x86/cpu: Avoid cpuinfo-induced IPIing of idle CPUs

Currently, accessing /proc/cpuinfo sends IPIs to idle CPUs in order to
learn their clock frequency.  Which is a bit strange, given that waking
them from idle likely significantly changes their clock frequency.
This commit therefore avoids sending /proc/cpuinfo-induced IPIs to
idle CPUs.

[ paulmck: Also check for idle in arch_freq_prepare_all(). ]
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: <x86@kernel.org>

1 files changed, 6 insertions, 0 deletions

diff --git a/arch/x86/kernel/cpu/aperfmperf.c b/arch/x86/kernel/cpu/aperfmperf.c
index dd3261dab0fb..22911deacb6e 100644
--- a/arch/x86/kernel/cpu/aperfmperf.c
+++ b/arch/x86/kernel/cpu/aperfmperf.c
@@ -14,6 +14,7 @@
 #include <linux/cpufreq.h>
 #include <linux/smp.h>
 #include <linux/sched/isolation.h>
+#include <linux/rcupdate.h>
 
 #include "cpu.h"
 
@@ -93,6 +94,9 @@ unsigned int aperfmperf_get_khz(int cpu)
 	if (!housekeeping_cpu(cpu, HK_FLAG_MISC))
 		return 0;
 
+	if (rcu_is_idle_cpu(cpu))
+		return 0; /* Idle CPUs are completely uninteresting. */
+
 	aperfmperf_snapshot_cpu(cpu, ktime_get(), true);
 	return per_cpu(samples.khz, cpu);
 }
@@ -112,6 +116,8 @@ void arch_freq_prepare_all(void)
 	for_each_online_cpu(cpu) {
 		if (!housekeeping_cpu(cpu, HK_FLAG_MISC))
 			continue;
+		if (rcu_is_idle_cpu(cpu))
+			continue; /* Idle CPUs are completely uninteresting. */
 		if (!aperfmperf_snapshot_cpu(cpu, now, false))
 			wait = true;
 	}