kprobes: kretprobe scalability improvement

kretprobe is using freelist to manage return-instances, but freelist,
as LIFO queue based on singly linked list, scales badly and reduces
the overall throughput of kretprobed routines, especially for high
contention scenarios.

Here's a typical throughput test of sys_prctl (counts in 10 seconds,
measured with perf stat -a -I 10000 -e syscalls:sys_enter_prctl):

OS: Debian 10 X86_64, Linux 6.5rc7 with freelist
HW: XEON 8336C x 2, 64 cores/128 threads, DDR4 3200MT/s

         1T       2T       4T       8T      16T      24T
   24150045 29317964 15446741 12494489 18287272 17708768
        32T      48T      64T      72T      96T     128T
   16200682 13737658 11645677 11269858 10470118  9931051

This patch introduces objpool to replace freelist. objpool is a
high performance queue, which can bring near-linear scalability
to kretprobed routines. Tests of kretprobe throughput show the
biggest ratio as 159x of original freelist. Here's the result:

                  1T         2T         4T         8T        16T
native:     41186213   82336866  164250978  328662645  658810299
freelist:   24150045   29317964   15446741   12494489   18287272
objpool:    23926730   48010314   96125218  191782984  385091769
                 32T        48T        64T        96T       128T
native:   1330338351 1969957941 2512291791 2615754135 2671040914
freelist:   16200682   13737658   11645677   10470118    9931051
objpool:   764481096 1147149781 1456220214 1502109662 1579015050

Testings on 96-core ARM64 output similarly, but with the biggest
ratio up to 448x:

OS: Debian 10 AARCH64, Linux 6.5rc7
HW: Kunpeng-920 96 cores/2 sockets/4 NUMA nodes, DDR4 2933 MT/s

                  1T         2T         4T         8T        16T
native: .   30066096   63569843  126194076  257447289  505800181
freelist:   16152090   11064397   11124068    7215768    5663013
objpool:    13997541   28032100   55726624  110099926  221498787
                 24T        32T        48T        64T        96T
native:    763305277 1015925192 1521075123 2033009392 3021013752
freelist:    5015810    4602893    3766792    3382478    2945292
objpool:   328192025  439439564  668534502  887401381 1319972072

Link: https://lore.kernel.org/all/20231017135654.82270-4-wuqiang.matt@bytedance.com/

Signed-off-by: wuqiang.matt <wuqiang.matt@bytedance.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>

include/linux/kprobes.h

@@ -26,8 +26,7 @@
 #include <linux/rcupdate.h>
 #include <linux/mutex.h>
 #include <linux/ftrace.h>
-#include <linux/refcount.h>
+#include <linux/objpool.h>
-#include <linux/freelist.h>
 #include <linux/rethook.h>
 #include <asm/kprobes.h>
 
@@ -141,7 +140,7 @@ static inline bool kprobe_ftrace(struct kprobe *p)
  */
 struct kretprobe_holder {
 	struct kretprobe	*rp;
-	refcount_t		ref;
+	struct objpool_head	pool;
 };
 
 struct kretprobe {
@@ -154,7 +153,6 @@ struct kretprobe {
 #ifdef CONFIG_KRETPROBE_ON_RETHOOK
 	struct rethook *rh;
 #else
-	struct freelist_head freelist;
 	struct kretprobe_holder *rph;
 #endif
 };
@@ -165,10 +163,7 @@ struct kretprobe_instance {
 #ifdef CONFIG_KRETPROBE_ON_RETHOOK
 	struct rethook_node node;
 #else
-	union {
+	struct rcu_head rcu;
-		struct freelist_node freelist;
-		struct rcu_head rcu;
-	};
 	struct llist_node llist;
 	struct kretprobe_holder *rph;
 	kprobe_opcode_t *ret_addr;

include/linux/rethook.h

@@ -6,11 +6,10 @@
 #define _LINUX_RETHOOK_H
 
 #include <linux/compiler.h>
-#include <linux/freelist.h>
+#include <linux/objpool.h>
 #include <linux/kallsyms.h>
 #include <linux/llist.h>
 #include <linux/rcupdate.h>
-#include <linux/refcount.h>
 
 struct rethook_node;
 
