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SubjectRe: [PATCH] x86: use cpufreq_quick_get() for /proc/cpuinfo "cpu MHz" again
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Hi Linus,

On 11/9/2017 11:38 AM, WANG Chao wrote:
> Commit 941f5f0f6ef5 (x86: CPU: Fix up "cpu MHz" in /proc/cpuinfo) caused
> a serious performance issue when reading from /proc/cpuinfo on system
> with aperfmperf.
>
> For each cpu, arch_freq_get_on_cpu() sleeps 20ms to get its frequency.
> On a system with 64 cpus, it takes 1.5s to finish running `cat
> /proc/cpuinfo`, while it previously was done in 15ms.

Honestly, I'm not sure what to do to address this ATM.

The last requested frequency is only available in the non-HWP case, so
it cannot be used universally.

> Some programs use /proc/cpuinfo during startup or runtime, eg. QEMU.
> This patch will restore the previous behavior and let user space happy.
>
> Fixes: 941f5f0f6ef5 (x86: CPU: Fix up "cpu MHz" in /proc/cpuinfo)
> Cc: stable@kernel.org # 4.13
> Signed-off-by: WANG Chao <chao.wang@ucloud.cn>
> ---
> arch/x86/kernel/cpu/Makefile | 2 +-
> arch/x86/kernel/cpu/proc.c | 4 +---
> 2 files changed, 2 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/kernel/cpu/Makefile b/arch/x86/kernel/cpu/Makefile
> index 236999c54edc..c60922a66385 100644
> --- a/arch/x86/kernel/cpu/Makefile
> +++ b/arch/x86/kernel/cpu/Makefile
> @@ -22,7 +22,7 @@ obj-y += common.o
> obj-y += rdrand.o
> obj-y += match.o
> obj-y += bugs.o
> -obj-y += aperfmperf.o
> +obj-$(CONFIG_CPU_FREQ) += aperfmperf.o
>
> obj-$(CONFIG_PROC_FS) += proc.o
> obj-$(CONFIG_X86_FEATURE_NAMES) += capflags.o powerflags.o
> diff --git a/arch/x86/kernel/cpu/proc.c b/arch/x86/kernel/cpu/proc.c
> index 4378a729b933..6b7e17bf0b71 100644
> --- a/arch/x86/kernel/cpu/proc.c
> +++ b/arch/x86/kernel/cpu/proc.c
> @@ -78,10 +78,8 @@ static int show_cpuinfo(struct seq_file *m, void *v)
> seq_printf(m, "microcode\t: 0x%x\n", c->microcode);
>
> if (cpu_has(c, X86_FEATURE_TSC)) {
> - unsigned int freq = arch_freq_get_on_cpu(cpu);
> + unsigned int freq = cpufreq_quick_get(cpu);
>
> - if (!freq)
> - freq = cpufreq_quick_get(cpu);
> if (!freq)
> freq = cpu_khz;
> seq_printf(m, "cpu MHz\t\t: %u.%03u\n",


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