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RE: [PATCH v2] cpufreq: x86: Make scaling_cur_freq behave more as expected

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  RE: [PATCH v2] cpufreq: x86: Make scaling_cur_freq behave more as expected "Doug Smythies" <dsmythies@telus.net> - 2017-08-01 01:50 +0200
    Re: [PATCH v2] cpufreq: x86: Make scaling_cur_freq behave more as expected "Rafael J. Wysocki" <rjw@rjwysocki.net> - 2017-08-01 03:00 +0200

#1700507 — RE: [PATCH v2] cpufreq: x86: Make scaling_cur_freq behave more as expected

On 2017.07.28 05:45 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 iteration
> of aperfmperf_snapshot_khz() which may be stale.
>
> To prevent that from happening, 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).

...[deleted the rest]...

This patch seems to work fine and addresses my complaints from last week.
Thanks.

... Doug

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#1700548

On Monday, July 31, 2017 04:46:42 PM Doug Smythies wrote:
> On 2017.07.28 05:45 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 iteration
> > of aperfmperf_snapshot_khz() which may be stale.
> >
> > To prevent that from happening, 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).
> 
> ...[deleted the rest]...
> 
> This patch seems to work fine and addresses my complaints from last week.
> Thanks.

Thanks for the confirmation!

Rafael

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