Re: [RFC v3 11/11] drm/i915: Gate engine stats collection with a static key

[Chris Wilson <chris@chris-wilson.co.uk>]

Quoting Tvrtko Ursulin (2017-09-13 13:18:19)
> From: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> 
> This reduces the cost of the software engine busyness tracking
> to a single no-op instruction when there are no listeners.
> 
> v2: Rebase and some comments.
> v3: Rebase.
> 
> Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
> ---
>  drivers/gpu/drm/i915/i915_pmu.c         | 54 +++++++++++++++++++++++--
>  drivers/gpu/drm/i915/intel_engine_cs.c  | 17 ++++++++
>  drivers/gpu/drm/i915/intel_ringbuffer.h | 70 ++++++++++++++++++++++-----------
>  3 files changed, 113 insertions(+), 28 deletions(-)
> 
> diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
> index 3c0c5d0549b3..d734879e67ee 100644
> --- a/drivers/gpu/drm/i915/i915_pmu.c
> +++ b/drivers/gpu/drm/i915/i915_pmu.c
> @@ -460,11 +460,17 @@ static void i915_pmu_enable(struct perf_event *event)
>                 GEM_BUG_ON(sample >= I915_PMU_SAMPLE_BITS);
>                 GEM_BUG_ON(engine->pmu.enable_count[sample] == ~0);
>                 if (engine->pmu.enable_count[sample]++ == 0) {
> +                       /*
> +                        * Enable engine busy stats tracking if needed or
> +                        * alternatively cancel the scheduled disabling of the
> +                        * same.
> +                        */
>                         if (engine_needs_busy_stats(engine) &&
>                             !engine->pmu.busy_stats) {
> -                               engine->pmu.busy_stats =
> -                                       intel_enable_engine_stats(engine) == 0;
> -                               WARN_ON_ONCE(!engine->pmu.busy_stats);
> +                               engine->pmu.busy_stats = true;
> +                               if (!cancel_delayed_work(&engine->pmu.disable_busy_stats))
> +                                       queue_work(i915->wq,
> +                                               &engine->pmu.enable_busy_stats);

The only users of i915->wq are supposed to be dependent on struct_mutex,
it is limited to a single job under the presumption that each worker
would be serialised by that mutex.
-Chris
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