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Subject[PATCH RFC] x86 / CPU: Add cpu isolation flag for avoiding disturbance from aperfmperf IPI
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Sensitive workloads like DPDK polling do not like any interrupts.

This patch adds flag 'freq' for boot option 'isolcpu': isolcpu=freq,<cpus>.

Users like show_cpuinfo() fallback to frequency from generic cpufreq
policy if arch-specific requesting method returns 0.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
Documentation/admin-guide/kernel-parameters.txt | 3 +++
arch/x86/kernel/cpu/aperfmperf.c | 12 +++++++++++-
include/linux/sched/isolation.h | 1 +
kernel/sched/isolation.c | 6 ++++++
4 files changed, 21 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 2b8ee90bb644..3797cd5c2b2e 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1900,6 +1900,9 @@
<cpu number> begins at 0 and the maximum value is
"number of CPUs in system - 1".

+ freq
+ Do not request current cpu frequency via IPI.
+
The format of <cpu-list> is described above.


diff --git a/arch/x86/kernel/cpu/aperfmperf.c b/arch/x86/kernel/cpu/aperfmperf.c
index 804c49493938..e8fb917980a8 100644
--- a/arch/x86/kernel/cpu/aperfmperf.c
+++ b/arch/x86/kernel/cpu/aperfmperf.c
@@ -14,6 +14,7 @@
#include <linux/percpu.h>
#include <linux/cpufreq.h>
#include <linux/smp.h>
+#include <linux/sched/isolation.h>

#include "cpu.h"

@@ -86,6 +87,9 @@ unsigned int aperfmperf_get_khz(int cpu)
if (!static_cpu_has(X86_FEATURE_APERFMPERF))
return 0;

+ if (!housekeeping_cpu(cpu, HK_FLAG_FREQ))
+ return 0;
+
aperfmperf_snapshot_cpu(cpu, ktime_get(), true);
return per_cpu(samples.khz, cpu);
}
@@ -102,9 +106,12 @@ void arch_freq_prepare_all(void)
if (!static_cpu_has(X86_FEATURE_APERFMPERF))
return;

- for_each_online_cpu(cpu)
+ for_each_online_cpu(cpu) {
+ if (!housekeeping_cpu(cpu, HK_FLAG_FREQ))
+ continue;
if (!aperfmperf_snapshot_cpu(cpu, now, false))
wait = true;
+ }

if (wait)
msleep(APERFMPERF_REFRESH_DELAY_MS);
@@ -118,6 +125,9 @@ unsigned int arch_freq_get_on_cpu(int cpu)
if (!static_cpu_has(X86_FEATURE_APERFMPERF))
return 0;

+ if (!housekeeping_cpu(cpu, HK_FLAG_FREQ))
+ return 0;
+
if (aperfmperf_snapshot_cpu(cpu, ktime_get(), true))
return per_cpu(samples.khz, cpu);

diff --git a/include/linux/sched/isolation.h b/include/linux/sched/isolation.h
index b0fb1446fe04..88e6013ac8a9 100644
--- a/include/linux/sched/isolation.h
+++ b/include/linux/sched/isolation.h
@@ -13,6 +13,7 @@ enum hk_flags {
HK_FLAG_TICK = (1 << 4),
HK_FLAG_DOMAIN = (1 << 5),
HK_FLAG_WQ = (1 << 6),
+ HK_FLAG_FREQ = (1 << 7),
};

#ifdef CONFIG_CPU_ISOLATION
diff --git a/kernel/sched/isolation.c b/kernel/sched/isolation.c
index b02d148e7672..456b5f1dfa11 100644
--- a/kernel/sched/isolation.c
+++ b/kernel/sched/isolation.c
@@ -140,6 +140,12 @@ static int __init housekeeping_isolcpus_setup(char *str)
continue;
}

+ if (!strncmp(str, "freq,", 5)) {
+ str += 5;
+ flags |= HK_FLAG_FREQ;
+ continue;
+ }
+
pr_warn("isolcpus: Error, unknown flag\n");
return 0;
}
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