[PATCH v2 10/13] perf/hw_breakpoint: Reduce contention with large number of tasks

Marco Elver
2022-06-28T10:00:15Z

While optimizing task_bp_pinned()'s runtime complexity to O(1) on
average helps reduce time spent in the critical section, we still suffer
due to serializing everything via 'nr_bp_mutex'. Indeed, a profile shows
that now contention is the biggest issue:

95.93% [kernel] [k] osq_lock
0.70% [kernel] [k] mutex_spin_on_owner
0.22% [kernel] [k] smp_cfm_core_cond
0.18% [kernel] [k] task_bp_pinned
0.18% [kernel] [k] rhashtable_jhash2
0.15% [kernel] [k] queued_spin_lock_slowpath

when running the breakpoint benchmark with (system with 256 CPUs):

| $> 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: 0.207 [sec]
|
| 108.267188 usecs/op
| 6929.100000 usecs/op/cpu

The main concern for synchronizing the breakpoint constraints data is
that a consistent snapshot of the per-CPU and per-task data is observed.

The access pattern is as follows:

1. If the target is a task: the task's pinned breakpoints are counted,
checked for space, and then appended to; only bp_cpuinfo::cpu_pinned
is used to check for conflicts with CPU-only breakpoints;
bp_cpuinfo::tsk_pinned are incremented/decremented, but otherwise
unused.

2. If the target is a CPU: bp_cpuinfo::cpu_pinned are counted, along
with bp_cpuinfo::tsk_pinned; after a successful check, cpu_pinned is
incremented. No per-task breakpoints are checked.

Since rhltable safely synchronizes insertions/deletions, we can allow
concurrency as follows:

1. If the target is a task: independent tasks may update and check the
constraints concurrently, but same-task target calls need to be
serialized; since bp_cpuinfo::tsk_pinned is only updated, but not
checked, these modifications can happen concurrently by switching
tsk_pinned to atomic_t.

2. If the target is a CPU: access to the per-CPU constraints needs to
be serialized with other CPU-target and task-target callers (to
stabilize the bp_cpuinfo::tsk_pinned snapshot).

We can allow the above concurrency by introducing a per-CPU constraints
data reader-writer lock (bp_cpuinfo_sem), and per-task mutexes (reuses
task_struct::perf_event_mutex):

1. If the target is a task: acquires perf_event_mutex, and acquires
bp_cpuinfo_sem as a reader. The choice of percpu-rwsem minimizes
contention in the presence of many read-lock but few write-lock
acquisitions: we assume many orders of magnitude more task target
breakpoints creations/destructions than CPU target breakpoints.

2. If the target is a CPU: acquires bp_cpuinfo_sem as a writer.

With these changes, contention with thousands of tasks is reduced to the
point where waiting on locking no longer dominates the profile:

| $> 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: 0.077 [sec]
|
| 40.201563 usecs/op
| 2572.900000 usecs/op/cpu

21.54% [kernel] [k] task_bp_pinned
20.18% [kernel] [k] rhashtable_jhash2
6.81% [kernel] [k] toggle_bp_slot
5.47% [kernel] [k] queued_spin_lock_slowpath
3.75% [kernel] [k] smp_cfm_core_cond
3.48% [kernel] [k] bcmp

On this particular setup that's a speedup of 2.7x.

We're also getting closer to the theoretical ideal performance through
optimizations in hw_breakpoint.c -- constraints accounting disabled:

| 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: 0.067 [sec]
|
| 35.286458 usecs/op
| 2258.333333 usecs/op/cpu

Which means the current implementation is ~12% slower than the
theoretical ideal.

For reference, performance without any breakpoints:

| $> bench -r 30 breakpoint thread -b 0 -p 64 -t 64
| # Running 'breakpoint/thread' benchmark:
| # Created/joined 30 threads with 0 breakpoints and 64 parallelism
| Total time: 0.060 [sec]
|
| 31.365625 usecs/op
| 2007.400000 usecs/op/cpu

On a system with 256 CPUs, the theoretical ideal is only ~12% slower
than no breakpoints at all; the current implementation is ~28% slower.

Signed-off-by: Marco Elver <elver@google.com>
---
v2:
* Use percpu-rwsem instead of rwlock.
* Use task_struct::perf_event_mutex. See code comment for reasoning.
==> Speedup of 2.7x (vs 2.5x in v1).
---
kernel/events/hw_breakpoint.c | 159 ++++++++++++++++++++++++++++------
1 file changed, 132 insertions(+), 27 deletions(-)

diff --git a/kernel/events/hw_breakpoint.c b/kernel/events/hw_breakpoint.c
index 63e39dc836bd..128ba3429223 100644
--- a/kernel/events/hw_breakpoint.c
+++ b/kernel/events/hw_breakpoint.c
@@ -19,6 +19,7 @@

