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

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Message-ID: <2989520.e9J7NaK4W3@rjwysocki.net>
Date: Fri, 29 Nov 2024 17:00:59 +0100
From: "Rafael J. Wysocki" <rjw@...ysocki.net>
To: Linux PM <linux-pm@...r.kernel.org>
Cc: LKML <linux-kernel@...r.kernel.org>, Lukasz Luba <lukasz.luba@....com>,
 Peter Zijlstra <peterz@...radead.org>,
 Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
 Dietmar Eggemann <dietmar.eggemann@....com>,
 Morten Rasmussen <morten.rasmussen@....com>,
 Vincent Guittot <vincent.guittot@...aro.org>,
 Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>,
 Pierre Gondois <pierre.gondois@....com>
Subject: [RFC][PATCH v021 4/9] sched/topology: Adjust cpufreq checks for EAS

From: Rafael J. Wysocki <rafael.j.wysocki@...el.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@...el.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.

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?

---
 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;