Messages in this thread | | | From | "Rafael J. Wysocki" <> | Subject | Re: [PATCH] cpufreq: x86: Make scaling_cur_freq behave more as expected | Date | Fri, 28 Jul 2017 14:26:56 +0200 |
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On Thursday, July 27, 2017 11:01:39 PM Doug Smythies wrote: > On 2017.07.27 17:13 Rafael J. Wysocki wrote: > > > From: Rafael J. Wysocki <rafael.j.wysocki@intel.com> > > > > After commit f8475cef9008 "x86: use common aperfmperf_khz_on_cpu() to > > calculate KHz using APERF/MPERF" the scaling_cur_freq policy attribute > > in sysfs only behaves as expected on x86 with APERF/MPERF registers > > available when it is read from at least twice in a row. > > > > The value returned by the first read may not be meaningful, because > > the computations in there use cached values from the previous > > aperfmperf_snapshot_khz() call which may be stale. However, the > > interface is expected to return meaningful values on every read, > > including the first one. > > > > To address this problem modify arch_freq_get_on_cpu() to call > > aperfmperf_snapshot_khz() twice, with a short delay between > > these calls, if the previous invocation of aperfmperf_snapshot_khz() > > was too far back in the past (specifically, more that 1s ago) and > > adjust aperfmperf_snapshot_khz() for that. > > > > Fixes: f8475cef9008 "x86: use common aperfmperf_khz_on_cpu() to calculate KHz using APERF/MPERF" > > Reported-by: Doug Smythies <dsmythies@telus.net> > > Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com> > > --- > > arch/x86/kernel/cpu/aperfmperf.c | 36 +++++++++++++++++++++++++++++------- > > 1 file changed, 29 insertions(+), 7 deletions(-) > > > > Index: linux-pm/arch/x86/kernel/cpu/aperfmperf.c > > ...[deleted the rest]... > > This proposed patch would be good. However, I can only try it maybe by Sunday. > I think the maximum time span means that this code: > > /* > * if (cpu_khz * aperf_delta) fits into ULLONG_MAX, then > * khz = (cpu_khz * aperf_delta) / mperf_delta > */ > if (div64_u64(ULLONG_MAX, cpu_khz) > aperf_delta) > s->khz = div64_u64((cpu_khz * aperf_delta), mperf_delta); > else /* khz = aperf_delta / (mperf_delta / cpu_khz) */ > s->khz = div64_u64(aperf_delta, > div64_u64(mperf_delta, cpu_khz)); > > Could be reduced to this: > > s->khz = div64_u64((cpu_khz * aperf_delta), mperf_delta); > > Because it could never overflow anymore.
Right, that's a good point.
I'll send a v2 with this change included shortly.
Thanks, Rafael
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