[PATCH 0/2] net: Use synchronous wakeups selectively

[PATCH 0/2] net: Use synchronous wakeups selectively

[Srikar Dronamraju]

The scheduler assumes in several wakeup paths that a task using WF_SYNC
is likely to yield the CPU shortly. However several networking wakeup
paths unconditionally use synchronous wakeups even when the waking task
continues execution.

During wakeup, with WF_SYNC flag set, because of the assumption that
current thread is ready to give up, the wakee thread will be migrated
from any core within the chip to the current LLC. If these operations
are frequent, and wakers are actually not going away, then it will lead
to load imbalance and hurt performance. This is especially true in
architectures where LLCs are small and number of LLCs per chip are more.

Running vllm workload was run on Power10 system
                                  No patch      with patch %diff
Inference Time (sec)              17.84         16.35     -8.35%
llm query bandwidth (tokens/sec)  14.78         16.21     +9.68%

Lower inference time and higher tokens/sec is better.

Srikar Dronamraju (2):
  net/socket: Record preference for synchronous wakeups
  net/sock: Propagate WF_SYNC only when requested

 include/net/sock.h |  1 +
 net/core/sock.c    | 31 +++++++++++++++++++------
 net/sctp/socket.c  | 10 +++++++--
 net/smc/af_smc.c   |  4 ++--
 net/smc/smc_rx.c   | 10 +++++++--
 net/socket.c       | 56 +++++++++++++++++++++++++++++++++++++++-------
 net/tipc/socket.c  | 22 +++++++++++++-----
 net/unix/af_unix.c | 26 ++++++++++++++-------
 8 files changed, 126 insertions(+), 34 deletions(-)

-- 
2.51.0

[PATCH 1/2] net/socket: Record preference for synchronous wakeups

[Srikar Dronamraju]

Scheduler differentiates between affine and non-affine wakeups by the
way of sync flags. Scheduler prefers to pull the tasks towards the waker
if the sync flag is set.

In some cases, socket APIs are blindly requesting sync wakeups. This may
cause load-balance issues and non-optimal performance.

Record whether the most recent blocking socket operation could benefit
from synchronous wakeups. Subsequent readiness notifications use this
hint to determine whether WF_SYNC should be propagated.

The flag is advisory and affects only wakeup placement decisions.

Signed-off-by: Srikar Dronamraju <srikar@linux.ibm.com>
---
 include/net/sock.h |  1 +
 net/socket.c       | 56 +++++++++++++++++++++++++++++++++++++++-------
 2 files changed, 49 insertions(+), 8 deletions(-)

diff --git a/include/net/sock.h b/include/net/sock.h
index 51185222aac2..acc6b1976dc4 100644
--- a/include/net/sock.h
+++ b/include/net/sock.h
@@ -1022,6 +1022,7 @@ enum sock_flags {
 	SOCK_RCVMARK, /* Receive SO_MARK  ancillary data with packet */
 	SOCK_RCVPRIORITY, /* Receive SO_PRIORITY ancillary data with packet */
 	SOCK_TIMESTAMPING_ANY, /* Copy of sk_tsflags & TSFLAGS_ANY */
+	SOCK_SYNC_WAKEUP, /* Prefer synchronous socket wakeups */
 };
 
 #define SK_FLAGS_TIMESTAMP ((1UL << SOCK_TIMESTAMP) | (1UL << SOCK_TIMESTAMPING_RX_SOFTWARE))
diff --git a/net/socket.c b/net/socket.c
index 63c69a0fa74e..0bcb57ae490e 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -1198,15 +1198,27 @@ static void sock_splice_eof(struct file *file)
 		ops->splice_eof(sock);
 }
 
+static inline void sock_update_sync_wakeup(struct sock *sk, bool nonblock)
+{
+	if (unlikely(!sk))
+		return;
+
+	if (nonblock) {
+		if (sock_flag(sk, SOCK_SYNC_WAKEUP))
+			sock_reset_flag(sk, SOCK_SYNC_WAKEUP);
+	} else {
+		if (!sock_flag(sk, SOCK_SYNC_WAKEUP))
+			sock_set_flag(sk, SOCK_SYNC_WAKEUP);
+	}
+}
+
 static ssize_t sock_read_iter(struct kiocb *iocb, struct iov_iter *to)
 {
 	struct file *file = iocb->ki_filp;
 	struct socket *sock = file->private_data;
 	struct msghdr msg = {.msg_iter = *to};
 	ssize_t res;
-
-	if (file->f_flags & O_NONBLOCK || (iocb->ki_flags & IOCB_NOWAIT))
-		msg.msg_flags = MSG_DONTWAIT;
+	bool nonblock;
 
 	if (iocb->ki_pos != 0)
 		return -ESPIPE;
@@ -1214,6 +1226,11 @@ static ssize_t sock_read_iter(struct kiocb *iocb, struct iov_iter *to)
 	if (!iov_iter_count(to))	/* Match SYS5 behaviour */
 		return 0;
 
