linux-kernel - [PATCH 1/3] arm64: topology: Improve AMU-based frequency calculation

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <20251104075544.3243606-2-yubowen8@huawei.com>
Date: Tue, 4 Nov 2025 15:55:42 +0800
From: Bowen Yu <yubowen8@...wei.com>
To: <linux-arm-kernel@...ts.infradead.org>, <linux-kernel@...r.kernel.org>,
	<catalin.marinas@....com>, <will@...nel.org>, <beata.michalska@....com>,
	<ptsm@...ux.microsoft.com>, <linuxarm@...wei.com>,
	<jonathan.cameron@...wei.com>
CC: <zhanjie9@...ilicon.com>, <prime.zeng@...ilicon.com>,
	<wanghuiqiang@...wei.com>, <xuwei5@...wei.com>, <zhenglifeng1@...wei.com>,
	<yubowen8@...wei.com>, <zhangpengjie2@...wei.com>
Subject: [PATCH 1/3] arm64: topology: Improve AMU-based frequency calculation

The current approach of reverse-calculating CPU frequency from capacity
values introduces quantization errors due to intermediate scaling of
arch_scale_freq_capacity, which results in the calculated frequency having
only 1/1024 resolution.

This patch:
1. Directly computes frequency using AMU counters in amu_scale_freq_tick():
freq = (core_cycles_delta * timer_freq) / (const_cycles_delta * 1000)
 - core_cycles_delta: Measured CPU cycles
 - timer_freq: Architectural timer frequency
 - const_cycles_delta: Reference cycles from fixed-frequency timer
2. Returns pre-computed avgfreq in arch_freq_get_on_cpu()

examples:
Before change
[root@...alhost ~]# cat /sys/devices/system/cpu/cpufreq/policy*/cpuinfo_avg_freq
2297851
2297851
2295312
2297851
2297851
2295312
2297851
2295312
2297851
2297851
2297851
2295312
2295312
2297851
2297851
2297851
2297851
2300390
2297851
2297851
2297851

After change
[root@...alhost ~]# cat /sys/devices/system/cpu/cpufreq/policy*/cpuinfo_avg_freq
2299177
2298117
2299188
2297330
2296530
2298817
2298434
2298986
2298596
2299395
2299560
2298446
2299108
2299294
2298707
2298453
2298632
2299218
2297962

Signed-off-by: Bowen Yu <yubowen8@...wei.com>
---
 arch/arm64/kernel/topology.c | 20 +++++++++++---------
 1 file changed, 11 insertions(+), 9 deletions(-)

diff --git a/arch/arm64/kernel/topology.c b/arch/arm64/kernel/topology.c
index 5d07ee85bdae..c0dbc27289ea 100644
--- a/arch/arm64/kernel/topology.c
+++ b/arch/arm64/kernel/topology.c
@@ -20,6 +20,7 @@
 #include <linux/percpu.h>
 #include <linux/sched/isolation.h>
 #include <linux/xarray.h>
+#include <linux/units.h>
 
 #include <asm/cpu.h>
 #include <asm/cputype.h>
@@ -144,6 +145,8 @@ int __init parse_acpi_topology(void)
  */
 static DEFINE_PER_CPU_READ_MOSTLY(unsigned long, arch_max_freq_scale) =  1UL << (2 * SCHED_CAPACITY_SHIFT);
 static cpumask_var_t amu_fie_cpus;
+static DEFINE_PER_CPU(unsigned long, core_delta);
+static DEFINE_PER_CPU(unsigned long, const_delta);
 
 struct amu_cntr_sample {
 	u64		arch_const_cycles_prev;
@@ -246,6 +249,7 @@ static void amu_scale_freq_tick(void)
 	 * arch_max_freq_scale and the use of SCHED_CAPACITY_SHIFT.
 	 */
 	scale = core_cnt - prev_core_cnt;
+	this_cpu_write(core_delta, scale);
 	scale *= this_cpu_read(arch_max_freq_scale);
 	scale = div64_u64(scale >> SCHED_CAPACITY_SHIFT,
 			  const_cnt - prev_const_cnt);
@@ -253,6 +257,7 @@ static void amu_scale_freq_tick(void)
 	scale = min_t(unsigned long, scale, SCHED_CAPACITY_SCALE);
 	this_cpu_write(arch_freq_scale, (unsigned long)scale);
 
+	this_cpu_write(const_delta, const_cnt - prev_const_cnt);
 	amu_sample->last_scale_update = jiffies;
 }
 
@@ -288,7 +293,7 @@ int arch_freq_get_on_cpu(int cpu)
 	unsigned int start_cpu = cpu;
 	unsigned long last_update;
 	unsigned int freq = 0;
-	u64 scale;
+	u64 delta_core_kHz;
 
 	if (!amu_fie_cpu_supported(cpu) || !arch_scale_freq_ref(cpu))
 		return -EOPNOTSUPP;
@@ -340,14 +345,11 @@ int arch_freq_get_on_cpu(int cpu)
 			break;
 		}
 	}
-	/*
-	 * Reversed computation to the one used to determine
-	 * the arch_freq_scale value
-	 * (see amu_scale_freq_tick for details)
-	 */
-	scale = arch_scale_freq_capacity(cpu);
-	freq = scale * arch_scale_freq_ref(cpu);
-	freq >>= SCHED_CAPACITY_SHIFT;
+
+	if (check_mul_overflow(per_cpu(core_delta, cpu), arch_timer_get_cntfrq(), &delta_core_kHz))
+		return -EINVAL;
+
+	freq = div_u64(delta_core_kHz, per_cpu(const_delta, cpu) * HZ_PER_KHZ);
 	return freq;
 }
 
-- 
2.33.0