From mboxrd@z Thu Jan  1 00:00:00 1970
From: WANG Chao <chao.wang@ucloud.cn>
Subject: Re: [PATCH] x86: use cpufreq_quick_get() for /proc/cpuinfo "cpu MHz"
 again
Date: Fri, 10 Nov 2017 17:21:02 +0800
Message-ID: <20171110092102.GA98700@WANG-Chaos-MacBook-Pro.local>
References: <20171109103814.70688-1-chao.wang@ucloud.cn>
 <6583ed1f-3ea3-c7fd-7e69-1430c8387e54@intel.com>
 <20171110072513.mp5p7o7pledumoyi@gmail.com>
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To: Ingo Molnar <mingo@kernel.org>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@intel.com>, Linus Torvalds <torvalds@linux-foundation.org>, linux-kernel@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, Vikas Shivappa <vikas.shivappa@linux.intel.com>, Kate Stewart <kstewart@linuxfoundation.org>, Len Brown <len.brown@intel.com>, Greg Kroah-Hartman <gregkh@linuxfoundation.org>, Philippe Ombredanne <pombredanne@nexb.com>, Mathias Krause <minipli@googlemail.com>, x86@kernel.org, "Rafael J. Wysocki" <rafael@kernel.org>, Linux PM <linux-pm@vger.kernel.org>

On 11/10/17 at 08:25P, Ingo Molnar wrote:
> 
> * Rafael J. Wysocki <rafael.j.wysocki@intel.com> wrote:
> 
> > 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.
> 
> This is a serious regression that needs to be fixed ASAP, because the slowdown is 
> utterly ridiculous on a 120 CPU system:
> 
>   fomalhaut:~> time cat /proc/cpuinfo  >/dev/null
> 
>   real    0m2.689s
>   user    0m0.001s
>   sys     0m0.007s

Because 4.14 is about to release, how about adding this patch to 4.14
now? We can work on a more sophisticated solution later.

Thanks,
WANG Chao