linux-kernel - Re: [PATCH v3 15/24] thermal: intel: hfi: Report the IPC class score of a CPU

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Message-ID: <20230331020741.GA1283@ranerica-svr.sc.intel.com>
Date:   Thu, 30 Mar 2023 19:07:41 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     "Rafael J. Wysocki" <rafael@...nel.org>
Cc:     "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Ricardo Neri <ricardo.neri@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Len Brown <len.brown@...el.com>, Mel Gorman <mgorman@...e.de>,
        "Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Lukasz Luba <lukasz.luba@....com>,
        Ionela Voinescu <ionela.voinescu@....com>, x86@...nel.org,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        "Tim C . Chen" <tim.c.chen@...el.com>
Subject: Re: [PATCH v3 15/24] thermal: intel: hfi: Report the IPC class score
 of a CPU

On Wed, Mar 29, 2023 at 02:17:01PM +0200, Rafael J. Wysocki wrote:
> On Wed, Mar 29, 2023 at 1:30 AM Ricardo Neri
> <ricardo.neri-calderon@...ux.intel.com> wrote:
> >
> > On Mon, Mar 27, 2023 at 06:50:13PM +0200, Rafael J. Wysocki wrote:
> > > On Tue, Feb 7, 2023 at 6:02 AM Ricardo Neri
> > > <ricardo.neri-calderon@...ux.intel.com> wrote:
> > > >
> > > > Implement the arch_get_ipcc_score() interface of the scheduler. Use the
> > > > performance capabilities of the extended Hardware Feedback Interface table
> > > > as the IPC score.
> > > >
> > > > Cc: Ben Segall <bsegall@...gle.com>
> > > > Cc: Daniel Bristot de Oliveira <bristot@...hat.com>
> > > > Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> > > > Cc: Ionela Voinescu <ionela.voinescu@....com>
> > > > Cc: Joel Fernandes (Google) <joel@...lfernandes.org>
> > > > Cc: Len Brown <len.brown@...el.com>
> > > > Cc: Lukasz Luba <lukasz.luba@....com>
> > > > Cc: Mel Gorman <mgorman@...e.de>
> > > > Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> > > > Cc: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
> > > > Cc: Steven Rostedt <rostedt@...dmis.org>
> > > > Cc: Tim C. Chen <tim.c.chen@...el.com>
> > > > Cc: Valentin Schneider <vschneid@...hat.com>
> > > > Cc: x86@...nel.org
> > > > Cc: linux-pm@...r.kernel.org
> > > > Cc: linux-kernel@...r.kernel.org
> > > > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
> > > > ---
> > > > Changes since v2:
> > > >  * None
> > > >
> > > > Changes since v1:
> > > >  * Adjusted the returned HFI class (which starts at 0) to match the
> > > >    scheduler IPCC class (which starts at 1). (PeterZ)
> > > >  * Used the new interface names.
> > > > ---
> > > >  arch/x86/include/asm/topology.h   |  2 ++
> > > >  drivers/thermal/intel/intel_hfi.c | 27 +++++++++++++++++++++++++++
> > > >  2 files changed, 29 insertions(+)
> > > >
> > > > diff --git a/arch/x86/include/asm/topology.h b/arch/x86/include/asm/topology.h
> > > > index ffcdac3f398f..c4fcd9c3c634 100644
> > > > --- a/arch/x86/include/asm/topology.h
> > > > +++ b/arch/x86/include/asm/topology.h
> > > > @@ -229,8 +229,10 @@ void init_freq_invariance_cppc(void);
> > > >
> > > >  #if defined(CONFIG_IPC_CLASSES) && defined(CONFIG_INTEL_HFI_THERMAL)
> > > >  void intel_hfi_update_ipcc(struct task_struct *curr);
> > > > +unsigned long intel_hfi_get_ipcc_score(unsigned short ipcc, int cpu);
> > > >
> > > >  #define arch_update_ipcc intel_hfi_update_ipcc
> > > > +#define arch_get_ipcc_score intel_hfi_get_ipcc_score
> > > >  #endif /* defined(CONFIG_IPC_CLASSES) && defined(CONFIG_INTEL_HFI_THERMAL) */
> > > >
> > > >  #endif /* _ASM_X86_TOPOLOGY_H */
> > > > diff --git a/drivers/thermal/intel/intel_hfi.c b/drivers/thermal/intel/intel_hfi.c
> > > > index 530dcf57e06e..fa9b4a678d92 100644
> > > > --- a/drivers/thermal/intel/intel_hfi.c
> > > > +++ b/drivers/thermal/intel/intel_hfi.c
> > > > @@ -206,6 +206,33 @@ void intel_hfi_update_ipcc(struct task_struct *curr)
> > > >         curr->ipcc = msr.split.classid + 1;
> > > >  }
> > > >
> > > > +unsigned long intel_hfi_get_ipcc_score(unsigned short ipcc, int cpu)
> > > > +{
> > > > +       unsigned short hfi_class;
> > >
> > > It looks like the variable above is only used to save a subtraction or
> > > addition of 1 to something going forward.
> > >
> > > > +       int *scores;
> > > > +
> > > > +       if (cpu < 0 || cpu >= nr_cpu_ids)
> > > > +               return -EINVAL;
> > > > +
> > > > +       if (ipcc == IPC_CLASS_UNCLASSIFIED)
> > > > +               return -EINVAL;
> > > > +
> > > > +       /*
> > > > +        * Scheduler IPC classes start at 1. HFI classes start at 0.
> > > > +        * See note intel_hfi_update_ipcc().
> > > > +        */
> > > > +       hfi_class = ipcc - 1;
> > > > +
> > > > +       if (hfi_class >= hfi_features.nr_classes)
> > >
> > > Personally, I would do
> > >
> > > if (ipcc >= hfi_features.nr_classes + 1)
> > >
> > > here and ->
> > >
> > > > +               return -EINVAL;
> > > > +
> > > > +       scores = per_cpu_ptr(hfi_ipcc_scores, cpu);
> > > > +       if (!scores)
> > > > +               return -ENODEV;
> > > > +
> > >
> > > -> scores[ipcc - 1]
> >
> > Sure, I can get rid of hfi_class.
> >
> > >
> > > below.
> > >
> > > > +       return READ_ONCE(scores[hfi_class]);
> > >
> > > And why does this need to use READ_ONCE()?
> >
> > This is the corresponding read of the WRITE_ONCE in patch 13. The CPU
> > handling the HFI interrupt, very likely a different CPU, updates
> > scores[hfi_class]. My intention is to let that write to complete before
> > reading the score here.
> 
> However, READ_ONCE()/WRITE_ONCE() only affect compiler optimizations
> AFAICS.  What if the CPUs running the code reorder the instructions?

True, this implementation may not be complete and may need a memory
barrier. I will look at it more closely.

> 
> In any case, IMV the reason why READ_ONCE() is used needs to be clear
> to the reviewers from the patch itself (and to a casual reader of the
> code from the code itself).

Sure. I will make sure this is clear in the commit message and/or
inline comments.

Thanks and BR,
Ricardo