[Intel-gfx] [PATCH 2/3] drm/i915/mtl: Add a PMU counter for total active ticks

[Tvrtko Ursulin]

On 22/09/2023 23:25, John.C.Harrison at Intel.com wrote:
> From: Umesh Nerlige Ramappa <umesh.nerlige.ramappa at intel.com>
> 
> Current engine busyness interface exposed by GuC has a few issues:
> 
> - The busyness of active engine is calculated using 2 values provided by
>    GuC and is prone to race between CPU reading those values and GuC
>    updating them. Any sort of HW synchronization would be at the cost of
>    scheduling latencies.
> 
> - GuC provides only 32 bit values for busyness and KMD has to run a
>    worker to extend the values to 64 bit. In addition KMD also needs to
>    extend the GT timestamp to 64 bits so that it can be used to calculate
>    active busyness for an engine.
> 
> To address these issues, GuC provides a new interface to calculate
> engine busyness. GuC accumulates the busyness ticks in a 64 bit value
> and also internally updates the busyness for an active context using a
> periodic timer. This simplifies the KMD implementation such that KMD
> only needs to relay the busyness value to the user.
> 
> In addition to fixing the interface, GuC also provides a periodically
> total active ticks that the GT has been running for. This counter is
> exposed to the user so that the % busyness can be calculated as follows:
> 
> busyness % = (engine active ticks/total active ticks) * 100.

AFAIU I915_PMU_TOTAL_ACTIVE_TICKS only runs when GT is awake, right?

So if GT is awake 10% of the time, and engine is busy that 100% of that 
time, which is 10% of the real/wall time, the busyness by this formula 
comes up as 100%. Which wouldn't be useful for intel_gpu_top and alike.

How to scale it back to wall time? Again AFAIU there is no info about 
tick frequency, so how does one know what a delta in total active ticks 
means?

Going back on the higher level, I am not convinced we need to add a new 
uapi just for MTL. If the tick period is known internally we could just 
use v2 internally and expose the current uapi using it.

Any timebase conversion error is unlikely to be relevant because 
userspace only looks at deltas over relatively short periods (seconds). 
Ie. I don't think that the clock drift error would accumulate so it 
would need to be really huge to be relevant over short sampling periods.

