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bpf: change bpf syacall to use u64 temp variables

RFC In review yalin wang 3 participants 3 messages Oct 19, 2015

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Networking Network drivers BPF BPF core
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yalin wang

author
#0 lore raw

RFC 1/1 bpf: change bpf syacall to use u64 temp variables

This patch change map_lookup_elem() and map_update_elem() function to use u64 temp variable if the key_size or value_size is less than u64, we don't need use kmalloc() for these small variables.

Signed-off-by yalin wang <[email protected]>
---
 kernel/bpf/syscall.c | 30 ++++++++++++++++++++----------
 1 file changed, 20 insertions(+), 10 deletions(-)
kernel/bpf/syscall.c +20 -10
index f640e5f..c82d7bf 100644
@@ -189,7 +189,8 @@ static int map_lookup_elem(union bpf_attr *attr)
 	void __user *uvalue = u64_to_ptr(attr->value);
 	int ufd = attr->map_fd;
 	struct bpf_map *map;
-	void *key, *value, *ptr;
+	u64 key_buf, value_buf;
+	void *key = &key_buf, *value = &value_buf, *ptr;
 	struct fd f;
 	int err;
 

@@ -202,7 +203,8 @@ static int map_lookup_elem(union bpf_attr *attr)
 		return PTR_ERR(map);
 
 	err = -ENOMEM;
-	key = kmalloc(map->key_size, GFP_USER);
+	if (map->key_size > sizeof(u64))
+		key = kmalloc(map->key_size, GFP_USER);
 	if (!key)
 		goto err_put;
 

@@ -211,7 +213,8 @@ static int map_lookup_elem(union bpf_attr *attr)
 		goto free_key;
 
 	err = -ENOMEM;
-	value = kmalloc(map->value_size, GFP_USER);
+	if (map->value_size > sizeof(u64))
+		value = kmalloc(map->value_size, GFP_USER);
 	if (!value)
 		goto free_key;
 

@@ -232,9 +235,11 @@ static int map_lookup_elem(union bpf_attr *attr)
 	err = 0;
 
 free_value:
-	kfree(value);
+	if (value != &value_buf)
+		kfree(value);
 free_key:
-	kfree(key);
+	if (key != &key_buf)
+		kfree(key);
 err_put:
 	fdput(f);
 	return err;

@@ -248,7 +253,8 @@ static int map_update_elem(union bpf_attr *attr)
 	void __user *uvalue = u64_to_ptr(attr->value);
 	int ufd = attr->map_fd;
 	struct bpf_map *map;
-	void *key, *value;
+	u64 key_buf, value_buf;
+	void *key = &key_buf, *value = &value_buf;
 	struct fd f;
 	int err;
 

@@ -261,7 +267,8 @@ static int map_update_elem(union bpf_attr *attr)
 		return PTR_ERR(map);
 
 	err = -ENOMEM;
-	key = kmalloc(map->key_size, GFP_USER);
+	if (map->key_size > sizeof(u64))
+		key = kmalloc(map->key_size, GFP_USER);
 	if (!key)
 		goto err_put;
 

@@ -270,7 +277,8 @@ static int map_update_elem(union bpf_attr *attr)
 		goto free_key;
 
 	err = -ENOMEM;
-	value = kmalloc(map->value_size, GFP_USER);
+	if (map->value_size > sizeof(u64))
+		value = kmalloc(map->value_size, GFP_USER);
 	if (!value)
 		goto free_key;
 

@@ -286,9 +294,11 @@ static int map_update_elem(union bpf_attr *attr)
 	rcu_read_unlock();
 
 free_value:
-	kfree(value);
+	if (value != &value_buf)
+		kfree(value);
 free_key:
-	kfree(key);
+	if (key != &key_buf)
+		kfree(key);
 err_put:
 	fdput(f);
 	return err;
Signature · 1 line
1.9.1
maintainer
#1 lore raw
Quoted 5 lines · yalin wang

This patch change map_lookup_elem() and map_update_elem() function to use u64 temp variable if the key_size or value_size is less than u64, we don't need use kmalloc() for these small variables.

Signed-off-by yalin wang <[email protected]>

From an application PoV that has to make the bpf(2) syscall, how much do we actually gain from this? I'm curious, did you perform some benchmarks that show a noticeable difference?

#2 lore raw
In reply to yalin wang
Quoted 24 lines · yalin wang

This patch change map_lookup_elem() and map_update_elem() function to use u64 temp variable if the key_size or value_size is less than u64, we don't need use kmalloc() for these small variables.

Signed-off-by yalin wang <[email protected]>
---
 kernel/bpf/syscall.c | 30 ++++++++++++++++++++----------
 1 file changed, 20 insertions(+), 10 deletions(-)
kernel/bpf/syscall.c +2 -1
index f640e5f..c82d7bf 100644
@@ -189,7 +189,8 @@ static int map_lookup_elem(union bpf_attr *attr)
 	void __user *uvalue = u64_to_ptr(attr->value);
 	int ufd = attr->map_fd;
 	struct bpf_map *map;
-	void *key, *value, *ptr;
+	u64 key_buf, value_buf;
+	void *key = &key_buf, *value = &value_buf, *ptr;
 	struct fd f;
 	int err;

kernel/bpf/syscall.c static int map_lookup_elem(union bpf_attr *attr) · line 207

Quoted 6 lines
kernel/bpf/syscall.c +2 -1
@@ -202,7 +203,8 @@ static int map_lookup_elem(union bpf_attr *attr)
 		return PTR_ERR(map);
 
 	err = -ENOMEM;
-	key = kmalloc(map->key_size, GFP_USER);
+	if (map->key_size > sizeof(u64))
+		key = kmalloc(map->key_size, GFP_USER);

I think it's a good optimization for common case. Performance numbers would be good to prove the point. Thanks