Re: [PATCH v2 00/12] uprobes: add batched register/unregister APIs and per-CPU RW semaphore

Peter Zijlstra
2024-07-02T11:54:55Z

+LKML

On Tue, Jul 02, 2024 at 12:23:53PM +0200, Peter Zijlstra wrote:
> On Mon, Jul 01, 2024 at 03:39:23PM -0700, Andrii Nakryiko wrote:
> > This patch set, ultimately, switches global uprobes_treelock from RW spinlock
> > to per-CPU RW semaphore, which has better performance and scales better under
> > contention and multiple parallel threads triggering lots of uprobes.
>
> Why not RCU + normal lock thing?

Something like the *completely* untested below.

---
diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 2c83ba776fc7..03b38f3f7be3 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -40,6 +40,7 @@ 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 */
+static seqcount_rwlock_t uprobes_seqcount = SEQCNT_RWLOCK_ZERO(uprobes_seqcount, &uprobes_treelock);

#define UPROBES_HASH_SZ 13
/* serialize uprobe->pending_list */
@@ -54,6 +55,7 @@ DEFINE_STATIC_PERCPU_RWSEM(dup_mmap_sem);
struct uprobe {
struct rb_node rb_node; /* node in the rb tree */
refcount_t ref;
+ struct rcu_head rcu;
struct rw_semaphore register_rwsem;
struct rw_semaphore consumer_rwsem;
struct list_head pending_list;
@@ -67,7 +69,7 @@ struct uprobe {
* The generic code assumes that it has two members of unknown type
* owned by the arch-specific code:
*
- * insn - copy_insn() saves the original instruction here for
+ * insn - copy_insn() saves the original instruction here for
* arch_uprobe_analyze_insn().
*
* ixol - potentially modified instruction to execute out of
@@ -593,6 +595,12 @@ static struct uprobe *get_uprobe(struct uprobe *uprobe)
return uprobe;
}

+static void uprobe_free_rcu(struct rcu_head *rcu)
+{
+ struct uprobe *uprobe = container_of(rcu, struct uprobe, rcu);
+ kfree(uprobe);
+}
+
static void put_uprobe(struct uprobe *uprobe)
{
if (refcount_dec_and_test(&uprobe->ref)) {
@@ -604,7 +612,8 @@ static void put_uprobe(struct uprobe *uprobe)
mutex_lock(&delayed_uprobe_lock);
delayed_uprobe_remove(uprobe, NULL);
mutex_unlock(&delayed_uprobe_lock);
- kfree(uprobe);
+
+ call_rcu(&uprobe->rcu, uprobe_free_rcu);
}
}

@@ -668,12 +677,25 @@ static struct uprobe *__find_uprobe(struct inode *inode, loff_t offset)
static struct uprobe *find_uprobe(struct inode *inode, loff_t offset)
{
struct uprobe *uprobe;
+ unsigned seq;

- read_lock(&uprobes_treelock);
- uprobe = __find_uprobe(inode, offset);
- read_unlock(&uprobes_treelock);
+ guard(rcu)();

- return uprobe;
+ do {
+ seq = read_seqcount_begin(&uprobes_seqcount);
+ uprobes = __find_uprobe(inode, offset);
+ if (uprobes) {
+ /*
+ * Lockless RB-tree lookups are prone to false-negatives.
+ * If they find something, it's good. If they do not find,
+ * it needs to be validated.
+ */
+ return uprobes;
+ }
+ } while (read_seqcount_retry(&uprobes_seqcount, seq));
+
+ /* Really didn't find anything. */
+ return NULL;
}

static struct uprobe *__insert_uprobe(struct uprobe *uprobe)
@@ -702,7 +724,9 @@ static struct uprobe *insert_uprobe(struct uprobe *uprobe)
struct uprobe *u;

write_lock(&uprobes_treelock);
+ write_seqcount_begin(&uprobes_seqcount);
u = __insert_uprobe(uprobe);
+ write_seqcount_end(&uprobes_seqcount);
write_unlock(&uprobes_treelock);

return u;
@@ -936,7 +960,9 @@ static void delete_uprobe(struct uprobe *uprobe)
return;

write_lock(&uprobes_treelock);
+ write_seqcount_begin(&uprobes_seqcount);
rb_erase(&uprobe->rb_node, &uprobes_tree);
+ write_seqcount_end(&uprobes_seqcount);
write_unlock(&uprobes_treelock);
RB_CLEAR_NODE(&uprobe->rb_node); /* for uprobe_is_active() */
put_uprobe(uprobe);