@@ -30,14 +29,12 @@ typedef void (*rethook_handler_t) (struct rethook_node *, void *, unsigned long,
 struct rethook {
 	void			*data;
 	rethook_handler_t	handler;
-	struct freelist_head	pool;
+	struct objpool_head	pool;
-	refcount_t		ref;
 	struct rcu_head		rcu;
 };
 
 /**
  * struct rethook_node - The rethook shadow-stack entry node.
- * @freelist: The freelist, linked to struct rethook::pool.
  * @rcu: The rcu_head for deferred freeing.
  * @llist: The llist, linked to a struct task_struct::rethooks.
  * @rethook: The pointer to the struct rethook.
@@ -48,20 +45,16 @@ struct rethook {
  * on each entry of the shadow stack.
  */
 struct rethook_node {
-	union {
+	struct rcu_head		rcu;
-		struct freelist_node freelist;
-		struct rcu_head      rcu;
-	};
 	struct llist_node	llist;
 	struct rethook		*rethook;
 	unsigned long		ret_addr;
 	unsigned long		frame;
 };
 
-struct rethook *rethook_alloc(void *data, rethook_handler_t handler);
+struct rethook *rethook_alloc(void *data, rethook_handler_t handler, int size, int num);
 void rethook_stop(struct rethook *rh);
 void rethook_free(struct rethook *rh);
-void rethook_add_node(struct rethook *rh, struct rethook_node *node);
 struct rethook_node *rethook_try_get(struct rethook *rh);
 void rethook_recycle(struct rethook_node *node);
 void rethook_hook(struct rethook_node *node, struct pt_regs *regs, bool mcount);
@@ -98,4 +91,3 @@ void rethook_flush_task(struct task_struct *tk);
 #endif
 
 #endif
-

kernel/kprobes.c

@@ -1877,13 +1877,27 @@ static struct notifier_block kprobe_exceptions_nb = {
 #ifdef CONFIG_KRETPROBES
 
 #if !defined(CONFIG_KRETPROBE_ON_RETHOOK)
+
+/* callbacks for objpool of kretprobe instances */
+static int kretprobe_init_inst(void *nod, void *context)
+{
+	struct kretprobe_instance *ri = nod;
+
+	ri->rph = context;
+	return 0;
+}
+static int kretprobe_fini_pool(struct objpool_head *head, void *context)
+{
+	kfree(context);
+	return 0;
+}
+
 static void free_rp_inst_rcu(struct rcu_head *head)
 {
 	struct kretprobe_instance *ri = container_of(head, struct kretprobe_instance, rcu);
+	struct kretprobe_holder *rph = ri->rph;
 
-	if (refcount_dec_and_test(&ri->rph->ref))
+	objpool_drop(ri, &rph->pool);
-		kfree(ri->rph);
-	kfree(ri);
 }
 NOKPROBE_SYMBOL(free_rp_inst_rcu);
 
@@ -1892,7 +1906,7 @@ static void recycle_rp_inst(struct kretprobe_instance *ri)
 	struct kretprobe *rp = get_kretprobe(ri);
 
 	if (likely(rp))
-		freelist_add(&ri->freelist, &rp->freelist);
+		objpool_push(ri, &rp->rph->pool);
 	else
 		call_rcu(&ri->rcu, free_rp_inst_rcu);
 }
@@ -1929,23 +1943,12 @@ NOKPROBE_SYMBOL(kprobe_flush_task);
 
 static inline void free_rp_inst(struct kretprobe *rp)
 {
-	struct kretprobe_instance *ri;
+	struct kretprobe_holder *rph = rp->rph;
-	struct freelist_node *node;
-	int count = 0;
 
-	node = rp->freelist.head;
+	if (!rph)
-	while (node) {
+		return;
-		ri = container_of(node, struct kretprobe_instance, freelist);
+	rp->rph = NULL;
-		node = node->next;
+	objpool_fini(&rph->pool);
-
-		kfree(ri);
-		count++;
-	}
-
-	if (refcount_sub_and_test(count, &rp->rph->ref)) {
-		kfree(rp->rph);
-		rp->rph = NULL;
-	}
 }
 
