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

Dixit, Ashutosh
2022-08-25T07:06:59Z

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

Hi Umesh,

Still reviewing but I have a question below.

> 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);
> +

/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);
> +

The question is why do we have 2 functions: __guc_context_update_clks()
(which we call periodically from guc_timestamp_ping()) and
guc_context_update_stats() (which we call non-periodically from
intel_context_get_total_runtime_ns()? Why don't we have just one function
which is called from both places? Or rather why don't we call
guc_context_update_stats() from both places?

If we don't call guc_context_update_stats() periodically from
guc_timestamp_ping() how e.g. does ce->stats.runtime.start_gt_clk get reset
to 0? If it gets reset to 0 in __guc_context_update_clks() then why do we
need to reset it in guc_context_update_stats()?

Also IMO guc->timestamp.lock should be taken by this single function,
(otherwise guc_context_update_stats() is modifying
ce->stats.runtime.start_gt_clk without taking the lock).

Thanks.
--
Ashutosh

> +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);
> + } else {
> + lrc_update_runtime(ce);
> + }
> +
> + 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);
> + return;
> + }
> +
> + __guc_context_update_clks(ce);
> + intel_context_unpin(ce);
> +}