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%3Ba=su...

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 stable@vger.kernel.org know about it.

From b29c6ef7bb1257853c1e31616d84f55e561cf631 Mon Sep 17 00:00:00 2001

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

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.

Fixes: 4815d3c56d1e (cpufreq: x86: Make scaling_cur_freq behave more as expected) Signed-off-by: Rafael J. Wysocki rafael.j.wysocki@intel.com Reviewed-by: WANG Chao chao.wang@ucloud.cn Signed-off-by: Linus Torvalds torvalds@linux-foundation.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org

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

--- a/arch/x86/kernel/cpu/aperfmperf.c +++ b/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)

Patches currently in stable-queue which might be from rafael.j.wysocki@intel.com 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