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Commit b29c6ef7 authored by Rafael J. Wysocki's avatar Rafael J. Wysocki Committed by Linus Torvalds
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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 941f5f0f ("x86: CPU: Fix up "cpu MHz"
in /proc/cpuinfo"), but it was not the reason for the revert and it
remains applicable.

Fixes: 4815d3c5 (cpufreq: x86: Make scaling_cur_freq behave more as expected)
Signed-off-by: default avatarRafael J. Wysocki <rafael.j.wysocki@intel.com>
Reviewed-by: default avatarWANG Chao <chao.wang@ucloud.cn>
Signed-off-by: default avatarLinus Torvalds <torvalds@linux-foundation.org>
parent 99306dfc

arch/x86/kernel/cpu/aperfmperf.c

+7 −4
Original line number Diff line number Diff line
@@ -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)