+	nonblock = (file->f_flags & O_NONBLOCK) || (iocb->ki_flags & IOCB_NOWAIT);
+	if (nonblock)
+		msg.msg_flags = MSG_DONTWAIT;
+
+	sock_update_sync_wakeup(sock->sk, nonblock);
 	res = sock_recvmsg(sock, &msg, msg.msg_flags);
 	*to = msg.msg_iter;
 	return res;
@@ -1225,13 +1242,17 @@ static ssize_t sock_write_iter(struct kiocb *iocb, struct iov_iter *from)
 	struct socket *sock = file->private_data;
 	struct msghdr msg = {.msg_iter = *from};
 	ssize_t res;
+	bool nonblock;
 
 	if (iocb->ki_pos != 0)
 		return -ESPIPE;
 
-	if (file->f_flags & O_NONBLOCK || (iocb->ki_flags & IOCB_NOWAIT))
+	nonblock = (file->f_flags & O_NONBLOCK) || (iocb->ki_flags & IOCB_NOWAIT);
+	if (nonblock)
 		msg.msg_flags = MSG_DONTWAIT;
 
+	sock_update_sync_wakeup(sock->sk, nonblock);
+
 	if (sock->type == SOCK_SEQPACKET)
 		msg.msg_flags |= MSG_EOR;
 
@@ -2221,6 +2242,7 @@ int __sys_sendto(int fd, void __user *buff, size_t len, unsigned int flags,
 	struct sockaddr_storage address;
 	int err;
 	struct msghdr msg;
+	bool nonblock;
 
 	err = import_ubuf(ITER_SOURCE, buff, len, &msg.msg_iter);
 	if (unlikely(err))
@@ -2246,8 +2268,11 @@ int __sys_sendto(int fd, void __user *buff, size_t len, unsigned int flags,
 		msg.msg_namelen = addr_len;
 	}
 	flags &= ~MSG_INTERNAL_SENDMSG_FLAGS;
-	if (sock->file->f_flags & O_NONBLOCK)
+	nonblock = (sock->file->f_flags & O_NONBLOCK);
+	if (nonblock)
 		flags |= MSG_DONTWAIT;
+
+	sock_update_sync_wakeup(sock->sk, nonblock);
 	msg.msg_flags = flags;
 	return __sock_sendmsg(sock, &msg);
 }
@@ -2284,6 +2309,7 @@ int __sys_recvfrom(int fd, void __user *ubuf, size_t size, unsigned int flags,
 	};
 	struct socket *sock;
 	int err, err2;
+	bool nonblock;
 
 	err = import_ubuf(ITER_DEST, ubuf, size, &msg.msg_iter);
 	if (unlikely(err))
@@ -2297,8 +2323,11 @@ int __sys_recvfrom(int fd, void __user *ubuf, size_t size, unsigned int flags,
 	if (unlikely(!sock))
 		return -ENOTSOCK;
 
-	if (sock->file->f_flags & O_NONBLOCK)
+	nonblock = (sock->file->f_flags & O_NONBLOCK);
+	if (nonblock)
 		flags |= MSG_DONTWAIT;
+
+	sock_update_sync_wakeup(sock->sk, nonblock);
 	err = sock_recvmsg(sock, &msg, flags);
 
 	if (err >= 0 && addr != NULL) {
@@ -2634,6 +2663,7 @@ static int ____sys_sendmsg(struct socket *sock, struct msghdr *msg_sys,
 	unsigned char *ctl_buf = ctl;
 	int ctl_len;
 	ssize_t err;
+	bool nonblock;
 
 	err = -ENOBUFS;
 
@@ -2666,8 +2696,12 @@ static int ____sys_sendmsg(struct socket *sock, struct msghdr *msg_sys,
 	flags &= ~MSG_INTERNAL_SENDMSG_FLAGS;
 	msg_sys->msg_flags = flags;
 
-	if (sock->file->f_flags & O_NONBLOCK)
+	nonblock = (sock->file->f_flags & O_NONBLOCK);
+	if (nonblock)
 		msg_sys->msg_flags |= MSG_DONTWAIT;
+
+	sock_update_sync_wakeup(sock->sk, nonblock);
+
 	/*
 	 * If this is sendmmsg() and current destination address is same as
 	 * previously succeeded address, omit asking LSM's decision.
@@ -2887,6 +2921,7 @@ static int ____sys_recvmsg(struct socket *sock, struct msghdr *msg_sys,
 	unsigned long cmsg_ptr;
 	int len;
 	ssize_t err;
+	bool nonblock;
 
 	msg_sys->msg_name = &addr;
 	cmsg_ptr = (unsigned long)msg_sys->msg_control;
@@ -2895,9 +2930,12 @@ static int ____sys_recvmsg(struct socket *sock, struct msghdr *msg_sys,
 	/* We assume all kernel code knows the size of sockaddr_storage */
 	msg_sys->msg_namelen = 0;
 
-	if (sock->file->f_flags & O_NONBLOCK)
+	nonblock = (sock->file->f_flags & O_NONBLOCK);
+	if (nonblock)
 		flags |= MSG_DONTWAIT;
 
+	sock_update_sync_wakeup(sock->sk, nonblock);
+
 	if (unlikely(nosec))
 		err = sock_recvmsg_nosec(sock, msg_sys, flags);
 	else
@@ -3056,6 +3094,8 @@ static int do_recvmmsg(int fd, struct mmsghdr __user *mmsg,
 		if (flags & MSG_WAITFORONE)
 			flags |= MSG_DONTWAIT;
 
+		sock_update_sync_wakeup(sock->sk, flags & MSG_WAITFORONE);
+
 		if (timeout) {
 			ktime_get_ts64(&timeout64);
 			*timeout = timespec64_sub(end_time, timeout64);
-- 
2.52.0

Re: [PATCH 1/2] net/socket: Record preference for synchronous wakeups

[Shrikanth Hegde]

Hi Srikar.

