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

Dmitry Vyukov
2022-06-28T14:53:38Z

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
>