linux-kernel - Re: [PATCH 2/4] perf/x86/intel: fix event update for auto-reload

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Date:   Tue, 19 Dec 2017 15:08:58 -0500
From:   "Liang, Kan" <kan.liang@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...hat.com, acme@...nel.org, linux-kernel@...r.kernel.org,
        tglx@...utronix.de, jolsa@...hat.com, eranian@...gle.com,
        ak@...ux.intel.com
Subject: Re: [PATCH 2/4] perf/x86/intel: fix event update for auto-reload



On 12/19/2017 1:58 PM, Peter Zijlstra wrote:
> On Mon, Dec 18, 2017 at 03:34:49AM -0800, kan.liang@...ux.intel.com wrote:
>>   arch/x86/events/core.c     | 14 ++++++++++++++
>>   arch/x86/events/intel/ds.c |  8 +++++++-
>>   2 files changed, 21 insertions(+), 1 deletion(-)
>>
>> diff --git a/arch/x86/events/core.c b/arch/x86/events/core.c
>> index 35552ea..f74e21d 100644
>> --- a/arch/x86/events/core.c
>> +++ b/arch/x86/events/core.c
>> @@ -100,6 +100,20 @@ u64 x86_perf_event_update(struct perf_event *event,
>>   	 * of the count.
>>   	 */
>>   	delta = (new_raw_count << shift) - (prev_raw_count << shift);
>> +
>> +	/*
>> +	 * Take auto-reload into account
>> +	 * For the auto-reload before the last time, it went through the
>> +	 * whole period (reload_val) every time.
>> +	 * Just simply add period * times to the event.
>> +	 *
>> +	 * For the last load, the elapsed delta (event-)time need to be
>> +	 * corrected by adding the period. Because the start point is -period.
>> +	 */
>> +	if (reload_times > 0) {
>> +		delta += (reload_val << shift);
>> +		local64_add(reload_val * (reload_times - 1), &event->count);
>> +	}
>>   	delta >>= shift;
>>   
>>   	local64_add(delta, &event->count);
>> diff --git a/arch/x86/events/intel/ds.c b/arch/x86/events/intel/ds.c
>> index 0b693b7..f0f6026 100644
>> --- a/arch/x86/events/intel/ds.c
>> +++ b/arch/x86/events/intel/ds.c
>> @@ -1256,11 +1256,17 @@ static void __intel_pmu_pebs_event(struct perf_event *event,
>>   				   void *base, void *top,
>>   				   int bit, int count)
>>   {
>> +	struct hw_perf_event *hwc = &event->hw;
>>   	struct perf_sample_data data;
>>   	struct pt_regs regs;
>>   	void *at = get_next_pebs_record_by_bit(base, top, bit);
>>   
>> -	if (!intel_pmu_save_and_restart(event, 0, 0) &&
>> +	/*
>> +	 * Now, auto-reload is only enabled in fixed period mode.
>> +	 * The reload value is always hwc->sample_period.
>> +	 * May need to change it, if auto-reload is enabled in freq mode later.
>> +	 */
>> +	if (!intel_pmu_save_and_restart(event, hwc->sample_period, count - 1) &&
>>   	    !(event->hw.flags & PERF_X86_EVENT_AUTO_RELOAD))
>>   		return;
>>   
> 
> This all looks very wrong... In auto reload we should never call
> intel_pmu_save_and_restore() in the first place I think.
> 
> Things like x86_perf_event_update() and x86_perf_event_set_period()
> simply _cannot_ do the right thing when we auto reload the counter.
>

I think it should be OK to call it in first place.
For x86_perf_event_update(), the reload_times will tell if it's auto 
reload. Both period_left and event->count are carefully recalculated for 
auto reload.
For x86_perf_event_set_period(), there is nothing special needed for 
auto reload. The period is fixed. The period_left from 
x86_perf_event_update() is already handled.


BTW: It should be 'count' not 'count - 1' which pass to
intel_pmu_save_and_restart(). I just found the issue. I will fix it in
V2 with other improvements if there are any.

Thanks,
Kan