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.wyso...@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.w...@ucloud.cn>
Thanks for the reviews, much appreciated!
