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x86 / CPU: Avoid unnecessary IPIs in arch_freq_get_on_cpu() 

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 941f5f0f6e ("x86: CPU: Fix up "cpu MHz"
in /proc/cpuinfo"), but it was not the reason for the revert and it
remains applicable.

Fixes: 4815d3c56d (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>
This commit is contained in:
Rafael J. Wysocki 2017-11-13 02:15:39 +01:00 committed by Linus Torvalds
parent 99306dfc06
commit b29c6ef7bb
1 changed files with 7 additions and 4 deletions
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11
arch/x86/kernel/cpu/aperfmperf.c
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@ -42,10 +42,6 @@ static void aperfmperf_snapshot_khz(void *dummy)
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 *dummy)
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 cpu)
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)