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SubjectRe: [PATCH 2/4] perf/x86/intel: fix event update for auto-reload
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On 12/19/2017 1:58 PM, Peter Zijlstra wrote:
> On Mon, Dec 18, 2017 at 03:34:49AM -0800, kan.liang@linux.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


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