Re: [PATCH] x86 / CPU: Always show current CPU frequency in /proc/cpuinfo

[Rafael J. Wysocki]

On Friday, November 17, 2017 5:27:07 AM CET WANG Chao wrote:
> 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@xxxxxxxxx>
> > > > > > 
> > > > > > 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@xxxxxxxxx>

Thanks for the reviews, much appreciated!