 /* This assumes the 'tsk' is the current task or the is not running. */
@@ -2087,19 +2090,17 @@ NOKPROBE_SYMBOL(__kretprobe_trampoline_handler)
 static int pre_handler_kretprobe(struct kprobe *p, struct pt_regs *regs)
 {
 	struct kretprobe *rp = container_of(p, struct kretprobe, kp);
+	struct kretprobe_holder *rph = rp->rph;
 	struct kretprobe_instance *ri;
-	struct freelist_node *fn;
 
-	fn = freelist_try_get(&rp->freelist);
+	ri = objpool_pop(&rph->pool);
-	if (!fn) {
+	if (!ri) {
 		rp->nmissed++;
 		return 0;
 	}
 
-	ri = container_of(fn, struct kretprobe_instance, freelist);
-
 	if (rp->entry_handler && rp->entry_handler(ri, regs)) {
-		freelist_add(&ri->freelist, &rp->freelist);
+		objpool_push(ri, &rph->pool);
 		return 0;
 	}
 
@@ -2193,7 +2194,6 @@ int kprobe_on_func_entry(kprobe_opcode_t *addr, const char *sym, unsigned long o
 int register_kretprobe(struct kretprobe *rp)
 {
 	int ret;
-	struct kretprobe_instance *inst;
 	int i;
 	void *addr;
 
@@ -2227,19 +2227,12 @@ int register_kretprobe(struct kretprobe *rp)
 		rp->maxactive = max_t(unsigned int, 10, 2*num_possible_cpus());
 
 #ifdef CONFIG_KRETPROBE_ON_RETHOOK
-	rp->rh = rethook_alloc((void *)rp, kretprobe_rethook_handler);
+	rp->rh = rethook_alloc((void *)rp, kretprobe_rethook_handler,
-	if (!rp->rh)
+				sizeof(struct kretprobe_instance) +
-		return -ENOMEM;
+				rp->data_size, rp->maxactive);
+	if (IS_ERR(rp->rh))
+		return PTR_ERR(rp->rh);
 
-	for (i = 0; i < rp->maxactive; i++) {
-		inst = kzalloc(struct_size(inst, data, rp->data_size), GFP_KERNEL);
-		if (inst == NULL) {
-			rethook_free(rp->rh);
-			rp->rh = NULL;
-			return -ENOMEM;
-		}
-		rethook_add_node(rp->rh, &inst->node);
-	}
 	rp->nmissed = 0;
 	/* Establish function entry probe point */
 	ret = register_kprobe(&rp->kp);
@@ -2248,24 +2241,18 @@ int register_kretprobe(struct kretprobe *rp)
 		rp->rh = NULL;
 	}
 #else	/* !CONFIG_KRETPROBE_ON_RETHOOK */
-	rp->freelist.head = NULL;
 	rp->rph = kzalloc(sizeof(struct kretprobe_holder), GFP_KERNEL);
 	if (!rp->rph)
 		return -ENOMEM;
 
-	rp->rph->rp = rp;
+	if (objpool_init(&rp->rph->pool, rp->maxactive, rp->data_size +
-	for (i = 0; i < rp->maxactive; i++) {
+			sizeof(struct kretprobe_instance), GFP_KERNEL,
-		inst = kzalloc(struct_size(inst, data, rp->data_size), GFP_KERNEL);
+			rp->rph, kretprobe_init_inst, kretprobe_fini_pool)) {
-		if (inst == NULL) {
+		kfree(rp->rph);
-			refcount_set(&rp->rph->ref, i);
+		rp->rph = NULL;
-			free_rp_inst(rp);
+		return -ENOMEM;
-			return -ENOMEM;
-		}
-		inst->rph = rp->rph;
-		freelist_add(&inst->freelist, &rp->freelist);
 	}
-	refcount_set(&rp->rph->ref, i);
+	rp->rph->rp = rp;
-
 	rp->nmissed = 0;
 	/* Establish function entry probe point */
 	ret = register_kprobe(&rp->kp);

