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

Marco Elver
2022-06-28T10:00:22Z

We can still see that a majority of the time is spent hashing task pointers:

...
16.98% [kernel] [k] rhashtable_jhash2
...

Doing the bookkeeping in toggle_bp_slots() is currently O(#cpus),
calling task_bp_pinned() for each CPU, even if task_bp_pinned() is
CPU-independent. The reason for this is to update the per-CPU
'tsk_pinned' histogram.

To optimize the CPU-independent case to O(1), keep a separate
CPU-independent 'tsk_pinned_all' histogram.

The major source of complexity are transitions between "all
CPU-independent task breakpoints" and "mixed CPU-independent and
CPU-dependent task breakpoints". The code comments list all cases that
require handling.

After this optimization:

| $> perf bench -r 100 breakpoint thread -b 4 -p 128 -t 512
| Total time: 1.758 [sec]
|
| 34.336621 usecs/op
| 4395.087500 usecs/op/cpu

38.08% [kernel] [k] queued_spin_lock_slowpath
10.81% [kernel] [k] smp_cfm_core_cond
3.01% [kernel] [k] update_sg_lb_stats
2.58% [kernel] [k] osq_lock
2.57% [kernel] [k] llist_reverse_order
1.45% [kernel] [k] find_next_bit
1.21% [kernel] [k] flush_tlb_func_common
1.01% [kernel] [k] arch_install_hw_breakpoint

Showing that the time spent hashing keys has become insignificant.

With the given benchmark parameters, that's an improvement of 12%
compared with the old O(#cpus) version.

And finally, using the less aggressive parameters from the preceding
changes, we now observe:

| $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
| Total time: 0.067 [sec]
|
| 35.292187 usecs/op
| 2258.700000 usecs/op/cpu

Which is an improvement of 12% compared to without the histogram
optimizations (baseline is 40 usecs/op). This is now on par with the
theoretical ideal (constraints disabled), and only 12% slower than no
breakpoints at all.

Signed-off-by: Marco Elver <elver@google.com>
---
v2:
* New patch.
---
kernel/events/hw_breakpoint.c | 152 +++++++++++++++++++++++++++-------
1 file changed, 121 insertions(+), 31 deletions(-)

diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index b5180a2ccfbf..31b24e42f2b5 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -66,6 +66,8 @@ static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)

/* Number of pinned CPU breakpoints globally. */
static struct bp_slots_histogram cpu_pinned[TYPE_MAX];
+/* Number of pinned CPU-independent task breakpoints. */
+static struct bp_slots_histogram tsk_pinned_all[TYPE_MAX];

/* Keep track of the breakpoints attached to tasks */
static struct rhltable task_bps_ht;
@@ -200,6 +202,8 @@ static __init int init_breakpoint_slots(void)
for (i = 0; i < TYPE_MAX; i++) {
if (!bp_slots_histogram_alloc(&cpu_pinned[i], i))
goto err;
+ if (!bp_slots_histogram_alloc(&tsk_pinned_all[i], i))
+ goto err;
}

return 0;
@@ -210,8 +214,10 @@ static __init int init_breakpoint_slots(void)
if (err_cpu == cpu)
break;
}
- for (i = 0; i < TYPE_MAX; i++)
+ for (i = 0; i < TYPE_MAX; i++) {
bp_slots_histogram_free(&cpu_pinned[i]);
+ bp_slots_histogram_free(&tsk_pinned_all[i]);
+ }

return -ENOMEM;
}
@@ -245,6 +251,26 @@ bp_slots_histogram_max(struct bp_slots_histogram *hist, enum bp_type_idx type)
return 0;
}

+static int
+bp_slots_histogram_max_merge(struct bp_slots_histogram *hist1, struct bp_slots_histogram *hist2,
+ enum bp_type_idx type)
+{
+ for (int i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
+ const int count1 = atomic_read(&hist1->count[i]);
+ const int count2 = atomic_read(&hist2->count[i]);
+
+ /* Catch unexpected writers; we want a stable snapshot. */
+ ASSERT_EXCLUSIVE_WRITER(hist1->count[i]);
+ ASSERT_EXCLUSIVE_WRITER(hist2->count[i]);
+ if (count1 + count2 > 0)
+ return i + 1;
+ WARN(count1 < 0, "inconsistent breakpoint slots histogram");
+ WARN(count2 < 0, "inconsistent breakpoint slots histogram");
+ }
+
+ return 0;
+}
+
#ifndef hw_breakpoint_weight
static inline int hw_breakpoint_weight(struct perf_event *bp)
{
@@ -273,7 +299,7 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
* toggle_bp_task_slot() to tsk_pinned, and we get a stable snapshot.
*/
lockdep_assert_held_write(&bp_cpuinfo_sem);
- return bp_slots_histogram_max(tsk_pinned, type);
+ return bp_slots_histogram_max_merge(tsk_pinned, &tsk_pinned_all[type], type);
}

/*
@@ -366,40 +392,22 @@ max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
return pinned_slots;
}