On 7/14/26 7:09 AM, Srikar Dronamraju wrote:
> Scheduler differentiates between affine and non-affine wakeups by the
> way of sync flags. Scheduler prefers to pull the tasks towards the waker
> if the sync flag is set.
> 
> In some cases, socket APIs are blindly requesting sync wakeups. This may
> cause load-balance issues and non-optimal performance.
> 
> Record whether the most recent blocking socket operation could benefit
> from synchronous wakeups. Subsequent readiness notifications use this
> hint to determine whether WF_SYNC should be propagated.
> 

What you mean by recent? Was it info on past set of packets?
Could you please explain the flow a bit?

Shouldn't it be
- if this socket has been defined as nonblock it shouldn't use sync
   always?

> The flag is advisory and affects only wakeup placement decisions.
> 
> Signed-off-by: Srikar Dronamraju <srikar@linux.ibm.com>
> ---
>   include/net/sock.h |  1 +
>   net/socket.c       | 56 +++++++++++++++++++++++++++++++++++++++-------
>   2 files changed, 49 insertions(+), 8 deletions(-)
> 
> diff --git a/include/net/sock.h b/include/net/sock.h
> index 51185222aac2..acc6b1976dc4 100644
> --- a/include/net/sock.h
> +++ b/include/net/sock.h
> @@ -1022,6 +1022,7 @@ enum sock_flags {
>   	SOCK_RCVMARK, /* Receive SO_MARK  ancillary data with packet */
>   	SOCK_RCVPRIORITY, /* Receive SO_PRIORITY ancillary data with packet */
>   	SOCK_TIMESTAMPING_ANY, /* Copy of sk_tsflags & TSFLAGS_ANY */
> +	SOCK_SYNC_WAKEUP, /* Prefer synchronous socket wakeups */
>   };
>   
>   #define SK_FLAGS_TIMESTAMP ((1UL << SOCK_TIMESTAMP) | (1UL << SOCK_TIMESTAMPING_RX_SOFTWARE))
> diff --git a/net/socket.c b/net/socket.c
> index 63c69a0fa74e..0bcb57ae490e 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -1198,15 +1198,27 @@ static void sock_splice_eof(struct file *file)
>   		ops->splice_eof(sock);
>   }
>   
> +static inline void sock_update_sync_wakeup(struct sock *sk, bool nonblock)
> +{
> +	if (unlikely(!sk))
> +		return;
> +
> +	if (nonblock) {
> +		if (sock_flag(sk, SOCK_SYNC_WAKEUP))
> +			sock_reset_flag(sk, SOCK_SYNC_WAKEUP);
> +	} else {
> +		if (!sock_flag(sk, SOCK_SYNC_WAKEUP))
> +			sock_set_flag(sk, SOCK_SYNC_WAKEUP);
> +	}
> +}

nit: You can combine two if statements. A bit easier to read.


static inline void sock_update_sync_wakeup(struct sock *sk, bool nonblock)
{
	if (unlikely(!sk))
		return;

	if (nonblock && sock_flag(sk, SOCK_SYNC_WAKEUP))
		sock_reset_flag(sk, SOCK_SYNC_WAKEUP);
	else if (!nonblock && !sock_flag(sk, SOCK_SYNC_WAKEUP))
		sock_set_flag(sk, SOCK_SYNC_WAKEUP);
}

