On a machine with 256 CPUs, running the recently added perf breakpoint
benchmark results in:

 | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
 |      Total time: 236.418 [sec]
 |
 |   123134.794271 usecs/op
 |  7880626.833333 usecs/op/cpu

The benchmark tests inherited breakpoint perf events across many
threads.

Looking at a perf profile, we can see that the majority of the time is
spent in various hw_breakpoint.c functions, which execute within the
'nr_bp_mutex' critical sections which then results in contention on that
mutex as well:

    37.27%  [kernel]       [k] osq_lock
    34.92%  [kernel]       [k] mutex_spin_on_owner
    12.15%  [kernel]       [k] toggle_bp_slot
    11.90%  [kernel]       [k] __reserve_bp_slot

The culprit here is task_bp_pinned(), which has a runtime complexity of
O(#tasks) due to storing all task breakpoints in the same list and
iterating through that list looking for a matching task. Clearly, this
does not scale to thousands of tasks.

Instead, make use of the "rhashtable" variant "rhltable" which stores
multiple items with the same key in a list. This results in average
runtime complexity of O(1) for task_bp_pinned().

With the optimization, the benchmark shows:

 | $> perf bench -r 30 breakpoint thread -b 4 -p 64 -t 64
 | # Running 'breakpoint/thread' benchmark:
 | # Created/joined 30 threads with 4 breakpoints and 64 parallelism
 |      Total time: 0.208 [sec]
 |
 |      108.422396 usecs/op
 |     6939.033333 usecs/op/cpu

On this particular setup that's a speedup of ~1135x.

While one option would be to make task_struct a breakpoint list node,
this would only further bloat task_struct for infrequently used data.
Furthermore, after all optimizations in this series, there's no evidence
it would result in better performance: later optimizations make the time
spent looking up entries in the hash table negligible (we'll reach the
theoretical ideal performance i.e. no constraints).

Signed-off-by: Marco Elver <elver@google.com>
Reviewed-by: Dmitry Vyukov <dvyukov@google.com>
---
v2:
* Commit message tweaks.
---
 include/linux/perf_event.h    |  3 +-
 kernel/events/hw_breakpoint.c | 56 ++++++++++++++++++++++-------------
 2 files changed, 37 insertions(+), 22 deletions(-)

diff --git a/include/linux/perf_event.h b/include/linux/perf_event.h
index 01231f1d976c..e27360436dc6 100644
--- a/include/linux/perf_event.h
+++ b/include/linux/perf_event.h

@@ -36,6 +36,7 @@ struct perf_guest_info_callbacks {
 };

 #ifdef CONFIG_HAVE_HW_BREAKPOINT
+#include <linux/rhashtable-types.h>
 #include <asm/hw_breakpoint.h>
 #endif

@@ -178,7 +179,7 @@ struct hw_perf_event {
                         * creation and event initalization.
                         */
                        struct arch_hw_breakpoint       info;
-                       struct list_head                bp_list;
+                       struct rhlist_head              bp_list;

                };
 #endif
                struct { /* amd_iommu */

diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index 6076c6346291..6d09edc80d19 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c

@@ -26,10 +26,10 @@
 #include <linux/irqflags.h>
 #include <linux/kdebug.h>
 #include <linux/kernel.h>
-#include <linux/list.h>
 #include <linux/mutex.h>
 #include <linux/notifier.h>
 #include <linux/percpu.h>
+#include <linux/rhashtable.h>
 #include <linux/sched.h>
 #include <linux/slab.h>

@@ -54,7 +54,13 @@ static struct bp_cpuinfo *get_bp_info(int cpu, enum bp_type_idx type)
 }

 /* Keep track of the breakpoints attached to tasks */
-static LIST_HEAD(bp_task_head);
+static struct rhltable task_bps_ht;
+static const struct rhashtable_params task_bps_ht_params = {
+       .head_offset = offsetof(struct hw_perf_event, bp_list),
+       .key_offset = offsetof(struct hw_perf_event, target),
+       .key_len = sizeof_field(struct hw_perf_event, target),
+       .automatic_shrinking = true,
+};

 static int constraints_initialized;

@@ -103,17 +109,23 @@ static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
  */
 static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type)
 {
-       struct task_struct *tsk = bp->hw.target;
+       struct rhlist_head *head, *pos;
        struct perf_event *iter;
        int count = 0;

-       list_for_each_entry(iter, &bp_task_head, hw.bp_list) {
-               if (iter->hw.target == tsk &&
-                   find_slot_idx(iter->attr.bp_type) == type &&
+       rcu_read_lock();
+       head = rhltable_lookup(&task_bps_ht, &bp->hw.target, task_bps_ht_params);
+       if (!head)
+               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);
        }

+out:
+       rcu_read_unlock();
        return count;
 }

@@ -186,7 +198,7 @@ static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
 /*
  * Add/remove the given breakpoint in our constraint table
  */
-static void
+static int
 toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
               int weight)
 {

@@ -199,7 +211,7 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
        /* Pinned counter cpu profiling */
        if (!bp->hw.target) {
                get_bp_info(bp->cpu, type)->cpu_pinned += weight;
-               return;
+               return 0;
        }

        /* Pinned counter task profiling */

@@ -207,9 +219,9 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
                toggle_bp_task_slot(bp, cpu, type, weight);

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

 __weak int arch_reserve_bp_slot(struct perf_event *bp)

@@ -307,9 +319,7 @@ static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type)
        if (ret)
                return ret;

-       toggle_bp_slot(bp, true, type, weight);
-
-       return 0;
+       return toggle_bp_slot(bp, true, type, weight);
 }

 int reserve_bp_slot(struct perf_event *bp)

@@ -334,7 +344,7 @@ static void __release_bp_slot(struct perf_event *bp, u64 bp_type)

        type = find_slot_idx(bp_type);
        weight = hw_breakpoint_weight(bp);
-       toggle_bp_slot(bp, false, type, weight);
+       WARN_ON(toggle_bp_slot(bp, false, type, weight));
 }

 void release_bp_slot(struct perf_event *bp)

@@ -707,7 +717,7 @@ static struct pmu perf_breakpoint = {
 int __init init_hw_breakpoint(void)
 {
        int cpu, err_cpu;
-       int i;
+       int i, ret;

        for (i = 0; i < TYPE_MAX; i++)
                nr_slots[i] = hw_breakpoint_slots(i);

@@ -718,18 +728,24 @@ int __init init_hw_breakpoint(void)

                        info->tsk_pinned = kcalloc(nr_slots[i], sizeof(int),
                                                        GFP_KERNEL);
-                       if (!info->tsk_pinned)
-                               goto err_alloc;
+                       if (!info->tsk_pinned) {
+                               ret = -ENOMEM;
+                               goto err;
+                       }
                }
        }

+       ret = rhltable_init(&task_bps_ht, &task_bps_ht_params);
+       if (ret)
+               goto err;
+
        constraints_initialized = 1;

        perf_pmu_register(&perf_breakpoint, "breakpoint", PERF_TYPE_BREAKPOINT);

        return register_die_notifier(&hw_breakpoint_exceptions_nb);

- err_alloc:
+err:
        for_each_possible_cpu(err_cpu) {
                for (i = 0; i < TYPE_MAX; i++)
                        kfree(get_bp_info(err_cpu, i)->tsk_pinned);

@@ -737,7 +753,5 @@ int __init init_hw_breakpoint(void)
                        break;
        }

-       return -ENOMEM;
+       return ret;
 }
-
-
--

2.37.0.rc0.161.g10f37bed90-goog