Re: [Intel-gfx] [PATCH 4/6] drm/i915/pmu: Add reference counting to the sampling timer

[Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>]

On 13/05/2023 00:44, Umesh Nerlige Ramappa wrote:
> On Fri, May 12, 2023 at 04:20:19PM -0700, Dixit, Ashutosh wrote:
>> On Fri, 12 May 2023 15:44:00 -0700, Umesh Nerlige Ramappa wrote:
>>>
>>> On Fri, May 12, 2023 at 03:29:03PM -0700, Dixit, Ashutosh wrote:
>>> > On Fri, 05 May 2023 17:58:14 -0700, Umesh Nerlige Ramappa wrote:
>>> >>
>>> >
>>> > Hi Umesh/Tvrtko,
>>> >
>>> >> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>> >>
>>> >> We do not want to have timers per tile and waste CPU cycles and 
>>> energy via
>>> >> multiple wake-up sources, for a relatively un-important task of PMU
>>> >> sampling, so keeping a single timer works well. But we also do not 
>>> want
>>> >> the first GT which goes idle to turn off the timer.
>>> >>
>>> >> Add some reference counting, via a mask of unparked GTs, to solve 
>>> this.
>>> >>
>>> >> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
>>> >> Signed-off-by: Umesh Nerlige Ramappa 
>>> <umesh.nerlige.ramappa@intel.com>
>>> >> ---
>>> >>  drivers/gpu/drm/i915/i915_pmu.c | 12 ++++++++++--
>>> >>  drivers/gpu/drm/i915/i915_pmu.h |  4 ++++
>>> >>  2 files changed, 14 insertions(+), 2 deletions(-)
>>> >>
>>> >> diff --git a/drivers/gpu/drm/i915/i915_pmu.c 
>>> b/drivers/gpu/drm/i915/i915_pmu.c
>>> >> index 2b63ee31e1b3..669a42e44082 100644
>>> >> --- a/drivers/gpu/drm/i915/i915_pmu.c
>>> >> +++ b/drivers/gpu/drm/i915/i915_pmu.c
>>> >> @@ -251,7 +251,9 @@ void i915_pmu_gt_parked(struct intel_gt *gt)
>>> >>     * Signal sampling timer to stop if only engine events are 
>>> enabled and
>>> >>     * GPU went idle.
>>> >>     */
>>> >> -    pmu->timer_enabled = pmu_needs_timer(pmu, false);
>>> >> +    pmu->unparked &= ~BIT(gt->info.id);
>>> >> +    if (pmu->unparked == 0)
>>> >> +        pmu->timer_enabled = pmu_needs_timer(pmu, false);
>>> >>
>>> >>    spin_unlock_irq(&pmu->lock);
>>> >>  }
>>> >> @@ -268,7 +270,10 @@ void i915_pmu_gt_unparked(struct intel_gt *gt)
>>> >>    /*
>>> >>     * Re-enable sampling timer when GPU goes active.
>>> >>     */
>>> >> -    __i915_pmu_maybe_start_timer(pmu);
>>> >> +    if (pmu->unparked == 0)
>>> >> +        __i915_pmu_maybe_start_timer(pmu);
>>> >> +
>>> >> +    pmu->unparked |= BIT(gt->info.id);
>>> >>
>>> >>    spin_unlock_irq(&pmu->lock);
>>> >>  }
>>> >> @@ -438,6 +443,9 @@ static enum hrtimer_restart i915_sample(struct 
>>> hrtimer *hrtimer)
>>> >>     */
>>> >>
>>> >>    for_each_gt(gt, i915, i) {
>>> >> +        if (!(pmu->unparked & BIT(i)))
>>> >> +            continue;
>>> >> +
>>> >
>>> > This is not correct. In this series we are at least sampling 
>>> frequencies
>>> > (calling frequency_sample) even when GT is parked. So these 3 lines 
>>> should be
>>> > deleted. engines_sample will get called and will return without doing
>>> > anything if engine events are disabled.
>>>
>>> Not sure I understand. This is checking pmu->'un'parked bits.
>>
>> Sorry, my bad. Not "engines_sample will get called and will return 
>> without
>> doing anything if engine events are disabled" but "engines_sample will 
>> get
>> called and will return without doing anything if GT is not awake". 
>> This is
>> the same as the previous behavior before this series.
>>
>> Umesh and I discussed this but writing this out in case Tvrtko takes a
>> look.
> 
> Sounds good, Dropping the check here in the new revision.

I think it is safe to not have the check, but I didn't quite understand 
the "this is not correct" part. I can only see the argument that it 
could be redundant, not that it is incorrect.

In which case I think it should better stay since it is way more 
efficient, given this gets called at 200Hz, than the *atomic* 
atomic_inc_not_zero (from intel_wakeref_get_if_active).

Regards,

Tvrtko