linux-kernel - Re: [RFC/RFT] [PATCH 03/10] cpufreq: intel_pstate: Utility functions to boost HWP performance limits

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Date:   Wed, 16 May 2018 08:41:13 -0700
From:   Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...e.de, lenb@...nel.org,
        rjw@...ysocki.net, mgorman@...hsingularity.net, x86@...nel.org,
        linux-pm@...r.kernel.org, viresh.kumar@...aro.org,
        juri.lelli@....com, linux-kernel@...r.kernel.org
Subject: Re: [RFC/RFT] [PATCH 03/10] cpufreq: intel_pstate: Utility
 functions to boost HWP performance limits

On Wed, 2018-05-16 at 09:22 +0200, Peter Zijlstra wrote:
> On Tue, May 15, 2018 at 09:49:04PM -0700, Srinivas Pandruvada wrote:
> > Setup necessary infrastructure to be able to boost HWP performance
> > on a
> > remote CPU. First initialize data structure to be able to use
> > smp_call_function_single_async(). The boost up function simply set
> > HWP
> > min to HWP max value and EPP to 0. The boost down function simply
> > restores
> > to last cached HWP Request value.
> > 
> > To avoid reading HWP Request MSR during dynamic update, the HWP
> > Request
> > MSR value is cached in the local memory. This caching is done
> > whenever
> > HWP request MSR is modified during driver init on in setpolicy()
> > callback
> > path.
> 
> The patch does two independent things; best to split that.
I had that split before, but thought no one is consuming the cached
value in that patch, so combined them. If this is not a problem, it is
better to split the patch.

Thanks,
Srinivas

> 
> 
> > diff --git a/drivers/cpufreq/intel_pstate.c
> > b/drivers/cpufreq/intel_pstate.c
> > index f686bbe..dc7dfa9 100644
> > --- a/drivers/cpufreq/intel_pstate.c
> > +++ b/drivers/cpufreq/intel_pstate.c
> > @@ -221,6 +221,9 @@ struct global_params {
> >   *			preference/bias
> >   * @epp_saved:		Saved EPP/EPB during system suspend
> > or CPU offline
> >   *			operation
> > + * @hwp_req_cached:	Cached value of the last HWP request
> > MSR
> 
> That's simply not true given the code below.
> 
> > @@ -763,6 +768,7 @@ static void intel_pstate_hwp_set(unsigned int
> > cpu)
> >  		intel_pstate_set_epb(cpu, epp);
> >  	}
> >  skip_epp:
> > +	cpu_data->hwp_req_cached = value;
> >  	wrmsrl_on_cpu(cpu, MSR_HWP_REQUEST, value);
> >  }
> >  
> > @@ -1381,6 +1387,39 @@ static void
> > intel_pstate_get_cpu_pstates(struct cpudata *cpu)
> >  	intel_pstate_set_min_pstate(cpu);
> >  }
> >  
> > +
> > +static inline void intel_pstate_hwp_boost_up(struct cpudata *cpu)
> > +{
> > +	u64 hwp_req;
> > +	u8 max;
> > +
> > +	max = (u8) (cpu->hwp_req_cached >> 8);
> > +
> > +	hwp_req = cpu->hwp_req_cached & ~GENMASK_ULL(31, 24);
> > +	hwp_req = (hwp_req & ~GENMASK_ULL(7, 0)) | max;
> > +
> > +	wrmsrl(MSR_HWP_REQUEST, hwp_req);
> > +}
> > +
> > +static inline void intel_pstate_hwp_boost_down(struct cpudata
> > *cpu)
> > +{
> > +	wrmsrl(MSR_HWP_REQUEST, cpu->hwp_req_cached);
> > +}
> 
> This is not a traditional msr shadow; that would be updated on every
> wrmsr. There is not a comment in sight explaining why this one is
> different.