kernel/trace/fprobe.c

@@ -187,7 +187,7 @@ static void fprobe_init(struct fprobe *fp)
 
 static int fprobe_init_rethook(struct fprobe *fp, int num)
 {
-	int i, size;
+	int size;
 
 	if (num < 0)
 		return -EINVAL;
@@ -205,26 +205,18 @@ static int fprobe_init_rethook(struct fprobe *fp, int num)
 	if (size < 0)
 		return -E2BIG;
 
-	fp->rethook = rethook_alloc((void *)fp, fprobe_exit_handler);
+	/* Initialize rethook */
-	if (!fp->rethook)
+	fp->rethook = rethook_alloc((void *)fp, fprobe_exit_handler,
-		return -ENOMEM;
+				sizeof(struct fprobe_rethook_node), size);
-	for (i = 0; i < size; i++) {
+	if (IS_ERR(fp->rethook))
-		struct fprobe_rethook_node *node;
+		return PTR_ERR(fp->rethook);
 
-		node = kzalloc(sizeof(*node) + fp->entry_data_size, GFP_KERNEL);
-		if (!node) {
-			rethook_free(fp->rethook);
-			fp->rethook = NULL;
-			return -ENOMEM;
-		}
-		rethook_add_node(fp->rethook, &node->node);
-	}
 	return 0;
 }
 
 static void fprobe_fail_cleanup(struct fprobe *fp)
 {
-	if (fp->rethook) {
+	if (!IS_ERR_OR_NULL(fp->rethook)) {
 		/* Don't need to cleanup rethook->handler because this is not used. */
 		rethook_free(fp->rethook);
 		fp->rethook = NULL;
@@ -379,14 +371,14 @@ int unregister_fprobe(struct fprobe *fp)
 	if (!fprobe_is_registered(fp))
 		return -EINVAL;
 
-	if (fp->rethook)
+	if (!IS_ERR_OR_NULL(fp->rethook))
 		rethook_stop(fp->rethook);
 
 	ret = unregister_ftrace_function(&fp->ops);
 	if (ret < 0)
 		return ret;
 
-	if (fp->rethook)
+	if (!IS_ERR_OR_NULL(fp->rethook))
 		rethook_free(fp->rethook);
 
 	ftrace_free_filter(&fp->ops);

kernel/trace/rethook.c

@@ -9,6 +9,7 @@
 #include <linux/rethook.h>
 #include <linux/slab.h>
 #include <linux/sort.h>
+#include <linux/smp.h>
 
 /* Return hook list (shadow stack by list) */
 
@@ -36,21 +37,7 @@ void rethook_flush_task(struct task_struct *tk)
 static void rethook_free_rcu(struct rcu_head *head)
 {
 	struct rethook *rh = container_of(head, struct rethook, rcu);
-	struct rethook_node *rhn;
+	objpool_fini(&rh->pool);
-	struct freelist_node *node;
-	int count = 1;
-
-	node = rh->pool.head;
-	while (node) {
-		rhn = container_of(node, struct rethook_node, freelist);
-		node = node->next;
-		kfree(rhn);
-		count++;
-	}
-
-	/* The rh->ref is the number of pooled node + 1 */
-	if (refcount_sub_and_test(count, &rh->ref))
-		kfree(rh);
 }
 
 /**
@@ -83,54 +70,62 @@ void rethook_free(struct rethook *rh)
 	call_rcu(&rh->rcu, rethook_free_rcu);
 }
 
-/**
+static int rethook_init_node(void *nod, void *context)
- * rethook_alloc() - Allocate struct rethook.
- * @data: a data to pass the @handler when hooking the return.
- * @handler: the return hook callback function.
- *
- * Allocate and initialize a new rethook with @data and @handler.
- * Return NULL if memory allocation fails or @handler is NULL.
- * Note that @handler == NULL means this rethook is going to be freed.
- */
-struct rethook *rethook_alloc(void *data, rethook_handler_t handler)
 {
-	struct rethook *rh = kzalloc(sizeof(struct rethook), GFP_KERNEL);
+	struct rethook_node *node = nod;
 
