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

[Doug Smythies]

On 2017.07.27 17:13 Rafael J. Wysocki wrote:

> From: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
>
> 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@xxxxxxxxx>
> Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> ---
> 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