Patch "x86 / CPU: Avoid unnecessary IPIs in arch_freq_get_on_cpu()" has been added to the 4.14-stable tree

This is a note to let you know that I've just added the patch titled

x86 / CPU: Avoid unnecessary IPIs in arch_freq_get_on_cpu()

to the 4.14-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary

The filename of the patch is:
x86-cpu-avoid-unnecessary-ipis-in-arch_freq_get_on_cpu.patch
and it can be found in the queue-4.14 subdirectory.

If you, or anyone else, feels it should not be added to the stable tree, please let <[email protected]> know about it.

From: "Rafael J. Wysocki" <[email protected]>
Date: Mon, 13 Nov 2017 02:15:39 +0100
Subject: x86 / CPU: Avoid unnecessary IPIs in arch_freq_get_on_cpu()

From: Rafael J. Wysocki <[email protected]>

commit b29c6ef7bb1257853c1e31616d84f55e561cf631 upstream.

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().

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.

---
 arch/x86/kernel/cpu/aperfmperf.c |   11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

@@ -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)

Patches currently in stable-queue which might be from [email protected] are

queue-4.14/x86-cpu-always-show-current-cpu-frequency-in-proc-cpuinfo.patch queue-4.14/x86-cpu-avoid-unnecessary-ipis-in-arch_freq_get_on_cpu.patch