#include <linux/hw_breakpoint.h>

+#include <linux/atomic.h>
#include <linux/bug.h>
#include <linux/cpu.h>
#include <linux/export.h>
@@ -28,6 +29,7 @@
#include <linux/kernel.h>
#include <linux/mutex.h>
#include <linux/notifier.h>
+#include <linux/percpu-rwsem.h>
#include <linux/percpu.h>
#include <linux/rhashtable.h>
#include <linux/sched.h>
@@ -41,9 +43,9 @@ struct bp_cpuinfo {
unsigned int cpu_pinned;
/* tsk_pinned[n] is the number of tasks having n+1 breakpoints */
#ifdef hw_breakpoint_slots
- unsigned int tsk_pinned[hw_breakpoint_slots(0)];
+ atomic_t tsk_pinned[hw_breakpoint_slots(0)];
#else
- unsigned int *tsk_pinned;
+ atomic_t *tsk_pinned;
#endif
};

@@ -65,8 +67,79 @@ static const struct rhashtable_params task_bps_ht_params = {

static bool constraints_initialized __ro_after_init;

-/* Serialize accesses to the above constraints */
-static DEFINE_MUTEX(nr_bp_mutex);
+/*
+ * Synchronizes accesses to the per-CPU constraints; the locking rules are:
+ *
+ * 1. Atomic updates to bp_cpuinfo::tsk_pinned only require a held read-lock
+ * (due to bp_slots_histogram::count being atomic, no update are lost).
+ *
+ * 2. Holding a write-lock is required for computations that require a
+ * stable snapshot of all bp_cpuinfo::tsk_pinned.
+ *
+ * 3. In all other cases, non-atomic accesses require the appropriately held
+ * lock (read-lock for read-only accesses; write-lock for reads/writes).
+ */
+DEFINE_STATIC_PERCPU_RWSEM(bp_cpuinfo_sem);
+
+/*
+ * Return mutex to serialize accesses to per-task lists in task_bps_ht. Since
+ * rhltable synchronizes concurrent insertions/deletions, independent tasks may
+ * insert/delete concurrently; therefore, a mutex per task is sufficient.
+ *
+ * Uses task_struct::perf_event_mutex, to avoid extending task_struct with a
+ * hw_breakpoint-only mutex, which may be infrequently used. The caveat here is
+ * that hw_breakpoint may contend with per-task perf event list management. The
+ * assumption is that perf usecases involving hw_breakpoints are very unlikely
+ * to result in unnecessary contention.
+ */
+static inline struct mutex *get_task_bps_mutex(struct perf_event *bp)
+{
+ struct task_struct *tsk = bp->hw.target;
+
+ return tsk ? &tsk->perf_event_mutex : NULL;
+}
+
+static struct mutex *bp_constraints_lock(struct perf_event *bp)
+{
+ struct mutex *tsk_mtx = get_task_bps_mutex(bp);
+
+ if (tsk_mtx) {
+ mutex_lock(tsk_mtx);
+ percpu_down_read(&bp_cpuinfo_sem);
+ } else {
+ percpu_down_write(&bp_cpuinfo_sem);
+ }
+
+ return tsk_mtx;
+}
+
+static void bp_constraints_unlock(struct mutex *tsk_mtx)
+{
+ if (tsk_mtx) {
+ percpu_up_read(&bp_cpuinfo_sem);
+ mutex_unlock(tsk_mtx);
+ } else {
+ percpu_up_write(&bp_cpuinfo_sem);
+ }
+}
+
+static bool bp_constraints_is_locked(struct perf_event *bp)
+{
+ struct mutex *tsk_mtx = get_task_bps_mutex(bp);
+
+ return percpu_is_write_locked(&bp_cpuinfo_sem) ||
+ (tsk_mtx ? mutex_is_locked(tsk_mtx) :
+ percpu_is_read_locked(&bp_cpuinfo_sem));
+}
+
+static inline void assert_bp_constraints_lock_held(struct perf_event *bp)
+{
+ struct mutex *tsk_mtx = get_task_bps_mutex(bp);
+
+ if (tsk_mtx)
+ lockdep_assert_held(tsk_mtx);
+ lockdep_assert_held(&bp_cpuinfo_sem);
+}

#ifdef hw_breakpoint_slots
/*
@@ -97,7 +170,7 @@ static __init int init_breakpoint_slots(void)
for (i = 0; i < TYPE_MAX; i++) {
struct bp_cpuinfo *info = get_bp_info(cpu, i);

- info->tsk_pinned = kcalloc(__nr_bp_slots[i], sizeof(int), GFP_KERNEL);
+ info->tsk_pinned = kcalloc(__nr_bp_slots[i], sizeof(atomic_t), GFP_KERNEL);
if (!info->tsk_pinned)
goto err;
}
@@ -137,11 +210,19 @@ static inline enum bp_type_idx find_slot_idx(u64 bp_type)
*/
static unsigned int max_task_bp_pinned(int cpu, enum bp_type_idx type)
{
- unsigned int *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
+ atomic_t *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
int i;

