Messages in this thread | | | From | "Doug Smythies" <> | Subject | RE: [PATCH] cpufreq: x86: Make scaling_cur_freq behave more as expected | Date | Thu, 27 Jul 2017 23:01:39 -0700 |
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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.
... Doug
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