Messages in this thread | | | From | "Rafael J. Wysocki" <> | Date | Thu, 9 Nov 2017 23:30:54 +0100 | Subject | Re: [PATCH] x86: use cpufreq_quick_get() for /proc/cpuinfo "cpu MHz" again |
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On Thu, Nov 9, 2017 at 5:06 PM, 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.
OK, here's an idea.
c_start() can run aperfmperf_snapshot_khz() on all CPUs upfront (say in parallel), then wait for a while (say 5 ms; the current 20 ms wait is overkill) and then aperfmperf_snapshot_khz() can be run once on each CPU in show_cpuinfo() without taking the "stale cache" threshold into account.
I'm going to try that and see how far I can get with it.
Thanks, Rafael
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