On Friday, February 1, 2019 5:54:37 PM CET Doug Smythies wrote:
> On 2019.01.30 16:05 Rafael J. Wysocki wrote:
>
> > From: Rafael J. Wysocki <rafael.j.wyso...@intel.com>
> >
> > The current iowait boosting mechanism in intel_pstate_update_util()
> > is quite aggressive, as it goes to the maximum P-state right away,
> > and may cause excessive amounts of energy to be used, which is not
> > desirable and arguably isn't necessary too.
> >
> > Follow commit a5a0809bc58e ("cpufreq: schedutil: Make iowait boost
> > more energy efficient") that reworked the analogous iowait boost
> > mechanism in the schedutil governor and make the iowait boosting
> > in intel_pstate_update_util() work along the same lines.
> >
> > Signed-off-by: Rafael J. Wysocki <rafael.j.wyso...@intel.com>
> > ---
> > drivers/cpufreq/intel_pstate.c | 46
> > +++++++++++++++++++++++++----------------
> > 1 file changed, 29 insertions(+), 17 deletions(-)
> >
> > Index: linux-pm/drivers/cpufreq/intel_pstate.c
> > ===================================================================
> > --- linux-pm.orig/drivers/cpufreq/intel_pstate.c
> > +++ linux-pm/drivers/cpufreq/intel_pstate.c
> > @@ -50,6 +50,8 @@
> > #define int_tofp(X) ((int64_t)(X) << FRAC_BITS)
> > #define fp_toint(X) ((X) >> FRAC_BITS)
> >
> > +#define ONE_EIGHTH_FP ((int64_t)1 << (FRAC_BITS - 3))
> > +
> > #define EXT_BITS 6
> > #define EXT_FRAC_BITS (EXT_BITS + FRAC_BITS)
> > #define fp_ext_toint(X) ((X) >> EXT_FRAC_BITS)
> > @@ -1678,17 +1680,14 @@ static inline int32_t get_avg_pstate(str
> > static inline int32_t get_target_pstate(struct cpudata *cpu)
> > {
> > struct sample *sample = &cpu->sample;
> > - int32_t busy_frac, boost;
> > + int32_t busy_frac;
> > int target, avg_pstate;
> >
> > busy_frac = div_fp(sample->mperf << cpu->aperf_mperf_shift,
> > sample->tsc);
> >
> > - boost = cpu->iowait_boost;
> > - cpu->iowait_boost >>= 1;
> > -
> > - if (busy_frac < boost)
> > - busy_frac = boost;
> > + if (busy_frac < cpu->iowait_boost)
> > + busy_frac = cpu->iowait_boost;
> >
> > sample->busy_scaled = busy_frac * 100;
> >
> > @@ -1767,22 +1766,35 @@ static void intel_pstate_update_util(str
> > if (smp_processor_id() != cpu->cpu)
> > return;
> >
> > + delta_ns = time - cpu->last_update;
> > if (flags & SCHED_CPUFREQ_IOWAIT) {
> > - cpu->iowait_boost = int_tofp(1);
> > - cpu->last_update = time;
> > - /*
> > - * The last time the busy was 100% so P-state was max anyway
> > - * so avoid overhead of computation.
> > - */
> > - if (fp_toint(cpu->sample.busy_scaled) == 100)
> > - return;
> > -
> > - goto set_pstate;
> > + /* Start over if the CPU may have been idle. */
> > + if (delta_ns > TICK_NSEC) {
> > + cpu->iowait_boost = ONE_EIGHTH_FP;
> > + } else if (cpu->iowait_boost) {
> > + cpu->iowait_boost <<= 1;
> > + if (cpu->iowait_boost >= int_tofp(1)) {
> > + cpu->iowait_boost = int_tofp(1);
> > + cpu->last_update = time;
> > + /*
> > + * The last time the busy was 100% so P-state
> > + * was max anyway, so avoid the overhead of
> > + * computation.
> > + */
> > + if (fp_toint(cpu->sample.busy_scaled) == 100)
> > + return;
>
> Hi Rafael,
>
> By exiting here, the trace, if enabled, is also bypassed.
> We want the trace sample.
Fair enough, but the return is there regardless of this patch.
Maybe it should be fixed separately?
> Also, there is a generic:
> "If the target ptstate is the same as before, then don't set it"
> later on.
> Suggest to delete this test and exit condition. (I see that this early
> exit was done before also.)
Well, exactly.
It is not unreasonable to boost the frequency right away for an IO-waiter
without waiting for the next sample time IMO.