+ /*
+ * At this point we want to have acquired the bp_cpuinfo_sem as a
+ * writer to ensure that there are no concurrent writers in
+ * toggle_bp_task_slot() to tsk_pinned, and we get a stable snapshot.
+ */
+ lockdep_assert_held_write(&bp_cpuinfo_sem);
+
for (i = hw_breakpoint_slots_cached(type) - 1; i >= 0; i--) {
- if (tsk_pinned[i] > 0)
+ ASSERT_EXCLUSIVE_WRITER(tsk_pinned[i]); /* Catch unexpected writers. */
+ if (atomic_read(&tsk_pinned[i]) > 0)
return i + 1;
}

@@ -158,6 +239,11 @@ static int task_bp_pinned(int cpu, struct perf_event *bp, enum bp_type_idx type)
struct perf_event *iter;
int count = 0;

+ /*
+ * We need a stable snapshot of the per-task breakpoint list.
+ */
+ assert_bp_constraints_lock_held(bp);
+
rcu_read_lock();
head = rhltable_lookup(&task_bps_ht, &bp->hw.target, task_bps_ht_params);
if (!head)
@@ -214,16 +300,25 @@ max_bp_pinned_slots(struct perf_event *bp, enum bp_type_idx type)
static void toggle_bp_task_slot(struct perf_event *bp, int cpu,
enum bp_type_idx type, int weight)
{
- unsigned int *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
+ atomic_t *tsk_pinned = get_bp_info(cpu, type)->tsk_pinned;
int old_idx, new_idx;

+ /*
+ * 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);
+
old_idx = task_bp_pinned(cpu, bp, type) - 1;
new_idx = old_idx + weight;

if (old_idx >= 0)
- tsk_pinned[old_idx]--;
+ atomic_dec(&tsk_pinned[old_idx]);
if (new_idx >= 0)
- tsk_pinned[new_idx]++;
+ atomic_inc(&tsk_pinned[new_idx]);
}

/*
@@ -241,6 +336,7 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,

/* Pinned counter cpu profiling */
if (!bp->hw.target) {
+ lockdep_assert_held_write(&bp_cpuinfo_sem);
get_bp_info(bp->cpu, type)->cpu_pinned += weight;
return 0;
}
@@ -249,6 +345,11 @@ toggle_bp_slot(struct perf_event *bp, bool enable, enum bp_type_idx type,
for_each_cpu(cpu, cpumask)
toggle_bp_task_slot(bp, cpu, type, weight);

+ /*
+ * Readers want a stable snapshot of the per-task breakpoint list.
+ */
+ assert_bp_constraints_lock_held(bp);
+
if (enable)
return rhltable_insert(&task_bps_ht, &bp->hw.bp_list, task_bps_ht_params);
else
@@ -354,14 +455,10 @@ static int __reserve_bp_slot(struct perf_event *bp, u64 bp_type)

int reserve_bp_slot(struct perf_event *bp)
{
- int ret;
-
- mutex_lock(&nr_bp_mutex);
-
- ret = __reserve_bp_slot(bp, bp->attr.bp_type);
-
- mutex_unlock(&nr_bp_mutex);
+ struct mutex *mtx = bp_constraints_lock(bp);
+ int ret = __reserve_bp_slot(bp, bp->attr.bp_type);

+ bp_constraints_unlock(mtx);
return ret;
}

@@ -379,12 +476,11 @@ static void __release_bp_slot(struct perf_event *bp, u64 bp_type)

void release_bp_slot(struct perf_event *bp)
{
- mutex_lock(&nr_bp_mutex);
+ struct mutex *mtx = bp_constraints_lock(bp);

arch_unregister_hw_breakpoint(bp);
__release_bp_slot(bp, bp->attr.bp_type);
-
- mutex_unlock(&nr_bp_mutex);
+ bp_constraints_unlock(mtx);
}

static int __modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
@@ -411,11 +507,10 @@ static int __modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)

static int modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
{
- int ret;
+ struct mutex *mtx = bp_constraints_lock(bp);
+ int ret = __modify_bp_slot(bp, old_type, new_type);

- mutex_lock(&nr_bp_mutex);
- ret = __modify_bp_slot(bp, old_type, new_type);
- mutex_unlock(&nr_bp_mutex);
+ bp_constraints_unlock(mtx);
return ret;
}

@@ -426,18 +521,28 @@ static int modify_bp_slot(struct perf_event *bp, u64 old_type, u64 new_type)
*/
int dbg_reserve_bp_slot(struct perf_event *bp)
{
- if (mutex_is_locked(&nr_bp_mutex))
+ int ret;
+
+ if (bp_constraints_is_locked(bp))
return -1;

- return __reserve_bp_slot(bp, bp->attr.bp_type);
+ /* Locks aren't held; disable lockdep assert checking. */
+ lockdep_off();
+ ret = __reserve_bp_slot(bp, bp->attr.bp_type);
+ lockdep_on();
+
+ return ret;
}

int dbg_release_bp_slot(struct perf_event *bp)
{
- if (mutex_is_locked(&nr_bp_mutex))
+ if (bp_constraints_is_locked(bp))
return -1;

+ /* Locks aren't held; disable lockdep assert checking. */
+ lockdep_off();
__release_bp_slot(bp, bp->attr.bp_type);
+ lockdep_on();

return 0;
}
--
2.37.0.rc0.161.g10f37bed90-goog