> +
>   static ssize_t sock_read_iter(struct kiocb *iocb, struct iov_iter *to)
>   {
>   	struct file *file = iocb->ki_filp;
>   	struct socket *sock = file->private_data;
>   	struct msghdr msg = {.msg_iter = *to};
>   	ssize_t res;
> -
> -	if (file->f_flags & O_NONBLOCK || (iocb->ki_flags & IOCB_NOWAIT))
> -		msg.msg_flags = MSG_DONTWAIT;
> +	bool nonblock;
>   
>   	if (iocb->ki_pos != 0)
>   		return -ESPIPE;
> @@ -1214,6 +1226,11 @@ static ssize_t sock_read_iter(struct kiocb *iocb, struct iov_iter *to)
>   	if (!iov_iter_count(to))	/* Match SYS5 behaviour */
>   		return 0;
>   
> +	nonblock = (file->f_flags & O_NONBLOCK) || (iocb->ki_flags & IOCB_NOWAIT);
> +	if (nonblock)
> +		msg.msg_flags = MSG_DONTWAIT;
> +
> +	sock_update_sync_wakeup(sock->sk, nonblock);
>   	res = sock_recvmsg(sock, &msg, msg.msg_flags);
>   	*to = msg.msg_iter;
>   	return res;
> @@ -1225,13 +1242,17 @@ static ssize_t sock_write_iter(struct kiocb *iocb, struct iov_iter *from)
>   	struct socket *sock = file->private_data;
>   	struct msghdr msg = {.msg_iter = *from};
>   	ssize_t res;
> +	bool nonblock;
>   
>   	if (iocb->ki_pos != 0)
>   		return -ESPIPE;
>   
> -	if (file->f_flags & O_NONBLOCK || (iocb->ki_flags & IOCB_NOWAIT))
> +	nonblock = (file->f_flags & O_NONBLOCK) || (iocb->ki_flags & IOCB_NOWAIT);
> +	if (nonblock)
>   		msg.msg_flags = MSG_DONTWAIT;
>   
> +	sock_update_sync_wakeup(sock->sk, nonblock);
> +
>   	if (sock->type == SOCK_SEQPACKET)
>   		msg.msg_flags |= MSG_EOR;
>   
> @@ -2221,6 +2242,7 @@ int __sys_sendto(int fd, void __user *buff, size_t len, unsigned int flags,
>   	struct sockaddr_storage address;
>   	int err;
>   	struct msghdr msg;
> +	bool nonblock;
>   
>   	err = import_ubuf(ITER_SOURCE, buff, len, &msg.msg_iter);
>   	if (unlikely(err))
> @@ -2246,8 +2268,11 @@ int __sys_sendto(int fd, void __user *buff, size_t len, unsigned int flags,
>   		msg.msg_namelen = addr_len;
>   	}
>   	flags &= ~MSG_INTERNAL_SENDMSG_FLAGS;
> -	if (sock->file->f_flags & O_NONBLOCK)
> +	nonblock = (sock->file->f_flags & O_NONBLOCK);
> +	if (nonblock)
>   		flags |= MSG_DONTWAIT;
> +
> +	sock_update_sync_wakeup(sock->sk, nonblock);
>   	msg.msg_flags = flags;
>   	return __sock_sendmsg(sock, &msg);
>   }
> @@ -2284,6 +2309,7 @@ int __sys_recvfrom(int fd, void __user *ubuf, size_t size, unsigned int flags,
>   	};
>   	struct socket *sock;
>   	int err, err2;
> +	bool nonblock;
>   
>   	err = import_ubuf(ITER_DEST, ubuf, size, &msg.msg_iter);
>   	if (unlikely(err))
> @@ -2297,8 +2323,11 @@ int __sys_recvfrom(int fd, void __user *ubuf, size_t size, unsigned int flags,
>   	if (unlikely(!sock))
>   		return -ENOTSOCK;
>   
> -	if (sock->file->f_flags & O_NONBLOCK)
> +	nonblock = (sock->file->f_flags & O_NONBLOCK);
> +	if (nonblock)
>   		flags |= MSG_DONTWAIT;
> +
> +	sock_update_sync_wakeup(sock->sk, nonblock);
>   	err = sock_recvmsg(sock, &msg, flags);
>   
>   	if (err >= 0 && addr != NULL) {
> @@ -2634,6 +2663,7 @@ static int ____sys_sendmsg(struct socket *sock, struct msghdr *msg_sys,
>   	unsigned char *ctl_buf = ctl;
>   	int ctl_len;
>   	ssize_t err;
> +	bool nonblock;
>   
>   	err = -ENOBUFS;
>   
> @@ -2666,8 +2696,12 @@ static int ____sys_sendmsg(struct socket *sock, struct msghdr *msg_sys,
>   	flags &= ~MSG_INTERNAL_SENDMSG_FLAGS;
>   	msg_sys->msg_flags = flags;
>   
> -	if (sock->file->f_flags & O_NONBLOCK)
> +	nonblock = (sock->file->f_flags & O_NONBLOCK);
> +	if (nonblock)
>   		msg_sys->msg_flags |= MSG_DONTWAIT;
> +
> +	sock_update_sync_wakeup(sock->sk, nonblock);
> +
>   	/*
>   	 * If this is sendmmsg() and current destination address is same as
>   	 * previously succeeded address, omit asking LSM's decision.
> @@ -2887,6 +2921,7 @@ static int ____sys_recvmsg(struct socket *sock, struct msghdr *msg_sys,
>   	unsigned long cmsg_ptr;
>   	int len;
>   	ssize_t err;
> +	bool nonblock;
>   
>   	msg_sys->msg_name = &addr;
>   	cmsg_ptr = (unsigned long)msg_sys->msg_control;
> @@ -2895,9 +2930,12 @@ static int ____sys_recvmsg(struct socket *sock, struct msghdr *msg_sys,
>   	/* We assume all kernel code knows the size of sockaddr_storage */
>   	msg_sys->msg_namelen = 0;
>   
> -	if (sock->file->f_flags & O_NONBLOCK)
> +	nonblock = (sock->file->f_flags & O_NONBLOCK);
> +	if (nonblock)
>   		flags |= MSG_DONTWAIT;
>   
> +	sock_update_sync_wakeup(sock->sk, nonblock);
> +
>   	if (unlikely(nosec))
>   		err = sock_recvmsg_nosec(sock, msg_sys, flags);
>   	else
> @@ -3056,6 +3094,8 @@ static int do_recvmmsg(int fd, struct mmsghdr __user *mmsg,
>   		if (flags & MSG_WAITFORONE)
>   			flags |= MSG_DONTWAIT;
>   
> +		sock_update_sync_wakeup(sock->sk, flags & MSG_WAITFORONE);
> +
>   		if (timeout) {
>   			ktime_get_ts64(&timeout64);
>   			*timeout = timespec64_sub(end_time, timeout64);

Re: [PATCH 1/2] net/socket: Record preference for synchronous wakeups

[Willem de Bruijn]

Srikar Dronamraju wrote:
> Scheduler differentiates between affine and non-affine wakeups by the
> way of sync flags. Scheduler prefers to pull the tasks towards the waker
> if the sync flag is set.
> 
> In some cases, socket APIs are blindly requesting sync wakeups. This may
> cause load-balance issues and non-optimal performance.
> 
> Record whether the most recent blocking socket operation could benefit

What is the heuristic that determines this?

