From: WANG Chao <chao.wang@ucloud.cn>
To: "Rafael J. Wysocki" <rjw@rjwysocki.net>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Linus Torvalds <torvalds@linux-foundation.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>,
the arch/x86 maintainers <x86@kernel.org>,
Linux PM <linux-pm@vger.kernel.org>,
"Rafael J. Wysocki" <rafael.j.wysocki@intel.com>
Subject: Re: [PATCH] x86 / CPU: Always show current CPU frequency in /proc/cpuinfo
Date: Fri, 17 Nov 2017 12:27:07 +0800 [thread overview]
Message-ID: <20171117042707.GA2069@WANG-Chaos-MacBook-Pro.local> (raw)
In-Reply-To: <3850698.eZd1pxJPHX@aspire.rjw.lan>
On 11/16/17 at 02:54P, Rafael J. Wysocki wrote:
> On Thursday, November 16, 2017 10:50:36 AM CET WANG Chao wrote:
> > On 11/16/17 at 01:24P, Rafael J. Wysocki wrote:
> > > On Wednesday, November 15, 2017 10:33:47 AM CET WANG Chao wrote:
> > > > On 11/15/17 at 02:13P, Rafael J. Wysocki wrote:
> > > > > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
> > > > >
> > > > > After commit 890da9cf0983 (Revert "x86: do not use cpufreq_quick_get()
> > > > > for /proc/cpuinfo "cpu MHz"") the "cpu MHz" number in /proc/cpuinfo
> > > > > on x86 can be either the nominal CPU frequency (which is constant)
> > > > > or the frequency most recently requested by a scaling governor in
> > > > > cpufreq, depending on the cpufreq configuration. That is somewhat
> > > > > inconsistent and is different from what it was before 4.13, so in
> > > > > order to restore the previous behavior, make it report the current
> > > > > CPU frequency like the scaling_cur_freq sysfs file in cpufreq.
> > > > >
> > > > > To that end, modify the /proc/cpuinfo implementation on x86 to use
> > > > > aperfmperf_snapshot_khz() to snapshot the APERF and MPERF feedback
> > > > > registers, if available, and use their values to compute the CPU
> > > > > frequency to be reported as "cpu MHz".
> > > > >
> > > > > However, do that carefully enough to avoid accumulating delays that
> > > > > lead to unacceptable access times for /proc/cpuinfo on systems with
> > > > > many CPUs. Run aperfmperf_snapshot_khz() once on all CPUs
> > > > > asynchronously at the /proc/cpuinfo open time, add a single delay
> > > > > upfront (if necessary) at that point and simply compute the current
> > > > > frequency while running show_cpuinfo() for each individual CPU.
> > > >
> > > > Hi, Rafael
> > > >
> > > > I tested your patch. It's much faster.
> > > >
> > > > But from what I got, calling aperfmperf_snapshot_khz() asynchronously
> > > > with 10ms sleep takes much longer than calling aperfmperf_snapshot_khz()
> > > > synchronously.
> > > >
> > > > Here's my result on 64 CPUs:
> > > >
> > > > - async aperfmperf_snapshot_khz() w/ 10ms sleep:
> > > >
> > > > # time cat /proc/cpuinfo > /dev/null
> > > > real 0m0.014s
> > > > user 0m0.000s
> > > > sys 0m0.002s
> > > >
> > > > - sync aperfmperf_snapshot_khz() w/o any sleep:
> > > >
> > > > # time cat /proc/cpuinfo > /dev/null
> > > > real 0m0.002s
> > > > user 0m0.000s
> > > > sys 0m0.002s
> > >
> > > Sure, but the delay is there, because without it the computed frequency
> > > may be way off for at least one of the CPUs.
> >
> > Thanks, I understand now. In this case, The 10ms delay turns out to be
> > the interval of measuring aperf and mperf and computing their deltas.
> >
> > Last question though, is 10ms best practice or can we make it shorter,
> > say 5ms?
>
> Experimentally, I found 5 ms to be slightly too short. It all depends on
> how accurate the numbers are expected to be, however, so there is some room
> for adjustments.
>
> Regardless, I'd prefer to start with 10 ms as that is what has been used in
> intel_pstate for quite a long time at least and adjust later if need be.
Sure. Thanks for your explanation.
This patch looks good to me.
Reviewed-by: WANG Chao <chao.wang@ucloud.cn>
next prev parent reply other threads:[~2017-11-17 4:28 UTC|newest]
Thread overview: 31+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-11-09 10:38 [PATCH] x86: use cpufreq_quick_get() for /proc/cpuinfo "cpu MHz" again WANG Chao
2017-11-09 16:06 ` Rafael J. Wysocki
2017-11-09 22:30 ` Rafael J. Wysocki
2017-11-10 0:06 ` Rafael J. Wysocki
2017-11-10 4:04 ` WANG Chao
2017-11-10 4:11 ` WANG Chao
2017-11-10 19:11 ` Linus Torvalds
2017-11-10 23:09 ` Rafael J. Wysocki
2017-11-14 22:47 ` Rafael J. Wysocki
2017-11-14 23:02 ` Linus Torvalds
2017-11-14 23:53 ` Thomas Gleixner
2017-11-15 0:04 ` Linus Torvalds
2017-11-15 0:06 ` Linus Torvalds
2017-11-15 0:30 ` Rafael J. Wysocki
2017-11-15 0:34 ` Linus Torvalds
2017-11-15 1:13 ` [PATCH] x86 / CPU: Always show current CPU frequency in /proc/cpuinfo Rafael J. Wysocki
2017-11-15 8:47 ` Thomas Gleixner
2017-11-15 9:33 ` WANG Chao
2017-11-16 0:24 ` Rafael J. Wysocki
2017-11-16 9:50 ` WANG Chao
2017-11-16 13:54 ` Rafael J. Wysocki
2017-11-17 4:27 ` WANG Chao [this message]
2017-11-17 13:33 ` Rafael J. Wysocki
2017-11-15 7:43 ` [PATCH] x86: use cpufreq_quick_get() for /proc/cpuinfo "cpu MHz" again Ingo Molnar
2017-11-15 7:54 ` Greg Kroah-Hartman
2017-11-15 17:27 ` Linus Torvalds
2017-11-15 18:05 ` Thomas Gleixner
2017-11-15 8:47 ` Thomas Gleixner
2017-11-15 0:06 ` Rafael J. Wysocki
2017-11-10 7:25 ` Ingo Molnar
2017-11-10 9:21 ` WANG Chao
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