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

Marco Elver
2022-06-28T10:00:21Z

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