x86/events/intel/core.c b/arch/x86/events/intel/core.c
>>>> index 09c26a4..bdc35f8 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 (event->attr.precise_ip)
>>>> +		return intel_pmu_pebs_read(event);
>>>
>>> check for (event->hw.flags & PERF_X86_EVENT_AUTO_RELOAD)
>>> would be more accurate?
>>>
>>
>> It will narrow down the events.
>> But considering the readability, I think it would be better to use
>> precise_ip.The exposed functions in ds.c should be generic functions for all
>> PEBS events, not specific case.
>> I think _AUTO_RELOAD looks too specific.
> 
> 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.
Also, it has to expose two inline PEBS specific functions. I think 
exposing one PEBS generic function would be better here.

Thanks,
Kan
> bool pebs_drain_before_read(struct perf_event *event)
> {
> 	return event->hw.flags & PERF_X86_EVENT_AUTO_RELOAD;
> }
> 
> thanks,
> jirka
>