Messages in this thread Patch in this message | | | Date | Tue, 28 Jun 2022 11:58:32 +0200 | Subject | [PATCH v2 12/13] perf/hw_breakpoint: Optimize max_bp_pinned_slots() for CPU-independent task targets | From | Marco Elver <> |
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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> --- 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
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