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

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

On Wed, 24 Aug 2022 22:03:19 -0700, Dixit, Ashutosh wrote:
>
> On Thu, 04 Aug 2022 16:21:25 -0700, Umesh Nerlige Ramappa wrote:
> >
>
> Hi Umesh,
>
> Still reviewing but I have a question below.

Please ignore this mail for now, mostly a result of my misunderstanding the
code. I will ask again if I have any questions. Thanks.

>
> > 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);
> > +}