-/*
- * Add a pinned breakpoint for the given task in our constraint table
- */
-static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
- enum bp_type_idx type, int weight)
-{
- struct bp_slots_histogram *tsk_pinned = &get_bp_info(cpu, type)->tsk_pinned;
-
- /*
- * If bp->hw.target, tsk_pinned is only modified, but not used
- * otherwise. We can permit concurrent updates as long as there are no
- * other uses: having acquired bp_cpuinfo_sem as a reader allows
- * concurrent updates here. Uses of tsk_pinned will require acquiring
- * bp_cpuinfo_sem as a writer to stabilize tsk_pinned's value.
- */
- lockdep_assert_held_read(&bp_cpuinfo_sem);
- bp_slots_histogram_add(tsk_pinned, task_bp_pinned(cpu, bp, type), weight);
-}
-
/*
* Add/remove the given breakpoint in our constraint table
*/
static int
-toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
- int weight)
+toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type, int weight)
{
- const struct cpumask *cpumask = cpumask_of_bp(bp);
- int cpu;
+ int cpu, next_tsk_pinned;

if (!enable)
weight = -weight;

- /* Pinned counter cpu profiling */
if (!bp->hw.target) {
+ /*
+ * Update the pinned CPU slots, in per-CPU bp_cpuinfo and in the
+ * global histogram.
+ */
struct bp_cpuinfo *info = get_bp_info(bp->cpu, type);

lockdep_assert_held_write(&bp_cpuinfo_sem);
@@ -408,9 +416,91 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
return 0;
}

- /* Pinned counter task profiling */
- for_each_cpu(cpu, cpumask)
- toggle_bp_task_slot(bp, cpu, type, weight);
+ /*
+ * If bp->hw.target, tsk_pinned is only modified, but not used
+ * otherwise. We can permit concurrent updates as long as there are no
+ * other uses: having acquired bp_cpuinfo_sem as a reader allows
+ * concurrent updates here. Uses of tsk_pinned will require acquiring
+ * bp_cpuinfo_sem as a writer to stabilize tsk_pinned's value.
+ */
+ lockdep_assert_held_read(&bp_cpuinfo_sem);
+
+ /*
+ * Update the pinned task slots, in per-CPU bp_cpuinfo and in the global
+ * histogram. We need to take care of 5 cases:
+ *
+ * 1. This breakpoint targets all CPUs (cpu < 0), and there may only
+ * exist other task breakpoints targeting all CPUs. In this case we
+ * can simply update the global slots histogram.
+ *
+ * 2. This breakpoint targets a specific CPU (cpu >= 0), but there may
+ * only exist other task breakpoints targeting all CPUs.
+ *
+ * a. On enable: remove the existing breakpoints from the global
+ * slots histogram and use the per-CPU histogram.
+ *
+ * b. On disable: re-insert the existing breakpoints into the global
+ * slots histogram and remove from per-CPU histogram.
+ *
+ * 3. Some other existing task breakpoints target specific CPUs. Only
+ * update the per-CPU slots histogram.
+ */
+
+ if (!enable) {
+ /*
+ * Remove before updating histograms so we can determine if this
+ * was the last task breakpoint for a specific CPU.
+ */
+ int ret = rhltable_remove(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
+
+ if (ret)
+ return ret;
+ }
+ /*
+ * Note: If !enable, next_tsk_pinned will not count the to-be-removed breakpoint.
+ */
+ next_tsk_pinned = task_bp_pinned(-1, bp, type);
+
+ if (next_tsk_pinned >= 0) {
+ if (bp->cpu < 0) { /* Case 1: fast path */
+ if (!enable)
+ next_tsk_pinned += hw_breakpoint_weight(bp);
+ bp_slots_histogram_add(&tsk_pinned_all[type], next_tsk_pinned, weight);
+ } else if (enable) { /* Case 2.a: slow path */
+ /* Add existing to per-CPU histograms. */
+ for_each_possible_cpu(cpu) {
+ bp_slots_histogram_add(&get_bp_info(cpu, type)->tsk_pinned,
+ 0, next_tsk_pinned);
+ }
+ /* Add this first CPU-pinned task breakpoint. */
+ bp_slots_histogram_add(&get_bp_info(bp->cpu, type)->tsk_pinned,
+ next_tsk_pinned, weight);
+ /* Rebalance global task pinned histogram. */
+ bp_slots_histogram_add(&tsk_pinned_all[type], next_tsk_pinned,
+ -next_tsk_pinned);
+ } else { /* Case 2.b: slow path */
+ /* Remove this last CPU-pinned task breakpoint. */
+ bp_slots_histogram_add(&get_bp_info(bp->cpu, type)->tsk_pinned,
+ next_tsk_pinned + hw_breakpoint_weight(bp), weight);
+ /* Remove all from per-CPU histograms. */
+ for_each_possible_cpu(cpu) {
+ bp_slots_histogram_add(&get_bp_info(cpu, type)->tsk_pinned,
+ next_tsk_pinned, -next_tsk_pinned);
+ }
+ /* Rebalance global task pinned histogram. */
+ bp_slots_histogram_add(&tsk_pinned_all[type], 0, next_tsk_pinned);
+ }
+ } else { /* Case 3: slow path */
+ const struct cpumask *cpumask = cpumask_of_bp(bp);
+
+ for_each_cpu(cpu, cpumask) {
+ next_tsk_pinned = task_bp_pinned(cpu, bp, type);
+ if (!enable)
+ next_tsk_pinned += hw_breakpoint_weight(bp);
+ bp_slots_histogram_add(&get_bp_info(cpu, type)->tsk_pinned,
+ next_tsk_pinned, weight);
+ }
+ }

/*
* Readers want a stable snapshot of the per-task breakpoint list.
@@ -419,8 +509,8 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,

if (enable)
return rhltable_insert(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
- else
- return rhltable_remove(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
+
+ return 0;
}

__weak int arch_reserve_bp_slot(struct perf_event *bp)
--
2.37.0.rc0.161.g10f37bed90-goog