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    SubjectRe: [PATCH v3 15/24] thermal: intel: hfi: Report the IPC class score of a CPU
    On Wed, Mar 29, 2023 at 1:30 AM Ricardo Neri
    <ricardo.neri-calderon@linux.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@linux.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@google.com>
    > > > Cc: Daniel Bristot de Oliveira <bristot@redhat.com>
    > > > Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
    > > > Cc: Ionela Voinescu <ionela.voinescu@arm.com>
    > > > Cc: Joel Fernandes (Google) <joel@joelfernandes.org>
    > > > Cc: Len Brown <len.brown@intel.com>
    > > > Cc: Lukasz Luba <lukasz.luba@arm.com>
    > > > Cc: Mel Gorman <mgorman@suse.de>
    > > > Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
    > > > Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
    > > > Cc: Steven Rostedt <rostedt@goodmis.org>
    > > > Cc: Tim C. Chen <tim.c.chen@intel.com>
    > > > Cc: Valentin Schneider <vschneid@redhat.com>
    > > > Cc: x86@kernel.org
    > > > Cc: linux-pm@vger.kernel.org
    > > > Cc: linux-kernel@vger.kernel.org
    > > > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.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?

    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).

    > >
    > > > +}
    > > > +
    > > > static int alloc_hfi_ipcc_scores(void)
    > > > {
    > > > if (!cpu_feature_enabled(X86_FEATURE_ITD))
    > > > --

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