linux-kernel - [PATCH v2 2/3] ACPI: processor: perflib: Avoid updating frequency QoS unnecessarily

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Message-ID: <4798709.31r3eYUQgx@kreacher>
Date:   Wed, 28 Dec 2022 22:24:10 +0100
From:   "Rafael J. Wysocki" <rjw@...ysocki.net>
To:     Linux PM <linux-pm@...r.kernel.org>
Cc:     Pratyush Yadav <ptyadav@...zon.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux ACPI <linux-acpi@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
Subject: [PATCH v2 2/3] ACPI: processor: perflib: Avoid updating frequency QoS unnecessarily

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

Modify acpi_processor_get_platform_limit() to avoid updating its
frequency QoS request when the _PPC return value has not changed
by comparing that value to the previous _PPC return value stored in
the performance_platform_limit field of the struct acpi_processor
corresponding to the given CPU.

While at it, do the _PPC return value check against the state count
earlier to avoid setting performance_platform_limit to an invalid value.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
---

v1 -> v2:
   * New patch.
   * Contains some changes from the previous [1/2] and a change to reset
     performance_platform_limit on QoS request creation.

---
 drivers/acpi/processor_perflib.c |   14 ++++++++++++--
 1 file changed, 12 insertions(+), 2 deletions(-)

Index: linux-pm/drivers/acpi/processor_perflib.c
===================================================================
--- linux-pm.orig/drivers/acpi/processor_perflib.c
+++ linux-pm/drivers/acpi/processor_perflib.c
@@ -76,13 +76,16 @@ static int acpi_processor_get_platform_l
 
 	index = ppc;
 
+	if (pr->performance_platform_limit == index ||
+	    ppc >= pr->performance->state_count)
+		return 0;
+
 	pr_debug("CPU %d: _PPC is %d - frequency %s limited\n", pr->id,
 		 index, index ? "is" : "is not");
 
 	pr->performance_platform_limit = index;
 
-	if (ppc >= pr->performance->state_count ||
-	    unlikely(!freq_qos_request_active(&pr->perflib_req)))
+	if (unlikely(!freq_qos_request_active(&pr->perflib_req)))
 		return 0;
 
 	/*
@@ -178,6 +181,13 @@ void acpi_processor_ppc_init(struct cpuf
 		if (!pr)
 			continue;
 
+		/*
+		 * Reset performance_platform_limit in case there is a stale
+		 * value in it, so as to make it match the "no limit" QoS value
+		 * below.
+		 */
+		pr->performance_platform_limit = 0;
+
 		ret = freq_qos_add_request(&policy->constraints,
 					   &pr->perflib_req,
 					   FREQ_QOS_MAX, INT_MAX);