Re: [Intel-gfx] [PATCH] i915/pmu: Wire GuC backend to per-client busyness

["Dixit, Ashutosh" <ashutosh.dixit@intel.com>]

On Thu, 04 Aug 2022 16:21:25 -0700, Umesh Nerlige Ramappa wrote:

Hi Umesh, I am fairly new to this code so some questions will be below will
be newbie questions, thanks for bearing with me.

> diff --git a/drivers/gpu/drm/i915/gt/intel_context.c b/drivers/gpu/drm/i915/gt/intel_context.c
> index 654a092ed3d6..e2d70a9fdac0 100644
> --- a/drivers/gpu/drm/i915/gt/intel_context.c
> +++ b/drivers/gpu/drm/i915/gt/intel_context.c
> @@ -576,16 +576,24 @@ void intel_context_bind_parent_child(struct intel_context *parent,
>	child->parallel.parent = parent;
>  }
>
> -u64 intel_context_get_total_runtime_ns(const struct intel_context *ce)
> +u64 intel_context_get_total_runtime_ns(struct intel_context *ce)
>  {
>	u64 total, active;
>
> +	if (ce->ops->update_stats)
> +		ce->ops->update_stats(ce);
> +
>	total = ce->stats.runtime.total;
>	if (ce->ops->flags & COPS_RUNTIME_CYCLES)
>		total *= ce->engine->gt->clock_period_ns;
>
>	active = READ_ONCE(ce->stats.active);
> -	if (active)
> +	/*
> +	 * When COPS_RUNTIME_ACTIVE_TOTAL is set for ce->cops, the backend
> +	 * already provides the total active time of the context, so skip this
> +	 * calculation when this flag is set.
> +	 */
> +	if (active && !(ce->ops->flags & COPS_RUNTIME_ACTIVE_TOTAL))
>		active = intel_context_clock() - active;
>
>	return total + active;

/snip/

> @@ -1396,6 +1399,10 @@ static void guc_timestamp_ping(struct work_struct *wrk)
>	with_intel_runtime_pm(&gt->i915->runtime_pm, wakeref)
>		__update_guc_busyness_stats(guc);
>
> +	/* adjust context stats for overflow */
> +	xa_for_each(&guc->context_lookup, index, ce)
> +		__guc_context_update_clks(ce);

What is the reason for calling __guc_context_update_clks() periodically
from guc_timestamp_ping() since it appears we should just be able to call
__guc_context_update_clks() from intel_context_get_total_runtime_ns() to
update 'active'? Is the reason for calling __guc_context_update_clks()
periodically that the calculations in __guc_context_update_clks() become
invalid if the counters overflow?

> +
>	intel_gt_reset_unlock(gt, srcu);
>
>	mod_delayed_work(system_highpri_wq, &guc->timestamp.work,
> @@ -1469,6 +1476,56 @@ void intel_guc_busyness_unpark(struct intel_gt *gt)
>			 guc->timestamp.ping_delay);
>  }
>
> +static void __guc_context_update_clks(struct intel_context *ce)
> +{
> +	struct intel_guc *guc = ce_to_guc(ce);
> +	struct intel_gt *gt = ce->engine->gt;
> +	u32 *pphwsp, last_switch, engine_id;
> +	u64 start_gt_clk, active;
> +	unsigned long flags;
> +	ktime_t unused;
> +
> +	spin_lock_irqsave(&guc->timestamp.lock, flags);
> +
> +	/*
> +	 * GPU updates ce->lrc_reg_state[CTX_TIMESTAMP] when context is switched
> +	 * out, however GuC updates PPHWSP offsets below. Hence KMD (CPU)
> +	 * relies on GuC and GPU for busyness calculations. Due to this, A
> +	 * potential race was highlighted in an earlier review that can lead to
> +	 * double accounting of busyness. While the solution to this is a wip,
> +	 * busyness is still usable for platforms running GuC submission.
> +	 */
> +	pphwsp = ((void *)ce->lrc_reg_state) - LRC_STATE_OFFSET;
> +	last_switch = READ_ONCE(pphwsp[PPHWSP_GUC_CONTEXT_USAGE_STAMP_LO]);
> +	engine_id = READ_ONCE(pphwsp[PPHWSP_GUC_CONTEXT_USAGE_ENGINE_ID]);
> +
> +	guc_update_pm_timestamp(guc, &unused);
> +
> +	if (engine_id != 0xffffffff && last_switch) {
> +		start_gt_clk = READ_ONCE(ce->stats.runtime.start_gt_clk);
> +		__extend_last_switch(guc, &start_gt_clk, last_switch);
> +		active = intel_gt_clock_interval_to_ns(gt, guc->timestamp.gt_stamp - start_gt_clk);
> +		WRITE_ONCE(ce->stats.runtime.start_gt_clk, start_gt_clk);
> +		WRITE_ONCE(ce->stats.active, active);

Should not need WRITE_ONCE to update regular memory. Not even sure we need
READ_ONCE above.

