Re: [RFC PATCH 15/23] thermal: intel: hfi: Report per-cpu class-specific performance scores

[Peter Zijlstra]

On Wed, Oct 05, 2022 at 04:59:59PM -0700, Ricardo Neri wrote:
> On Tue, Sep 27, 2022 at 01:59:15PM +0200, Peter Zijlstra wrote:
> > On Fri, Sep 09, 2022 at 04:11:57PM -0700, Ricardo Neri wrote:
> > > Support the arch_get_task_class_score() interface of the scheduler. Use the
> > > data that Intel Thread Director provides to inform the scheduler the
> > > performance of a class of tasks when placed on a given CPU.
> > > 
> > 
> > > +static void get_one_hfi_cap(struct hfi_instance *hfi_instance, s16 index,
> > > +			    struct hfi_cpu_data *hfi_caps, int class)
> > > +{
> > > +	struct hfi_cpu_data *caps;
> > > +
> > > +	/* Find the capabilities of @cpu */
> > > +	caps = hfi_instance->data + index * hfi_features.cpu_stride +
> > > +	       class * hfi_features.class_stride;
> > > +	memcpy(hfi_caps, caps, sizeof(*hfi_caps));
> > > +}
> > > +
> > > +int intel_hfi_get_task_class_score(int class, int cpu)
> > > +{
> > > +	struct hfi_cpu_info *info = &per_cpu(hfi_cpu_info, cpu);
> > > +	struct hfi_instance *instance;
> > > +	struct hfi_cpu_data caps;
> > > +	unsigned long flags;
> > > +	int cap;
> > > +
> > > +	if (cpu < 0 || cpu >= nr_cpu_ids)
> > > +		return -EINVAL;
> > > +
> > > +	if (class == TASK_CLASS_UNCLASSIFIED)
> > > +		return -EINVAL;
> > > +
> > > +	if (class >= (int)hfi_features.nr_classes)
> > > +		return -EINVAL;
> > > +
> > > +	instance = info->hfi_instance;
> > > +	if (!instance)
> > > +		return -ENOENT;
> > > +
> > > +	raw_spin_lock_irqsave(&instance->table_lock, flags);
> > > +	get_one_hfi_cap(instance, info->index, &caps, class);
> > > +	cap = caps.perf_cap;
> > > +	raw_spin_unlock_irqrestore(&instance->table_lock, flags);
> > > +
> > > +	return cap;
> > > +}
> > 
> > Does any of that data actually ever change? Isn't the class score fixed
> > per CPU type?
> 
> Yes, data can change. The Intel SDM Vol 3 Section 14.6.7 states that the
> table can be updated during runtime.

I find the SDM is often unreadable gibberish, this part doesn't dissapoint.

There's a ton of might and maybe there; what does it actually do and how
often does it do it? Given the thermal interrupt is such a shitshow, we
really, as in *REALLY* don't want this to happen at any frequency at
all.

And if it barely happens, why do we care if it happens at all?