Re: [PATCH v2 12/13] perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent task targets

[Dmitry Vyukov <dvyukov@google.com>]

On Tue, 28 Jun 2022 at 11:59, Marco Elver <elver@google.com> wrote:
>
> Running the perf benchmark with (note: more aggressive parameters vs.
> preceding changes, but same 256 CPUs host):
>
>  | $> perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512
>  | # Running 'breakpoint/thread' benchmark:
>  | # Created/joined 100 threads with 4 breakpoints and 128 parallelism
>  |      Total time: 1.989 [sec]
>  |
>  |       38.854160 usecs/op
>  |     4973.332500 usecs/op/cpu
>
>     20.43%  [kernel]       [k] queued_spin_lock_slowpath
>     18.75%  [kernel]       [k] osq_lock
>     16.98%  [kernel]       [k] rhashtable_jhash2
>      8.34%  [kernel]       [k] task_bp_pinned
>      4.23%  [kernel]       [k] smp_cfm_core_cond
>      3.65%  [kernel]       [k] bcmp
>      2.83%  [kernel]       [k] toggle_bp_slot
>      1.87%  [kernel]       [k] find_next_bit
>      1.49%  [kernel]       [k] __reserve_bp_slot
>
> We can see that a majority of the time is now spent hashing task
> pointers to index into task_bps_ht in task_bp_pinned().
>
> Obtaining the max_bp_pinned_slots() for CPU-independent task targets
> currently is O(#cpus), and calls task_bp_pinned() for each CPU, even if
> the result of task_bp_pinned() is CPU-independent.
>
> The loop in max_bp_pinned_slots() wants to compute the maximum slots
> across all CPUs. If task_bp_pinned() is CPU-independent, we can do so by
> obtaining the max slots across all CPUs and adding task_bp_pinned().
>
> To do so in O(1), use a bp_slots_histogram for CPU-pinned slots.
>
> After this optimization:
>
>  | $> perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512
>  | # Running 'breakpoint/thread' benchmark:
>  | # Created/joined 100 threads with 4 breakpoints and 128 parallelism
>  |      Total time: 1.930 [sec]
>  |
>  |       37.697832 usecs/op
>  |     4825.322500 usecs/op/cpu
>
>     19.13%  [kernel]       [k] queued_spin_lock_slowpath
>     18.21%  [kernel]       [k] rhashtable_jhash2
>     15.46%  [kernel]       [k] osq_lock
>      6.27%  [kernel]       [k] toggle_bp_slot
>      5.91%  [kernel]       [k] task_bp_pinned
>      5.05%  [kernel]       [k] smp_cfm_core_cond
>      1.78%  [kernel]       [k] update_sg_lb_stats
>      1.36%  [kernel]       [k] llist_reverse_order
>      1.34%  [kernel]       [k] find_next_bit
>      1.19%  [kernel]       [k] bcmp
>
> Suggesting that time spent in task_bp_pinned() has been reduced.
> However, we're still hashing too much, which will be addressed in the
> subsequent change.
>
> Signed-off-by: Marco Elver <elver@google.com>

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

> ---
> v2:
> * New patch.
> ---
>  kernel/events/hw_breakpoint.c | 45 +++++++++++++++++++++++++++++++----
>  1 file changed, 41 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
> index 18886f115abc..b5180a2ccfbf 100644
> --- a/kernel/events/hw_breakpoint.c
> +++ b/kernel/events/hw_breakpoint.c
> @@ -64,6 +64,9 @@ static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
>         return per_cpu_ptr(bp_cpuinfo + type, cpu);
>  }
>
> +/* Number of pinned CPU breakpoints globally. */
> +static struct bp_slots_histogram cpu_pinned[TYPE_MAX];
> +
>  /* Keep track of the breakpoints attached to tasks */
>  static struct rhltable task_bps_ht;
>  static const struct rhashtable_params task_bps_ht_params = {
> @@ -194,6 +197,10 @@ static __init int init_breakpoint_slots(void)
>                                 goto err;
>                 }
>         }
> +       for (i = 0; i < TYPE_MAX; i++) {
> +               if (!bp_slots_histogram_alloc(&cpu_pinned[i], i))
> +                       goto err;
> +       }
>
>         return 0;
>  err:
> @@ -203,6 +210,8 @@ static __init int init_breakpoint_slots(void)
>                 if (err_cpu == cpu)
>                         break;
>         }
> +       for (i = 0; i < TYPE_MAX; i++)
> +               bp_slots_histogram_free(&cpu_pinned[i]);
>
>         return -ENOMEM;
>  }
> @@ -270,6 +279,9 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
>  /*
>   * Count the number of breakpoints of the same type and same task.
>   * The given event must be not on the list.
> + *
> + * If @cpu is -1, but the result of task_bp_pinned() is not CPU-independent,
> + * returns a negative value.
>   */
>  static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type)
>  {
> @@ -288,9 +300,18 @@ static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type)
>                 goto out;
>
>         rhl_for_each_entry_rcu(iter, pos, head, hw.bp_list) {
> -               if (find_slot_idx(iter->attr.bp_type) == type &&
> -                   (iter->cpu < 0 || cpu == iter->cpu))
> -                       count += hw_breakpoint_weight(iter);
> +               if (find_slot_idx(iter->attr.bp_type) != type)
> +                       continue;
> +
> +               if (iter->cpu >= 0) {
> +                       if (cpu == -1) {
> +                               count = -1;
> +                               goto out;
> +                       } else if (cpu != iter->cpu)
> +                               continue;
> +               }
> +
> +               count += hw_breakpoint_weight(iter);
>         }
>
>  out:
> @@ -316,6 +337,19 @@ max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
>         int pinned_slots = 0;
>         int cpu;
>
> +       if (bp->hw.target && bp->cpu < 0) {
> +               int max_pinned = task_bp_pinned(-1, bp, type);
> +
> +               if (max_pinned >= 0) {
> +                       /*
> +                        * Fast path: task_bp_pinned() is CPU-independent and
> +                        * returns the same value for any CPU.
> +                        */
> +                       max_pinned += bp_slots_histogram_max(&cpu_pinned[type], type);
> +                       return max_pinned;
> +               }
> +       }
> +
>         for_each_cpu(cpu, cpumask) {
>                 struct bp_cpuinfo *info = get_bp_info(cpu, type);
>                 int nr;
> @@ -366,8 +400,11 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
>
>         /* Pinned counter cpu profiling */
>         if (!bp->hw.target) {
> +               struct bp_cpuinfo *info = get_bp_info(bp->cpu, type);
> +
>                 lockdep_assert_held_write(&bp_cpuinfo_sem);
> -               get_bp_info(bp->cpu, type)->cpu_pinned += weight;
> +               bp_slots_histogram_add(&cpu_pinned[type], info->cpu_pinned, weight);
> +               info->cpu_pinned += weight;
>                 return 0;
>         }
>
> --
> 2.37.0.rc0.161.g10f37bed90-goog
>