If respinning, please state that explicitly in the commit message.


> from synchronous wakeups. Subsequent readiness notifications use this
> hint to determine whether WF_SYNC should be propagated.
> 
> The flag is advisory and affects only wakeup placement decisions.
> 
> Signed-off-by: Srikar Dronamraju <srikar@linux.ibm.com>

[PATCH 2/2] net/sock: Propagate WF_SYNC only when requested

[Srikar Dronamraju]

Use SOCK_SYNC_WAKEUP to select between synchronous and asynchronous wakeup
wakeup APIs. This avoids propagating WF_SYNC when no blocking waiter is
expected. All wakeup locations in networking code that currently issue
synchronous poll-style wakeups unconditionally are updated.

Signed-off-by: Srikar Dronamraju <srikar@linux.ibm.com>
---
 net/core/sock.c    | 31 ++++++++++++++++++++++++-------
 net/sctp/socket.c  | 10 ++++++++--
 net/smc/af_smc.c   |  4 ++--
 net/smc/smc_rx.c   | 10 ++++++++--
 net/tipc/socket.c  | 22 +++++++++++++++++-----
 net/unix/af_unix.c | 26 ++++++++++++++++++--------
 6 files changed, 77 insertions(+), 26 deletions(-)

diff --git a/net/core/sock.c b/net/core/sock.c
index 8a59bfaa8096..a214e883b14b 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -3652,9 +3652,15 @@ void sock_def_readable(struct sock *sk)
 
 	rcu_read_lock();
 	wq = rcu_dereference(sk->sk_wq);
-	if (skwq_has_sleeper(wq))
-		wake_up_interruptible_sync_poll(&wq->wait, EPOLLIN | EPOLLPRI |
+	if (skwq_has_sleeper(wq)) {
+		if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
+			wake_up_interruptible_sync_poll(&wq->wait, EPOLLIN | EPOLLPRI |
+						EPOLLRDNORM | EPOLLRDBAND);
+		} else {
+			wake_up_interruptible_poll(&wq->wait, EPOLLIN | EPOLLPRI |
 						EPOLLRDNORM | EPOLLRDBAND);
+		}
+	}
 	sk_wake_async_rcu(sk, SOCK_WAKE_WAITD, POLL_IN);
 	rcu_read_unlock();
 }
@@ -3670,9 +3676,15 @@ static void sock_def_write_space(struct sock *sk)
 	 */
 	if (sock_writeable(sk)) {
 		wq = rcu_dereference(sk->sk_wq);
-		if (skwq_has_sleeper(wq))
-			wake_up_interruptible_sync_poll(&wq->wait, EPOLLOUT |
+		if (skwq_has_sleeper(wq)) {
+			if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
+				wake_up_interruptible_sync_poll(&wq->wait, EPOLLOUT |
+						EPOLLWRNORM | EPOLLWRBAND);
+			} else {
+				wake_up_interruptible_poll(&wq->wait, EPOLLOUT |
 						EPOLLWRNORM | EPOLLWRBAND);
+			}
+		}
 
 		/* Should agree with poll, otherwise some programs break */
 		sk_wake_async_rcu(sk, SOCK_WAKE_SPACE, POLL_OUT);
@@ -3695,10 +3707,15 @@ static void sock_def_write_space_wfree(struct sock *sk, int wmem_alloc)
 
 		/* rely on refcount_sub from sock_wfree() */
 		smp_mb__after_atomic();
-		if (wq && waitqueue_active(&wq->wait))
-			wake_up_interruptible_sync_poll(&wq->wait, EPOLLOUT |
+		if (wq && waitqueue_active(&wq->wait)) {
+			if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
+				wake_up_interruptible_sync_poll(&wq->wait, EPOLLOUT |
 						EPOLLWRNORM | EPOLLWRBAND);
-
+			} else {
+				wake_up_interruptible_poll(&wq->wait, EPOLLOUT |
+						EPOLLWRNORM | EPOLLWRBAND);
+			}
+		}
 		/* Should agree with poll, otherwise some programs break */
 		sk_wake_async_rcu(sk, SOCK_WAKE_SPACE, POLL_OUT);
 	}
diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index c7b9e325ec1c..9cb3432f065a 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -9348,9 +9348,15 @@ void sctp_data_ready(struct sock *sk)
 