> +	} else {
> +		lrc_update_runtime(ce);

As was being discussed, should not need this here in this function. See
below too.

> +	}
> +
> +	spin_unlock_irqrestore(&guc->timestamp.lock, flags);
> +}
> +
> +static void guc_context_update_stats(struct intel_context *ce)
> +{
> +	if (!intel_context_pin_if_active(ce)) {
> +		WRITE_ONCE(ce->stats.runtime.start_gt_clk, 0);
> +		WRITE_ONCE(ce->stats.active, 0);

Why do these need to be initialized to 0? Looks like the calculations in
__guc_context_update_clks() will work even if we don't do this? Also I
didn't follow the 'if (!intel_context_pin_if_active(ce))' check.

> +		return;
> +	}
> +
> +	__guc_context_update_clks(ce);
> +	intel_context_unpin(ce);
> +}
> +
>  static inline bool
>  submission_disabled(struct intel_guc *guc)
>  {
> @@ -2723,6 +2780,7 @@ static void guc_context_unpin(struct intel_context *ce)
>  {
>	struct intel_guc *guc = ce_to_guc(ce);
>
> +	lrc_update_runtime(ce);

How about moving this into lrc_unpin() since that gets called from all guc
context types (parent/child/virtual).

>	unpin_guc_id(guc, ce);
>	lrc_unpin(ce);
>
> @@ -3344,6 +3402,7 @@ static void remove_from_context(struct i915_request *rq)
>  }
>
>  static const struct intel_context_ops guc_context_ops = {
> +	.flags = COPS_RUNTIME_CYCLES | COPS_RUNTIME_ACTIVE_TOTAL,
>	.alloc = guc_context_alloc,
>
>	.pre_pin = guc_context_pre_pin,
> @@ -3360,6 +3419,8 @@ static const struct intel_context_ops guc_context_ops = {
>
>	.sched_disable = guc_context_sched_disable,
>
> +	.update_stats = guc_context_update_stats,
> +
>	.reset = lrc_reset,
>	.destroy = guc_context_destroy,
>
> @@ -3593,6 +3654,7 @@ static int guc_virtual_context_alloc(struct intel_context *ce)
>  }
>
>  static const struct intel_context_ops virtual_guc_context_ops = {
> +	.flags = COPS_RUNTIME_CYCLES | COPS_RUNTIME_ACTIVE_TOTAL,
>	.alloc = guc_virtual_context_alloc,
>
>	.pre_pin = guc_virtual_context_pre_pin,
> @@ -3608,6 +3670,7 @@ static const struct intel_context_ops virtual_guc_context_ops = {
>	.exit = guc_virtual_context_exit,
>
>	.sched_disable = guc_context_sched_disable,
> +	.update_stats = guc_context_update_stats,
>
>	.destroy = guc_context_destroy,
>
> diff --git a/drivers/gpu/drm/i915/i915_drm_client.c b/drivers/gpu/drm/i915/i915_drm_client.c
> index b09d1d386574..8d81119fff14 100644
> --- a/drivers/gpu/drm/i915/i915_drm_client.c
> +++ b/drivers/gpu/drm/i915/i915_drm_client.c
> @@ -147,11 +147,7 @@ void i915_drm_client_fdinfo(struct seq_file *m, struct file *f)
>		   PCI_SLOT(pdev->devfn), PCI_FUNC(pdev->devfn));
>	seq_printf(m, "drm-client-id:\t%u\n", client->id);
>
> -	/*
> -	 * Temporarily skip showing client engine information with GuC submission till
> -	 * fetching engine busyness is implemented in the GuC submission backend
> -	 */
> -	if (GRAPHICS_VER(i915) < 8 || intel_uc_uses_guc_submission(&i915->gt0.uc))
> +	if (GRAPHICS_VER(i915) < 8)
>		return;
>
>	for (i = 0; i < ARRAY_SIZE(uabi_class_names); i++)
> --
> 2.37.1
>