-	if (!rh || !handler) {
+	node->rethook = context;
-		kfree(rh);
+	return 0;
-		return NULL;
+}
-	}
 
-	rh->data = data;
+static int rethook_fini_pool(struct objpool_head *head, void *context)
-	rh->handler = handler;
+{
-	rh->pool.head = NULL;
+	kfree(context);
-	refcount_set(&rh->ref, 1);
+	return 0;
-
-	return rh;
 }
 
 /**
- * rethook_add_node() - Add a new node to the rethook.
+ * rethook_alloc() - Allocate struct rethook.
- * @rh: the struct rethook.
+ * @data: a data to pass the @handler when hooking the return.
- * @node: the struct rethook_node to be added.
+ * @handler: the return hook callback function, must NOT be NULL
+ * @size: node size: rethook node and additional data
+ * @num: number of rethook nodes to be preallocated
  *
- * Add @node to @rh. User must allocate @node (as a part of user's
+ * Allocate and initialize a new rethook with @data and @handler.
- * data structure.) The @node fields are initialized in this function.
+ * Return pointer of new rethook, or error codes for failures.
+ *
+ * Note that @handler == NULL means this rethook is going to be freed.
  */
-void rethook_add_node(struct rethook *rh, struct rethook_node *node)
+struct rethook *rethook_alloc(void *data, rethook_handler_t handler,
+			      int size, int num)
 {
-	node->rethook = rh;
+	struct rethook *rh;
-	freelist_add(&node->freelist, &rh->pool);
+
-	refcount_inc(&rh->ref);
+	if (!handler || num <= 0 || size < sizeof(struct rethook_node))
+		return ERR_PTR(-EINVAL);
+
+	rh = kzalloc(sizeof(struct rethook), GFP_KERNEL);
+	if (!rh)
+		return ERR_PTR(-ENOMEM);
+
+	rh->data = data;
+	rh->handler = handler;
+
+	/* initialize the objpool for rethook nodes */
+	if (objpool_init(&rh->pool, num, size, GFP_KERNEL, rh,
+			 rethook_init_node, rethook_fini_pool)) {
+		kfree(rh);
+		return ERR_PTR(-ENOMEM);
+	}
+	return rh;
 }
 
 static void free_rethook_node_rcu(struct rcu_head *head)
 {
 	struct rethook_node *node = container_of(head, struct rethook_node, rcu);
+	struct rethook *rh = node->rethook;
 
-	if (refcount_dec_and_test(&node->rethook->ref))
+	objpool_drop(node, &rh->pool);
-		kfree(node->rethook);
-	kfree(node);
 }
 
 /**
@@ -145,7 +140,7 @@ void rethook_recycle(struct rethook_node *node)
 	lockdep_assert_preemption_disabled();
 
 	if (likely(READ_ONCE(node->rethook->handler)))
-		freelist_add(&node->freelist, &node->rethook->pool);
+		objpool_push(node, &node->rethook->pool);
 	else
 		call_rcu(&node->rcu, free_rethook_node_rcu);
 }
@@ -161,7 +156,6 @@ NOKPROBE_SYMBOL(rethook_recycle);
 struct rethook_node *rethook_try_get(struct rethook *rh)
 {
 	rethook_handler_t handler = READ_ONCE(rh->handler);
-	struct freelist_node *fn;
 
 	lockdep_assert_preemption_disabled();
 
@@ -178,11 +172,7 @@ struct rethook_node *rethook_try_get(struct rethook *rh)
 	if (unlikely(!rcu_is_watching()))
 		return NULL;
 
-	fn = freelist_try_get(&rh->pool);
+	return (struct rethook_node *)objpool_pop(&rh->pool);
-	if (!fn)
-		return NULL;
-
-	return container_of(fn, struct rethook_node, freelist);
 }
 NOKPROBE_SYMBOL(rethook_try_get);