[PATCH v2 12/12] uprobes: switch uprobes_treelock to per-CPU RW semaphore

Andrii Nakryiko
2024-07-01T22:40:18Z

With all the batch uprobe APIs work we are now finally ready to reap the
benefits. Switch uprobes_treelock from reader-writer spinlock to a much
more efficient and scalable per-CPU RW semaphore.

Benchmarks and numbers time. I've used BPF selftests' bench tool,
trig-uprobe-nop benchmark specifically, to see how uprobe total
throughput scales with number of competing threads (mapped to individual
CPUs). Here are results:

# threads BEFORE (mln/s) AFTER (mln/s)
--------- -------------- -------------
1 3.131 3.140
2 3.394 3.601
3 3.630 3.960
4 3.317 3.551
5 3.448 3.464
6 3.345 3.283
7 3.469 3.444
8 3.182 3.258
9 3.138 3.139
10 2.999 3.212
11 2.903 3.183
12 2.802 3.027
13 2.792 3.027
14 2.695 3.086
15 2.822 2.965
16 2.679 2.939
17 2.622 2.888
18 2.628 2.914
19 2.702 2.836
20 2.561 2.837

One can see that per-CPU RW semaphore-based implementation scales better
with number of CPUs (especially that single CPU throughput is basically
the same).

Note, scalability is still limited by register_rwsem and this will
hopefully be address in follow up patch set(s).

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
kernel/events/uprobes.c | 30 +++++++++++++++---------------
1 file changed, 15 insertions(+), 15 deletions(-)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index bb480a2400e1..1d76551e5e23 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -39,7 +39,7 @@ static struct rb_root uprobes_tree = RB_ROOT;
*/
#define no_uprobe_events() RB_EMPTY_ROOT(&uprobes_tree)

-static DEFINE_RWLOCK(uprobes_treelock); /* serialize rbtree access */
+DEFINE_STATIC_PERCPU_RWSEM(uprobes_treelock); /* serialize rbtree access */

#define UPROBES_HASH_SZ 13
/* serialize uprobe->pending_list */
@@ -684,7 +684,7 @@ static void __put_uprobe(struct uprobe *uprobe, bool tree_locked)
bool destroy;

if (!tree_locked)
- write_lock(&uprobes_treelock);
+ percpu_down_write(&uprobes_treelock);
/*
* We might race with find_uprobe()->__get_uprobe() executed
* from inside read-locked uprobes_treelock, which can bump
@@ -708,7 +708,7 @@ static void __put_uprobe(struct uprobe *uprobe, bool tree_locked)
if (destroy && uprobe_is_active(uprobe))
rb_erase(&uprobe->rb_node, &uprobes_tree);
if (!tree_locked)
- write_unlock(&uprobes_treelock);
+ percpu_up_write(&uprobes_treelock);

/*
* Beyond here we don't need RCU protection, we are either the
@@ -816,9 +816,9 @@ static struct uprobe *find_uprobe(struct inode *inode, loff_t offset)
{
struct uprobe *uprobe;

- read_lock(&uprobes_treelock);
+ percpu_down_read(&uprobes_treelock);
uprobe = __find_uprobe(inode, offset);
- read_unlock(&uprobes_treelock);
+ percpu_up_read(&uprobes_treelock);

return uprobe;
}
@@ -1205,7 +1205,7 @@ void uprobe_unregister_batch(struct inode *inode, int cnt, uprobe_consumer_fn ge
up_write(&uprobe->register_rwsem);
}

- write_lock(&uprobes_treelock);
+ percpu_down_write(&uprobes_treelock);
for (i = 0; i < cnt; i++) {
uc = get_uprobe_consumer(i, ctx);
uprobe = uc->uprobe;
@@ -1216,7 +1216,7 @@ void uprobe_unregister_batch(struct inode *inode, int cnt, uprobe_consumer_fn ge
__put_uprobe(uprobe, true);
uc->uprobe = NULL;
}
- write_unlock(&uprobes_treelock);
+ percpu_up_write(&uprobes_treelock);
}

static struct uprobe_consumer *uprobe_consumer_identity(size_t idx, void *ctx)
@@ -1321,7 +1321,7 @@ int uprobe_register_batch(struct inode *inode, int cnt,
}

ret = 0;
- write_lock(&uprobes_treelock);
+ percpu_down_write(&uprobes_treelock);
for (i = 0; i < cnt; i++) {
struct uprobe *cur_uprobe;

@@ -1344,7 +1344,7 @@ int uprobe_register_batch(struct inode *inode, int cnt,
}
}
unlock_treelock:
- write_unlock(&uprobes_treelock);
+ percpu_up_write(&uprobes_treelock);
if (ret)
goto cleanup_uprobes;

@@ -1376,7 +1376,7 @@ int uprobe_register_batch(struct inode *inode, int cnt,
}
cleanup_uprobes:
/* put all the successfully allocated/reused uprobes */
- write_lock(&uprobes_treelock);
+ percpu_down_write(&uprobes_treelock);
for (i = 0; i < cnt; i++) {
uc = get_uprobe_consumer(i, ctx);

@@ -1384,7 +1384,7 @@ int uprobe_register_batch(struct inode *inode, int cnt,
__put_uprobe(uc->uprobe, true);
uc->uprobe = NULL;
}
- write_unlock(&uprobes_treelock);
+ percpu_up_write(&uprobes_treelock);
return ret;
}

@@ -1492,7 +1492,7 @@ static void build_probe_list(struct inode *inode,
min = vaddr_to_offset(vma, start);
max = min + (end - start) - 1;

- read_lock(&uprobes_treelock);
+ percpu_down_read(&uprobes_treelock);
n = find_node_in_range(inode, min, max);
if (n) {
for (t = n; t; t = rb_prev(t)) {
@@ -1510,7 +1510,7 @@ static void build_probe_list(struct inode *inode,
list_add(&u->pending_list, head);
}
}
- read_unlock(&uprobes_treelock);
+ percpu_up_read(&uprobes_treelock);
}

/* @vma contains reference counter, not the probed instruction. */
@@ -1601,9 +1601,9 @@ vma_has_uprobes(struct vm_area_struct *vma, unsigned long start, unsigned long e
min = vaddr_to_offset(vma, start);
max = min + (end - start) - 1;

- read_lock(&uprobes_treelock);
+ percpu_down_read(&uprobes_treelock);
n = find_node_in_range(inode, min, max);
- read_unlock(&uprobes_treelock);
+ percpu_up_read(&uprobes_treelock);

return !!n;
}
--
2.43.0