 	rcu_read_lock();
 	wq = rcu_dereference(sk->sk_wq);
-	if (skwq_has_sleeper(wq))
-		wake_up_interruptible_sync_poll(&wq->wait, EPOLLIN |
+	if (skwq_has_sleeper(wq)) {
+		if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
+			wake_up_interruptible_sync_poll(&wq->wait, EPOLLIN |
+					EPOLLRDNORM | EPOLLRDBAND);
+		} else {
+			wake_up_interruptible_poll(&wq->wait, EPOLLIN |
 						EPOLLRDNORM | EPOLLRDBAND);
+		}
+	}
 	sk_wake_async_rcu(sk, SOCK_WAKE_WAITD, POLL_IN);
 	rcu_read_unlock();
 }
diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
index b5db69073e20..1a6ea2e30769 100644
--- a/net/smc/af_smc.c
+++ b/net/smc/af_smc.c
@@ -819,10 +819,10 @@ static void smc_fback_wakeup_waitqueue(struct smc_sock *smc, void *key)
 		wake_up_interruptible_all(&wq->wait);
 	} else {
 		flags = key_to_poll(key);
-		if (flags & (EPOLLIN | EPOLLOUT))
+		if (flags & (EPOLLIN | EPOLLOUT) && sock_flag(&smc->sk, SOCK_SYNC_WAKEUP))
 			/* sk_data_ready or sk_write_space */
 			wake_up_interruptible_sync_poll(&wq->wait, flags);
-		else if (flags & EPOLLERR)
+		else
 			/* sk_error_report */
 			wake_up_interruptible_poll(&wq->wait, flags);
 	}
diff --git a/net/smc/smc_rx.c b/net/smc/smc_rx.c
index c1d9b923938d..4e288a2364d2 100644
--- a/net/smc/smc_rx.c
+++ b/net/smc/smc_rx.c
@@ -39,9 +39,15 @@ static void smc_rx_wake_up(struct sock *sk)
 	/* called already in smc_listen_work() */
 	rcu_read_lock();
 	wq = rcu_dereference(sk->sk_wq);
-	if (skwq_has_sleeper(wq))
-		wake_up_interruptible_sync_poll(&wq->wait, EPOLLIN | EPOLLPRI |
+	if (skwq_has_sleeper(wq)) {
+		if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
+			wake_up_interruptible_sync_poll(&wq->wait, EPOLLIN | EPOLLPRI |
 						EPOLLRDNORM | EPOLLRDBAND);
+		} else {
+			wake_up_interruptible_poll(&wq->wait, EPOLLIN | EPOLLPRI |
+						EPOLLRDNORM | EPOLLRDBAND);
+		}
+	}
 	sk_wake_async_rcu(sk, SOCK_WAKE_WAITD, POLL_IN);
 	if ((sk->sk_shutdown == SHUTDOWN_MASK) ||
 	    (sk->sk_state == SMC_CLOSED))
diff --git a/net/tipc/socket.c b/net/tipc/socket.c
index e564341e0216..9fa83a89882c 100644
--- a/net/tipc/socket.c
+++ b/net/tipc/socket.c
@@ -2116,9 +2116,15 @@ static void tipc_write_space(struct sock *sk)
 
 	rcu_read_lock();
 	wq = rcu_dereference(sk->sk_wq);
-	if (skwq_has_sleeper(wq))
-		wake_up_interruptible_sync_poll(&wq->wait, EPOLLOUT |
+	if (skwq_has_sleeper(wq)) {
+		if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
+			wake_up_interruptible_sync_poll(&wq->wait, EPOLLOUT |
 						EPOLLWRNORM | EPOLLWRBAND);
+		} else {
+			wake_up_interruptible_poll(&wq->wait, EPOLLOUT |
+						EPOLLWRNORM | EPOLLWRBAND);
+		}
+	}
 	rcu_read_unlock();
 }
 
@@ -2134,9 +2140,15 @@ static void tipc_data_ready(struct sock *sk)
 
 	rcu_read_lock();
 	wq = rcu_dereference(sk->sk_wq);
-	if (skwq_has_sleeper(wq))
-		wake_up_interruptible_sync_poll(&wq->wait, EPOLLIN |
-						EPOLLRDNORM | EPOLLRDBAND);
+	if (skwq_has_sleeper(wq)) {
+		if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
+			wake_up_interruptible_sync_poll(&wq->wait, EPOLLIN |
+							EPOLLRDNORM | EPOLLRDBAND);
+		} else {
+			wake_up_interruptible_poll(&wq->wait, EPOLLIN |
+							EPOLLRDNORM | EPOLLRDBAND);
+		}
+	}
 	rcu_read_unlock();
 }
 
diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
index f7a9d55eee8a..15ebcc2d9d58 100644
--- a/net/unix/af_unix.c
+++ b/net/unix/af_unix.c
@@ -601,9 +601,15 @@ static void unix_write_space(struct sock *sk)
 	rcu_read_lock();
 	if (unix_writable(sk, READ_ONCE(sk->sk_state))) {
 		wq = rcu_dereference(sk->sk_wq);
-		if (skwq_has_sleeper(wq))
-			wake_up_interruptible_sync_poll(&wq->wait,
-				EPOLLOUT | EPOLLWRNORM | EPOLLWRBAND);
+		if (skwq_has_sleeper(wq)) {
+			if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
+				wake_up_interruptible_sync_poll(&wq->wait,
+					EPOLLOUT | EPOLLWRNORM | EPOLLWRBAND);
+			} else {
+				wake_up_interruptible_poll(&wq->wait,
+					EPOLLOUT | EPOLLWRNORM | EPOLLWRBAND);
+			}
+		}
 		sk_wake_async_rcu(sk, SOCK_WAKE_SPACE, POLL_OUT);
 	}
 	rcu_read_unlock();
@@ -2603,11 +2609,15 @@ int __unix_dgram_recvmsg(struct sock *sk, struct msghdr *msg, size_t size,
 		goto out;
 	}
 
