[Patch] Scm: Remove unnecessary pid & credential references in Unix socket's send and receive path

Tim Chen
2011-08-17T23:56:05Z

scm_destroy_cred at the end of unix_dgram_send* functions. One pair of
increment and decrement of the reference counts can be removed so we
only need to increment the ref counts once.

By incorporating these changes, for hackbench running on a 4 socket
NHM-EX machine with 40 cores, the execution of hackbench on
50 groups of 20 threads sped up by factor of 2.

Hackbench command used for testing:
./hackbench 50 thread 2000

Reviews and comments are appreciated.

Thanks.

Tim

---

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
--
diff --git a/include/net/scm.h b/include/net/scm.h
index 745460f..e24ec88 100644
--- a/include/net/scm.h
+++ b/include/net/scm.h
@@ -53,6 +53,14 @@ static __inline__ void scm_set_cred(struct scm_cookie *scm,
cred_to_ucred(pid, cred, &scm->creds);
}

+static __inline__ void scm_set_cred_noref(struct scm_cookie *scm,
+ struct pid *pid, const struct cred *cred)
+{
+ scm->pid = pid;
+ scm->cred = cred;
+ cred_to_ucred(pid, cred, &scm->creds);
+}
+
static __inline__ void scm_destroy_cred(struct scm_cookie *scm)
{
put_pid(scm->pid);
@@ -70,6 +78,15 @@ static __inline__ void scm_destroy(struct scm_cookie *scm)
__scm_destroy(scm);
}

+static __inline__ void scm_release(struct scm_cookie *scm)
+{
+ /* keep ref on pid and cred */
+ scm->pid = NULL;
+ scm->cred = NULL;
+ if (scm && scm->fp)
+ __scm_destroy(scm);
+}
+
static __inline__ int scm_send(struct socket *sock, struct msghdr *msg,
struct scm_cookie *scm)
{
@@ -108,15 +125,14 @@ static __inline__ void scm_recv(struct socket *sock, struct msghdr *msg,
if (!msg->msg_control) {
if (test_bit(SOCK_PASSCRED, &sock->flags) || scm->fp)
msg->msg_flags |= MSG_CTRUNC;
- scm_destroy(scm);
+ if (scm && scm->fp)
+ __scm_destroy(scm);
return;
}

if (test_bit(SOCK_PASSCRED, &sock->flags))
put_cmsg(msg, SOL_SOCKET, SCM_CREDENTIALS, sizeof(scm->creds), &scm->creds);

- scm_destroy_cred(scm);
-
scm_passec(sock, msg, scm);

if (!scm->fp)
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index 0722a25..bd85c06 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -1382,11 +1382,17 @@ static int unix_attach_fds(struct scm_cookie *scm, struct sk_buff *skb)
return max_level;
}

-static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb, bool send_fds)
+static int unix_scm_to_skb(struct scm_cookie *scm, struct sk_buff *skb,
+ bool send_fds, bool ref)
{
int err = 0;
- UNIXCB(skb).pid = get_pid(scm->pid);
- UNIXCB(skb).cred = get_cred(scm->cred);
+ if (ref) {
+ UNIXCB(skb).pid = get_pid(scm->pid);
+ UNIXCB(skb).cred = get_cred(scm->cred);
+ } else {
+ UNIXCB(skb).pid = scm->pid;
+ UNIXCB(skb).cred = scm->cred;
+ }
UNIXCB(skb).fp = NULL;
if (scm->fp && send_fds)
err = unix_attach_fds(scm, skb);
@@ -1411,7 +1417,7 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
int namelen = 0; /* fake GCC */
int err;
unsigned hash;
- struct sk_buff *skb;
+ struct sk_buff *skb = NULL;
long timeo;
struct scm_cookie tmp_scm;
int max_level;
@@ -1452,7 +1458,7 @@ static int unix_dgram_sendmsg(struct kiocb *kiocb, struct socket *sock,
if (skb == NULL)
goto out;

- err = unix_scm_to_skb(siocb->scm, skb, true);
+ err = unix_scm_to_skb(siocb->scm, skb, true, false);
if (err < 0)
goto out_free;
max_level = err + 1;
@@ -1548,7 +1554,7 @@ restart:
unix_state_unlock(other);
other->sk_data_ready(other, len);
sock_put(other);
- scm_destroy(siocb->scm);
+ scm_release(siocb->scm);
return len;

out_unlock:
@@ -1558,7 +1564,8 @@ out_free:
out:
if (other)
sock_put(other);
- scm_destroy(siocb->scm);
+ if (skb == NULL)
+ scm_destroy(siocb->scm);
return err;
}

@@ -1570,7 +1577,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
struct sock *sk = sock->sk;
struct sock *other = NULL;
int err, size;
- struct sk_buff *skb;
+ struct sk_buff *skb = NULL;
int sent = 0;
struct scm_cookie tmp_scm;
bool fds_sent = false;
@@ -1635,11 +1642,14 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
size = min_t(int, size, skb_tailroom(skb));

- /* Only send the fds in the first buffer */
- err = unix_scm_to_skb(siocb->scm, skb, !fds_sent);
+ /* Only send the fds and no ref to pid in the first buffer */
+ if (fds_sent)
+ err = unix_scm_to_skb(siocb->scm, skb, !fds_sent, true);
+ else
+ err = unix_scm_to_skb(siocb->scm, skb, !fds_sent, false);
if (err < 0) {
kfree_skb(skb);
- goto out_err;
+ goto out;
}
max_level = err + 1;
fds_sent = true;
@@ -1647,7 +1657,7 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
err = memcpy_fromiovec(skb_put(skb, size), msg->msg_iov, size);
if (err) {
kfree_skb(skb);
- goto out_err;
+ goto out;
}

unix_state_lock(other);
@@ -1664,7 +1674,10 @@ static int unix_stream_sendmsg(struct kiocb *kiocb, struct socket *sock,
sent += size;
}

- scm_destroy(siocb->scm);
+ if (skb)
+ scm_release(siocb->scm);
+ else
+ scm_destroy(siocb->scm);
siocb->scm = NULL;

return sent;
@@ -1677,7 +1690,9 @@ pipe_err:
send_sig(SIGPIPE, current, 0);
err = -EPIPE;
out_err:
- scm_destroy(siocb->scm);
+ if (skb == NULL)
+ scm_destroy(siocb->scm);
+out:
siocb->scm = NULL;
return sent ? : err;
}
@@ -1781,7 +1796,7 @@ static int unix_dgram_recvmsg(struct kiocb *iocb, struct socket *sock,
siocb->scm = &tmp_scm;
memset(&tmp_scm, 0, sizeof(tmp_scm));
}
- scm_set_cred(siocb->scm, UNIXCB(skb).pid, UNIXCB(skb).cred);
+ scm_set_cred_noref(siocb->scm, UNIXCB(skb).pid, UNIXCB(skb).cred);
unix_set_secdata(siocb->scm, skb);

if (!(flags & MSG_PEEK)) {
@@ -1943,7 +1958,8 @@ static int unix_stream_recvmsg(struct kiocb *iocb, struct socket *sock,