Re: [RFC][PATCH v021 4/9] sched/topology: Adjust cpufreq checks for EAS

[Christian Loehle <christian.loehle@arm.com>]

On 11/29/24 16:00, Rafael J. Wysocki wrote:
> From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> 
> Make it possible to use EAS with cpufreq drivers that implement the
> :setpolicy() callback instead of using generic cpufreq governors.
> 
> This is going to be necessary for using EAS with intel_pstate in its
> default configuration.
> 
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> ---
> 
> This is the minimum of what's needed, but I'd really prefer to move
> the cpufreq vs EAS checks into cpufreq because messing around cpufreq
> internals in topology.c feels like a butcher shop kind of exercise.

Makes sense, something like cpufreq_eas_capable().

> 
> Besides, as I said before, I remain unconvinced about the usefulness
> of these checks at all.  Yes, one is supposed to get the best results
> from EAS when running schedutil, but what if they just want to try
> something else with EAS?  What if they can get better results with
> that other thing, surprisingly enough?

How do you imagine this to work then?
I assume we don't make any 'resulting-OPP-guesses' like
sugov_effective_cpu_perf() for any of the setpolicy governors.
Neither for dbs and I guess userspace.
What about standard powersave and performance?
Do we just have a cpufreq callback to ask which OPP to use for
the energy calculation? Assume lowest/highest?
(I don't think there is hardware where lowest/highest makes a
difference, so maybe not bothering with the complexity could
be an option, too.)

> 
> ---
>  kernel/sched/topology.c |   10 +++++++---
>  1 file changed, 7 insertions(+), 3 deletions(-)
> 
> Index: linux-pm/kernel/sched/topology.c
> ===================================================================
> --- linux-pm.orig/kernel/sched/topology.c
> +++ linux-pm/kernel/sched/topology.c
> @@ -217,6 +217,7 @@ static bool sched_is_eas_possible(const
>  	bool any_asym_capacity = false;
>  	struct cpufreq_policy *policy;
>  	struct cpufreq_governor *gov;
> +	bool cpufreq_ok;
>  	int i;
>  
>  	/* EAS is enabled for asymmetric CPU capacity topologies. */
> @@ -251,7 +252,7 @@ static bool sched_is_eas_possible(const
>  		return false;
>  	}
>  
> -	/* Do not attempt EAS if schedutil is not being used. */
> +	/* Do not attempt EAS if cpufreq is not configured adequately */
>  	for_each_cpu(i, cpu_mask) {
>  		policy = cpufreq_cpu_get(i);
>  		if (!policy) {
> @@ -261,11 +262,14 @@ static bool sched_is_eas_possible(const
>  			}
>  			return false;
>  		}
> +		/* Require schedutil or a "setpolicy" driver */
>  		gov = policy->governor;
> +		cpufreq_ok = gov == &schedutil_gov ||
> +				(!gov && policy->policy != CPUFREQ_POLICY_UNKNOWN);
>  		cpufreq_cpu_put(policy);
> -		if (gov != &schedutil_gov) {
> +		if (!cpufreq_ok) {
>  			if (sched_debug()) {
> -				pr_info("rd %*pbl: Checking EAS, schedutil is mandatory\n",
> +				pr_info("rd %*pbl: Checking EAS, unsuitable cpufreq governor\n",
>  					cpumask_pr_args(cpu_mask));
>  			}
>  			return false;

The logic here looks fine to me FWIW.