Re: [PATCH v2 11/13] perf/hw_breakpoint: Introduce bp_slots_histogram

[Dmitry Vyukov <dvyukov@google.com>]

On Tue, 28 Jun 2022 at 11:59, Marco Elver <elver@google.com> wrote:
>
> Factor out the existing `atomic_t count[N]` into its own struct called
> 'bp_slots_histogram', to generalize and make its intent clearer in
> preparation of reusing elsewhere. The basic idea of bucketing "total
> uses of N slots" resembles a histogram, so calling it such seems most
> intuitive.
>
> No functional change.
>
> Signed-off-by: Marco Elver <elver@google.com>

Reviewed-by: Dmitry Vyukov <dvyukov@google.com>

> ---
> v2:
> * New patch.
> ---
>  kernel/events/hw_breakpoint.c | 94 +++++++++++++++++++++++------------
>  1 file changed, 62 insertions(+), 32 deletions(-)
>
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index 128ba3429223..18886f115abc 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -36,19 +36,27 @@
>  #include <linux/slab.h>
>
>  /*
> - * Constraints data
> + * Datastructure to track the total uses of N slots across tasks or CPUs;
> + * bp_slots_histogram::count[N] is the number of assigned N+1 breakpoint slots.
>   */
> -struct bp_cpuinfo {
> -       /* Number of pinned cpu breakpoints in a cpu */
> -       unsigned int    cpu_pinned;
> -       /* tsk_pinned[n] is the number of tasks having n+1 breakpoints */
> +struct bp_slots_histogram {
>  #ifdef hw_breakpoint_slots
> -       atomic_t        tsk_pinned[hw_breakpoint_slots(0)];
> +       atomic_t count[hw_breakpoint_slots(0)];
>  #else
> -       atomic_t        *tsk_pinned;
> +       atomic_t *count;
>  #endif
>  };
>
> +/*
> + * Per-CPU constraints data.
> + */
> +struct bp_cpuinfo {
> +       /* Number of pinned CPU breakpoints in a CPU. */
> +       unsigned int                    cpu_pinned;
> +       /* Histogram of pinned task breakpoints in a CPU. */
> +       struct bp_slots_histogram       tsk_pinned;
> +};
> +
>  static DEFINE_PER_CPU(struct bp_cpuinfo, bp_cpuinfo[TYPE_MAX]);
>
>  static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
> @@ -159,6 +167,18 @@ static inline int hw_breakpoint_slots_cached(int type)
>         return __nr_bp_slots[type];
>  }
>
> +static __init bool
> +bp_slots_histogram_alloc(struct bp_slots_histogram *hist, enum bp_type_idx type)
> +{
> +       hist->count = kcalloc(hw_breakpoint_slots_cached(type), sizeof(*hist->count), GFP_KERNEL);
> +       return hist->count;
> +}
> +
> +static __init void bp_slots_histogram_free(struct bp_slots_histogram *hist)
> +{
> +       kfree(hist->count);
> +}
> +
>  static __init int init_breakpoint_slots(void)
>  {
>         int i, cpu, err_cpu;
> @@ -170,8 +190,7 @@ static __init int init_breakpoint_slots(void)
>                 for (i = 0; i < TYPE_MAX; i++) {
>                         struct bp_cpuinfo *info = get_bp_info(cpu, i);
>
> -                       info->tsk_pinned = kcalloc(__nr_bp_slots[i], sizeof(atomic_t), GFP_KERNEL);
> -                       if (!info->tsk_pinned)
> +                       if (!bp_slots_histogram_alloc(&info->tsk_pinned, i))
>                                 goto err;
>                 }
>         }
> @@ -180,7 +199,7 @@ static __init int init_breakpoint_slots(void)
>  err:
>         for_each_possible_cpu(err_cpu) {
>                 for (i = 0; i < TYPE_MAX; i++)
> -                       kfree(get_bp_info(err_cpu, i)->tsk_pinned);
> +                       bp_slots_histogram_free(&get_bp_info(err_cpu, i)->tsk_pinned);
>                 if (err_cpu == cpu)
>                         break;
>         }
> @@ -189,6 +208,34 @@ static __init int init_breakpoint_slots(void)
>  }
>  #endif
>
> +static inline void
> +bp_slots_histogram_add(struct bp_slots_histogram *hist, int old, int val)
> +{
> +       const int old_idx = old - 1;
> +       const int new_idx = old_idx + val;
> +
> +       if (old_idx >= 0)
> +               atomic_dec(&hist->count[old_idx]);
> +       if (new_idx >= 0)
> +               atomic_inc(&hist->count[new_idx]);
> +}
> +
> +static int
> +bp_slots_histogram_max(struct bp_slots_histogram *hist, enum bp_type_idx type)
> +{
> +       for (int i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
> +               const int count = atomic_read(&hist->count[i]);
> +
> +               /* Catch unexpected writers; we want a stable snapshot. */
> +               ASSERT_EXCLUSIVE_WRITER(hist->count[i]);
> +               if (count > 0)
> +                       return i + 1;
> +               WARN(count < 0, "inconsistent breakpoint slots histogram");
> +       }
> +
> +       return 0;
> +}
> +
>  #ifndef hw_breakpoint_weight
>  static inline int hw_breakpoint_weight(struct perf_event *bp)
>  {
> @@ -205,13 +252,11 @@ static inline enum bp_type_idx find_slot_idx(u64 bp_type)
>  }
>
>  /*
> - * Report the maximum number of pinned breakpoints a task
> - * have in this cpu
> + * Return the maximum number of pinned breakpoints a task has in this CPU.
>   */
>  static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
>  {
> -       atomic_t *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
> -       int i;
> +       struct bp_slots_histogram *tsk_pinned = &get_bp_info(cpu, type)->tsk_pinned;
>
>         /*
>          * At this point we want to have acquired the bp_cpuinfo_sem as a
> @@ -219,14 +264,7 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
>          * toggle_bp_task_slot() to tsk_pinned, and we get a stable snapshot.
>          */
>         lockdep_assert_held_write(&bp_cpuinfo_sem);
> -
> -       for (i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
> -               ASSERT_EXCLUSIVE_WRITER(tsk_pinned[i]); /* Catch unexpected writers. */
> -               if (atomic_read(&tsk_pinned[i]) > 0)
> -                       return i + 1;
> -       }
> -
> -       return 0;
> +       return bp_slots_histogram_max(tsk_pinned, type);
>  }
>
>  /*
> @@ -300,8 +338,7 @@ max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
>  static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
>                                 enum bp_type_idx type, int weight)
>  {
> -       atomic_t *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
> -       int old_idx, new_idx;
> +       struct bp_slots_histogram *tsk_pinned = &get_bp_info(cpu, type)->tsk_pinned;
>
>         /*
>          * If bp->hw.target, tsk_pinned is only modified, but not used
> @@ -311,14 +348,7 @@ static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
>          * bp_cpuinfo_sem as a writer to stabilize tsk_pinned's value.
>          */
>         lockdep_assert_held_read(&bp_cpuinfo_sem);
> -
> -       old_idx = task_bp_pinned(cpu, bp, type) - 1;
> -       new_idx = old_idx + weight;
> -
> -       if (old_idx >= 0)
> -               atomic_dec(&tsk_pinned[old_idx]);
> -       if (new_idx >= 0)
> -               atomic_inc(&tsk_pinned[new_idx]);
> +       bp_slots_histogram_add(tsk_pinned, task_bp_pinned(cpu, bp, type), weight);
>  }
>
>  /*
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>