Re: [PATCH v4 14/24] thermal: intel: hfi: Store per-CPU IPCC scores

[Ricardo Neri]

On Thu, Jun 29, 2023 at 08:53:31PM +0200, Rafael J. Wysocki wrote:
> On Tue, Jun 13, 2023 at 6:23 AM Ricardo Neri
> <ricardo.neri-calderon@xxxxxxxxxxxxxxx> wrote:
> >
> > The scheduler reads the IPCC scores when balancing load. These reads can
> > occur frequently and originate from many CPUs. Hardware may also
> > occasionally update the HFI table. Controlling access with locks would
> > cause contention.
> >
> > Cache the IPCC scores in separate per-CPU variables that the scheduler can
> > use. Use a seqcount to synchronize memory accesses to these cached values.
> > This eliminates the need for locks, as the sequence counter provides the
> > memory ordering required to prevent the use of stale data.
> >
> > The HFI delayed workqueue guarantees that only one CPU writes the cached
> > IPCC scores. The frequency of updates is low (every CONFIG_HZ jiffies or
> > less), and the number of writes per update is in the order of tens. Writes
> > should not starve reads.
> >
> > Only cache IPCC scores in this changeset. A subsequent changeset will
> > use these scores.
> >
> > Cc: Ben Segall <bsegall@xxxxxxxxxx>
> > Cc: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
> > Cc: Dietmar Eggemann <dietmar.eggemann@xxxxxxx>
> > Cc: Ionela Voinescu <ionela.voinescu@xxxxxxx>
> > Cc: Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
> > Cc: Len Brown <len.brown@xxxxxxxxx>
> > Cc: Lukasz Luba <lukasz.luba@xxxxxxx>
> > Cc: Mel Gorman <mgorman@xxxxxxx>
> > Cc: Perry Yuan <Perry.Yuan@xxxxxxx>
> > Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> > Cc: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Cc: Tim C. Chen <tim.c.chen@xxxxxxxxx>
> > Cc: Valentin Schneider <vschneid@xxxxxxxxxx>
> > Cc: Zhao Liu <zhao1.liu@xxxxxxxxxxxxxxx>
> > Cc: x86@xxxxxxxxxx
> > Cc: linux-pm@xxxxxxxxxxxxxxx
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Suggested-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
> > ---
> > Changes since v3:
> >  * As Rafael requested, I reworked the memory ordering of the cached IPCC
> >    scores. I selected a seqcount as is less expensive than a memory
> >    barrier, which is not necessary anyways.
> >  * Made alloc_hfi_ipcc_scores() return -ENOMEM on allocation failure.
> >    (Rafael)
> >  * Added a comment to describe hfi_ipcc_scores. (Rafael)
> >
> > Changes since v2:
> >  * Only create these per-CPU variables when Intel Thread Director is
> >    supported.
> >
> > Changes since v1:
> >  * Added this patch.
> > ---
> >  drivers/thermal/intel/intel_hfi.c | 66 +++++++++++++++++++++++++++++++
> >  1 file changed, 66 insertions(+)
> >
> > diff --git a/drivers/thermal/intel/intel_hfi.c b/drivers/thermal/intel/intel_hfi.c
> > index 20ee4264dcd4..d822ed0bb5c1 100644
> > --- a/drivers/thermal/intel/intel_hfi.c
> > +++ b/drivers/thermal/intel/intel_hfi.c
> > @@ -29,9 +29,11 @@
> >  #include <linux/kernel.h>
> >  #include <linux/math.h>
> >  #include <linux/mutex.h>
> > +#include <linux/percpu.h>
> >  #include <linux/percpu-defs.h>
> >  #include <linux/printk.h>
> >  #include <linux/processor.h>
> > +#include <linux/seqlock.h>
> >  #include <linux/slab.h>
> >  #include <linux/spinlock.h>
> >  #include <linux/string.h>
> > @@ -180,6 +182,62 @@ static struct workqueue_struct *hfi_updates_wq;
> >  #define HFI_UPDATE_INTERVAL            HZ
> >  #define HFI_MAX_THERM_NOTIFY_COUNT     16
> >
> > +/* A cache of the HFI perf capabilities for lockless access. */
> > +static int __percpu *hfi_ipcc_scores;
> > +/* Sequence counter for hfi_ipcc_scores */
> > +static seqcount_t hfi_ipcc_seqcount = SEQCNT_ZERO(hfi_ipcc_seqcount);
> > +
> > +static int alloc_hfi_ipcc_scores(void)
> > +{
> > +       if (!cpu_feature_enabled(X86_FEATURE_ITD))
> > +               return 0;
> > +
> > +       hfi_ipcc_scores = __alloc_percpu(sizeof(*hfi_ipcc_scores) *
> > +                                        hfi_features.nr_classes,
> > +                                        sizeof(*hfi_ipcc_scores));
> > +
> > +       return hfi_ipcc_scores ? 0 : -ENOMEM;
> > +}
> 
> This doesn't need to return an int.  It could be a bool function
> returning !!hfi_ipcc_scores (or false for the feature missing case).

Sure Rafael, I can make this change.

> 
> Apart from this minor thing, the patch looks reasonable to me.

Thank you for your review!