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

[Rafael J. Wysocki]

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).

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

> +
> +static void set_hfi_ipcc_scores(struct hfi_instance *hfi_instance)
> +{
> +       int cpu;
> +
> +       if (!cpu_feature_enabled(X86_FEATURE_ITD))
> +               return;
> +
> +       /*
> +        * Serialize with writes to the HFI table. It also protects the write
> +        * loop against seqcount readers running in interrupt context.
> +        */
> +       raw_spin_lock_irq(&hfi_instance->table_lock);
> +       /*
> +        * The seqcount implies store-release semantics to order stores with
> +        * lockless loads from the seqcount read side. It also implies a
> +        * compiler barrier.
> +        */
> +       write_seqcount_begin(&hfi_ipcc_seqcount);
> +       for_each_cpu(cpu, hfi_instance->cpus) {
> +               int c, *scores;
> +               s16 index;
> +
> +               index = per_cpu(hfi_cpu_info, cpu).index;
> +               scores = per_cpu_ptr(hfi_ipcc_scores, cpu);
> +
> +               for (c = 0;  c < hfi_features.nr_classes; c++) {
> +                       struct hfi_cpu_data *caps;
> +
> +                       caps = hfi_instance->data +
> +                              index * hfi_features.cpu_stride +
> +                              c * hfi_features.class_stride;
> +                       scores[c] = caps->perf_cap;
> +               }
> +       }
> +
> +       write_seqcount_end(&hfi_ipcc_seqcount);
> +       raw_spin_unlock_irq(&hfi_instance->table_lock);
> +}
> +
>  /**
>   * intel_hfi_read_classid() - Read the currrent classid
>   * @classid:   Variable to which the classid will be written.
> @@ -275,6 +333,8 @@ static void update_capabilities(struct hfi_instance *hfi_instance)
>                 thermal_genl_cpu_capability_event(cpu_count, &cpu_caps[i]);
>
>         kfree(cpu_caps);
> +
> +       set_hfi_ipcc_scores(hfi_instance);
>  out:
>         mutex_unlock(&hfi_instance_lock);
>  }
> @@ -618,8 +678,14 @@ void __init intel_hfi_init(void)
>         if (!hfi_updates_wq)
>                 goto err_nomem;
>
> +       if (alloc_hfi_ipcc_scores())
> +               goto err_ipcc;
> +
>         return;
>
> +err_ipcc:
> +       destroy_workqueue(hfi_updates_wq);
> +
>  err_nomem:
>         for (j = 0; j < i; ++j) {
>                 hfi_instance = &hfi_instances[j];
> --
> 2.25.1
>