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: NRafael J. Wysocki <rafael.j.wysocki@intel.com> Reviewed-by: NWANG Chao <chao.wang@ucloud.cn> Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>
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