-	if (wq_has_sleeper(&u->peer_wait))
-		wake_up_interruptible_sync_poll(&u->peer_wait,
-						EPOLLOUT | EPOLLWRNORM |
-						EPOLLWRBAND);
-
+	if (wq_has_sleeper(&u->peer_wait)) {
+		if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
+			wake_up_interruptible_sync_poll(&u->peer_wait,
+						EPOLLOUT | EPOLLWRNORM | EPOLLWRBAND);
+		} else {
+			wake_up_interruptible_poll(&u->peer_wait,
+						EPOLLOUT | EPOLLWRNORM | EPOLLWRBAND);
+		}
+	}
 	if (msg->msg_name) {
 		unix_copy_addr(msg, skb->sk);
 
-- 
2.52.0

Re: [PATCH 2/2] net/sock: Propagate WF_SYNC only when requested

[Shrikanth Hegde]

Hi Srikar,

On 7/14/26 7:09 AM, Srikar Dronamraju wrote:
> Use SOCK_SYNC_WAKEUP to select between synchronous and asynchronous wakeup
> wakeup APIs. This avoids propagating WF_SYNC when no blocking waiter is
> expected. All wakeup locations in networking code that currently issue
> synchronous poll-style wakeups unconditionally are updated.
> 

You can also add the performance data in the cover-letter to this patch.

