[PATCH] x86 / CPU: Avoid unnecessary IPIs in arch_freq_get_on_cpu()

["Rafael J. Wysocki" <rjw@rjwysocki.net>]

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

Even though aperfmperf_snapshot_khz() caches the samples.khz value
to return if called again in a sufficiently short time, its caller,
arch_freq_get_on_cpu(), still uses smp_call_function_single() to run
it which may allow user space to trigger an IPI storm by reading from
the scaling_cur_freq cpufreq sysfs file in a tight loop.

To avoid that, move the decision on whether or not to return the
cached samples.khz value to arch_freq_get_on_cpu().

Fixes: 4815d3c56d1e (cpufreq: x86: Make scaling_cur_freq behave more as expected)
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
---

This change was part of commit 941f5f0f6ef5 (x86: CPU: Fix up "cpu MHz" in
/proc/cpuinfo), but it was not the reason for the revert and it remains
applicable.

Thanks,
Rafael

---
 arch/x86/kernel/cpu/Makefile     |    2 +-
 arch/x86/kernel/cpu/aperfmperf.c |   11 +++++++----
 arch/x86/kernel/cpu/proc.c       |    4 +++-
 3 files changed, 11 insertions(+), 6 deletions(-)

Index: linux-pm/arch/x86/kernel/cpu/aperfmperf.c
===================================================================
--- linux-pm.orig/arch/x86/kernel/cpu/aperfmperf.c
+++ linux-pm/arch/x86/kernel/cpu/aperfmperf.c
@@ -42,10 +42,6 @@ static void aperfmperf_snapshot_khz(void
 	s64 time_delta = ktime_ms_delta(now, s->time);
 	unsigned long flags;
 
-	/* Don't bother re-computing within the cache threshold time. */
-	if (time_delta < APERFMPERF_CACHE_THRESHOLD_MS)
-		return;
-
 	local_irq_save(flags);
 	rdmsrl(MSR_IA32_APERF, aperf);
 	rdmsrl(MSR_IA32_MPERF, mperf);
@@ -74,6 +70,7 @@ static void aperfmperf_snapshot_khz(void
 
 unsigned int arch_freq_get_on_cpu(int cpu)
 {
+	s64 time_delta;
 	unsigned int khz;
 
 	if (!cpu_khz)
@@ -82,6 +79,12 @@ unsigned int arch_freq_get_on_cpu(int cp
 	if (!static_cpu_has(X86_FEATURE_APERFMPERF))
 		return 0;
 
+	/* Don't bother re-computing within the cache threshold time. */
+	time_delta = ktime_ms_delta(ktime_get(), per_cpu(samples.time, cpu));
+	khz = per_cpu(samples.khz, cpu);
+	if (khz && time_delta < APERFMPERF_CACHE_THRESHOLD_MS)
+		return khz;
+
 	smp_call_function_single(cpu, aperfmperf_snapshot_khz, NULL, 1);
 	khz = per_cpu(samples.khz, cpu);
 	if (khz)