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

From: Dmitry Vyukov
Date: Tue Jun 28 2022 - 11:41:46 EST

On Tue, 28 Jun 2022 at 11:59, Marco Elver <elver@xxxxxxxxxx> 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@xxxxxxxxxx>

Reviewed-by: Dmitry Vyukov <dvyukov@xxxxxxxxxx>

> ---
> 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
>