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    Subject[PATCH v3 13/14] perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent task targets
    From
    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>
    ---
    v3:
    * Update hw_breakpoint_is_used() to include global cpu_pinned.

    v2:
    * New patch.
    ---
    kernel/events/hw_breakpoint.c | 57 ++++++++++++++++++++++++++++++++---
    1 file changed, 53 insertions(+), 4 deletions(-)

    diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
    index 03ebecf048c0..a489f31fe147 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;
    }

    @@ -804,6 +841,18 @@ bool hw_breakpoint_is_used(void)
    }
    }

    + for (int type = 0; type < TYPE_MAX; ++type) {
    + for (int slot = 0; slot < hw_breakpoint_slots_cached(type); ++slot) {
    + /*
    + * Warn, because if there are CPU pinned counters,
    + * should never get here; bp_cpuinfo::cpu_pinned should
    + * be consistent with the global cpu_pinned histogram.
    + */
    + if (WARN_ON(atomic_read(&cpu_pinned[type].count[slot])))
    + return true;
    + }
    + }
    +
    return false;
    }

    --
    2.37.0.rc0.161.g10f37bed90-goog
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