Re: [PATCH 4/4] cpufreq: intel_pstate: enable boost for Skylake Xeon

[Eero Tamminen]

Hi,

On 28.07.2018 17:14, Srinivas Pandruvada wrote:
> On Fri, 2018-07-27 at 22:34 -0700, Francisco Jerez wrote:
> > Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx> writes:
> >
> > > Enable HWP boost on Skylake server and workstations.
> >
> > Please revert this series, it led to significant energy usage and
> > graphics performance regressions [1].
> Which SKX platform is targeted to graphics?

Patch that Chris pointed out is this:
+static const struct x86_cpu_id intel_pstate_hwp_boost_ids[] = {
+       ICPU(INTEL_FAM6_SKYLAKE_X, core_funcs),
+       ICPU(INTEL_FAM6_SKYLAKE_DESKTOP, core_funcs),
+       {}
+};

The regressing platforms in our test system were:
- SKL 6600K i5 / GT2
- SKL 6770HQ i7 / GT4e

SKL-U i5 / GT3e device wasn't impacted, so I assume U devices don't 
match INTEL_FAM6_SKYLAKE_DESKTOP.



	- Eero

> >   The reasons are roughly the ones
> > we discussed by e-mail off-list last April: This causes the
> > intel_pstate
> > driver to decrease the EPP to zero
> No. You didn't check this series. We are not using EPP at all.
> The boost mechanism used here is not boost to max.
>
> Thanks,
> Srinivas
>
> > when the workload blocks on IO
> > frequently enough, which for the regressing benchmarks detailed in
> > [1]
> > is a symptom of the workload being heavily IO-bound, which means they
> > won't benefit at all from the EPP boost since they aren't
> > significantly
> > CPU-bound, and they will suffer a decrease in parallelism due to the
> > active CPU core using a larger fraction of the TDP in order to
> > achieve
> > the same work, causing the GPU to have a lower power budget
> > available,
> > leading to a decrease in system performance.
> >
> > You may want to give a shot to my previous suggestion of using [2] in
> > order to detect whether the system is IO-bound, which you can use as
> > an
> > indicator that the optimization implemented in this series cannot
> > possibly improve performance and can be expected to hurt energy
> > usage.
> >
> > Thanks.
> >
> > [1] https://bugs.freedesktop.org/show_bug.cgi?id=107410
> > [2] https://patchwork.kernel.org/patch/10312259/
> >
> > > Reported-by: Mel Gorman <mgorman@xxxxxxxxxxxxxxxxxxx>
> > > Tested-by: Giovanni Gherdovich <ggherdovich@xxxxxxx>
> > > Signed-off-by: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxx
> > > .com>
> > > ---
> > >   drivers/cpufreq/intel_pstate.c | 10 ++++++++++
> > >   1 file changed, 10 insertions(+)
> > >
> > > diff --git a/drivers/cpufreq/intel_pstate.c
> > > b/drivers/cpufreq/intel_pstate.c
> > > index 70bf63bb4e0e..01c8da1f99db 100644
> > > --- a/drivers/cpufreq/intel_pstate.c
> > > +++ b/drivers/cpufreq/intel_pstate.c
> > > @@ -1794,6 +1794,12 @@ static const struct x86_cpu_id
> > > intel_pstate_cpu_ee_disable_ids[] = {
> > >   	{}
> > >   };
> > >   
> > > +static const struct x86_cpu_id intel_pstate_hwp_boost_ids[]
> > > __initconst = {
> > > +	ICPU(INTEL_FAM6_SKYLAKE_X, core_funcs),
> > > +	ICPU(INTEL_FAM6_SKYLAKE_DESKTOP, core_funcs),
> > > +	{}
> > > +};
> > > +
> > >   static int intel_pstate_init_cpu(unsigned int cpunum)
> > >   {
> > >   	struct cpudata *cpu;
> > > @@ -1824,6 +1830,10 @@ static int intel_pstate_init_cpu(unsigned
> > > int cpunum)
> > >   			intel_pstate_disable_ee(cpunum);
> > >   
> > >   		intel_pstate_hwp_enable(cpu);
> > > +
> > > +		id = x86_match_cpu(intel_pstate_hwp_boost_ids);
> > > +		if (id)
> > > +			hwp_boost = true;
> > >   	}
> > >   
> > >   	intel_pstate_get_cpu_pstates(cpu);
> > > --
> > > 2.13.6