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

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Date:   Tue, 19 Dec 2017 15:24:49 -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 3:08 PM, Liang, Kan wrote:
> 
> 
> 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.

Think a bit more. You are right. We cannot rely on count to tell us if 
it's auto reload.
The count could also be 1 if auto reload is enabled.

I will fix it in V2.

Thanks,
Kan

> 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
> 
>