Regards,

Tvrtko

> 
> Implement the new interface and start by adding a new counter for total
> active ticks.
> 
> Signed-off-by: Umesh Nerlige Ramappa <umesh.nerlige.ramappa at intel.com>
> Signed-off-by: John Harrison <John.C.Harrison at Intel.com>
> ---
>   .../gpu/drm/i915/gt/uc/intel_guc_submission.c | 24 +++++++++++++++++++
>   .../gpu/drm/i915/gt/uc/intel_guc_submission.h |  1 +
>   drivers/gpu/drm/i915/i915_pmu.c               |  6 +++++
>   include/uapi/drm/i915_drm.h                   |  2 ++
>   4 files changed, 33 insertions(+)
> 
> diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> index 88465d701c278..0c1fee5360777 100644
> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.c
> @@ -1607,6 +1607,30 @@ static ktime_t busy_v2_guc_engine_busyness(struct intel_engine_cs *engine, ktime
>   	return ns_to_ktime(total);
>   }
>   
> +static u64 busy_v1_intel_guc_total_active_ticks(struct intel_guc *guc)
> +{
> +	return guc->busy.v1.gt_stamp;
> +}
> +
> +static u64 busy_v2_intel_guc_total_active_ticks(struct intel_guc *guc)
> +{
> +	u64 ticks_gt;
> +
> +	__busy_v2_get_engine_usage_record(guc, NULL, NULL, NULL, &ticks_gt);
> +
> +	return ticks_gt;
> +}
> +
> +u64 intel_guc_total_active_ticks(struct intel_gt *gt)
> +{
> +	struct intel_guc *guc = &gt->uc.guc;
> +
> +	if (GUC_SUBMIT_VER(guc) < MAKE_GUC_VER(1, 3, 1))
> +		return busy_v1_intel_guc_total_active_ticks(guc);
> +	else
> +		return busy_v2_intel_guc_total_active_ticks(guc);
> +}
> +
>   static int busy_v2_guc_action_enable_usage_stats_device(struct intel_guc *guc)
>   {
>   	u32 offset = guc_engine_usage_offset_v2_device(guc);
> diff --git a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.h b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.h
> index c57b29cdb1a64..f6d42838825f2 100644
> --- a/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.h
> +++ b/drivers/gpu/drm/i915/gt/uc/intel_guc_submission.h
> @@ -30,6 +30,7 @@ void intel_guc_dump_active_requests(struct intel_engine_cs *engine,
>   				    struct drm_printer *m);
>   void intel_guc_busyness_park(struct intel_gt *gt);
>   void intel_guc_busyness_unpark(struct intel_gt *gt);
> +u64 intel_guc_total_active_ticks(struct intel_gt *gt);
>   
>   bool intel_guc_virtual_engine_has_heartbeat(const struct intel_engine_cs *ve);
>   
> diff --git a/drivers/gpu/drm/i915/i915_pmu.c b/drivers/gpu/drm/i915/i915_pmu.c
> index d35973b411863..4f52636eb4a80 100644
> --- a/drivers/gpu/drm/i915/i915_pmu.c
> +++ b/drivers/gpu/drm/i915/i915_pmu.c
> @@ -563,6 +563,8 @@ config_status(struct drm_i915_private *i915, u64 config)
>   		break;
>   	case I915_PMU_SOFTWARE_GT_AWAKE_TIME:
>   		break;
> +	case I915_PMU_TOTAL_ACTIVE_TICKS:
> +		break;
>   	default:
>   		return -ENOENT;
>   	}
> @@ -678,6 +680,9 @@ static u64 __i915_pmu_event_read(struct perf_event *event)
>   		case I915_PMU_SOFTWARE_GT_AWAKE_TIME:
>   			val = ktime_to_ns(intel_gt_get_awake_time(to_gt(i915)));
>   			break;
> +		case I915_PMU_TOTAL_ACTIVE_TICKS:
> +			val = intel_guc_total_active_ticks(i915->gt[gt_id]);
> +			break;
>   		}
>   	}
>   
> @@ -986,6 +991,7 @@ create_event_attributes(struct i915_pmu *pmu)
>   		__global_event(2, "interrupts", NULL),
>   		__event(3, "rc6-residency", "ns"),
>   		__event(4, "software-gt-awake-time", "ns"),
> +		__event(5, "total-active-ticks", NULL),
>   	};
>   	static const struct {
>   		enum drm_i915_pmu_engine_sample sample;
> diff --git a/include/uapi/drm/i915_drm.h b/include/uapi/drm/i915_drm.h
> index 7000e5910a1d7..e26dd27ff4a5f 100644
> --- a/include/uapi/drm/i915_drm.h
> +++ b/include/uapi/drm/i915_drm.h
> @@ -296,6 +296,7 @@ enum drm_i915_pmu_engine_sample {
>   #define I915_PMU_INTERRUPTS		__I915_PMU_OTHER(2)
>   #define I915_PMU_RC6_RESIDENCY		__I915_PMU_OTHER(3)
>   #define I915_PMU_SOFTWARE_GT_AWAKE_TIME	__I915_PMU_OTHER(4)
> +#define I915_PMU_TOTAL_ACTIVE_TICKS	__I915_PMU_OTHER(5)
>   
>   #define I915_PMU_LAST /* Deprecated - do not use */ I915_PMU_RC6_RESIDENCY
>   
> @@ -304,6 +305,7 @@ enum drm_i915_pmu_engine_sample {
>   #define __I915_PMU_INTERRUPTS(gt)		___I915_PMU_OTHER(gt, 2)
>   #define __I915_PMU_RC6_RESIDENCY(gt)		___I915_PMU_OTHER(gt, 3)
>   #define __I915_PMU_SOFTWARE_GT_AWAKE_TIME(gt)	___I915_PMU_OTHER(gt, 4)
> +#define __I915_PMU_TOTAL_ACTIVE_TICKS(gt)	___I915_PMU_OTHER(gt, 5)
>   
>   /* Each region is a minimum of 16k, and there are at most 255 of them.
>    */