> Signed-off-by: Srikar Dronamraju <srikar@linux.ibm.com>
> ---
>   net/core/sock.c    | 31 ++++++++++++++++++++++++-------
>   net/sctp/socket.c  | 10 ++++++++--
>   net/smc/af_smc.c   |  4 ++--
>   net/smc/smc_rx.c   | 10 ++++++++--
>   net/tipc/socket.c  | 22 +++++++++++++++++-----
>   net/unix/af_unix.c | 26 ++++++++++++++++++--------
>   6 files changed, 77 insertions(+), 26 deletions(-)
> 
> diff --git a/net/core/sock.c b/net/core/sock.c
> index 8a59bfaa8096..a214e883b14b 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -3652,9 +3652,15 @@ void sock_def_readable(struct sock *sk)
>   
>   	rcu_read_lock();
>   	wq = rcu_dereference(sk->sk_wq);
> -	if (skwq_has_sleeper(wq))
> -		wake_up_interruptible_sync_poll(&wq->wait, EPOLLIN | EPOLLPRI |
> +	if (skwq_has_sleeper(wq)) {
> +		if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
> +			wake_up_interruptible_sync_poll(&wq->wait, EPOLLIN | EPOLLPRI |
> +						EPOLLRDNORM | EPOLLRDBAND);
> +		} else {
> +			wake_up_interruptible_poll(&wq->wait, EPOLLIN | EPOLLPRI |
>   						EPOLLRDNORM | EPOLLRDBAND);
> +		}
> +	}
>   	sk_wake_async_rcu(sk, SOCK_WAKE_WAITD, POLL_IN);
>   	rcu_read_unlock();
>   }
> @@ -3670,9 +3676,15 @@ static void sock_def_write_space(struct sock *sk)
>   	 */
>   	if (sock_writeable(sk)) {
>   		wq = rcu_dereference(sk->sk_wq);
> -		if (skwq_has_sleeper(wq))
> -			wake_up_interruptible_sync_poll(&wq->wait, EPOLLOUT |
> +		if (skwq_has_sleeper(wq)) {
> +			if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
> +				wake_up_interruptible_sync_poll(&wq->wait, EPOLLOUT |
> +						EPOLLWRNORM | EPOLLWRBAND);
> +			} else {
> +				wake_up_interruptible_poll(&wq->wait, EPOLLOUT |
>   						EPOLLWRNORM | EPOLLWRBAND);
> +			}
> +		}
>   
>   		/* Should agree with poll, otherwise some programs break */
>   		sk_wake_async_rcu(sk, SOCK_WAKE_SPACE, POLL_OUT);
> @@ -3695,10 +3707,15 @@ static void sock_def_write_space_wfree(struct sock *sk, int wmem_alloc)
>   
>   		/* rely on refcount_sub from sock_wfree() */
>   		smp_mb__after_atomic();
> -		if (wq && waitqueue_active(&wq->wait))
> -			wake_up_interruptible_sync_poll(&wq->wait, EPOLLOUT |
> +		if (wq && waitqueue_active(&wq->wait)) {
> +			if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
> +				wake_up_interruptible_sync_poll(&wq->wait, EPOLLOUT |
>   						EPOLLWRNORM | EPOLLWRBAND);
> -
> +			} else {
> +				wake_up_interruptible_poll(&wq->wait, EPOLLOUT |
> +						EPOLLWRNORM | EPOLLWRBAND);
> +			}
> +		}
>   		/* Should agree with poll, otherwise some programs break */
>   		sk_wake_async_rcu(sk, SOCK_WAKE_SPACE, POLL_OUT);
>   	}
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index c7b9e325ec1c..9cb3432f065a 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -9348,9 +9348,15 @@ void sctp_data_ready(struct sock *sk)
>   
>   	rcu_read_lock();
>   	wq = rcu_dereference(sk->sk_wq);
> -	if (skwq_has_sleeper(wq))
> -		wake_up_interruptible_sync_poll(&wq->wait, EPOLLIN |
> +	if (skwq_has_sleeper(wq)) {
> +		if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
> +			wake_up_interruptible_sync_poll(&wq->wait, EPOLLIN |
> +					EPOLLRDNORM | EPOLLRDBAND);
> +		} else {
> +			wake_up_interruptible_poll(&wq->wait, EPOLLIN |
>   						EPOLLRDNORM | EPOLLRDBAND);
> +		}
> +	}
>   	sk_wake_async_rcu(sk, SOCK_WAKE_WAITD, POLL_IN);
>   	rcu_read_unlock();
>   }
> diff --git a/net/smc/af_smc.c b/net/smc/af_smc.c
> index b5db69073e20..1a6ea2e30769 100644
> --- a/net/smc/af_smc.c
> +++ b/net/smc/af_smc.c
> @@ -819,10 +819,10 @@ static void smc_fback_wakeup_waitqueue(struct smc_sock *smc, void *key)
>   		wake_up_interruptible_all(&wq->wait);
>   	} else {
>   		flags = key_to_poll(key);
> -		if (flags & (EPOLLIN | EPOLLOUT))
> +		if (flags & (EPOLLIN | EPOLLOUT) && sock_flag(&smc->sk, SOCK_SYNC_WAKEUP))
>   			/* sk_data_ready or sk_write_space */
>   			wake_up_interruptible_sync_poll(&wq->wait, flags);
> -		else if (flags & EPOLLERR)
> +		else
>   			/* sk_error_report */
>   			wake_up_interruptible_poll(&wq->wait, flags);
>   	}
> diff --git a/net/smc/smc_rx.c b/net/smc/smc_rx.c
> index c1d9b923938d..4e288a2364d2 100644
> --- a/net/smc/smc_rx.c
> +++ b/net/smc/smc_rx.c
> @@ -39,9 +39,15 @@ static void smc_rx_wake_up(struct sock *sk)
>   	/* called already in smc_listen_work() */
>   	rcu_read_lock();
>   	wq = rcu_dereference(sk->sk_wq);
> -	if (skwq_has_sleeper(wq))
> -		wake_up_interruptible_sync_poll(&wq->wait, EPOLLIN | EPOLLPRI |
> +	if (skwq_has_sleeper(wq)) {
> +		if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
> +			wake_up_interruptible_sync_poll(&wq->wait, EPOLLIN | EPOLLPRI |
>   						EPOLLRDNORM | EPOLLRDBAND);
> +		} else {
> +			wake_up_interruptible_poll(&wq->wait, EPOLLIN | EPOLLPRI |
> +						EPOLLRDNORM | EPOLLRDBAND);
> +		}
> +	}
>   	sk_wake_async_rcu(sk, SOCK_WAKE_WAITD, POLL_IN);
>   	if ((sk->sk_shutdown == SHUTDOWN_MASK) ||
>   	    (sk->sk_state == SMC_CLOSED))
> diff --git a/net/tipc/socket.c b/net/tipc/socket.c
> index e564341e0216..9fa83a89882c 100644
> --- a/net/tipc/socket.c
> +++ b/net/tipc/socket.c
> @@ -2116,9 +2116,15 @@ static void tipc_write_space(struct sock *sk)
>   
>   	rcu_read_lock();
>   	wq = rcu_dereference(sk->sk_wq);
> -	if (skwq_has_sleeper(wq))
> -		wake_up_interruptible_sync_poll(&wq->wait, EPOLLOUT |
> +	if (skwq_has_sleeper(wq)) {
> +		if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
> +			wake_up_interruptible_sync_poll(&wq->wait, EPOLLOUT |
>   						EPOLLWRNORM | EPOLLWRBAND);
> +		} else {
> +			wake_up_interruptible_poll(&wq->wait, EPOLLOUT |
> +						EPOLLWRNORM | EPOLLWRBAND);
> +		}
> +	}
>   	rcu_read_unlock();
>   }
>   
> @@ -2134,9 +2140,15 @@ static void tipc_data_ready(struct sock *sk)
>   
>   	rcu_read_lock();
>   	wq = rcu_dereference(sk->sk_wq);
> -	if (skwq_has_sleeper(wq))
> -		wake_up_interruptible_sync_poll(&wq->wait, EPOLLIN |
> -						EPOLLRDNORM | EPOLLRDBAND);
> +	if (skwq_has_sleeper(wq)) {
> +		if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
> +			wake_up_interruptible_sync_poll(&wq->wait, EPOLLIN |
> +							EPOLLRDNORM | EPOLLRDBAND);
> +		} else {
> +			wake_up_interruptible_poll(&wq->wait, EPOLLIN |
> +							EPOLLRDNORM | EPOLLRDBAND);
> +		}
> +	}
>   	rcu_read_unlock();
>   }
>   
> diff --git a/net/unix/af_unix.c b/net/unix/af_unix.c
> index f7a9d55eee8a..15ebcc2d9d58 100644
> --- a/net/unix/af_unix.c
> +++ b/net/unix/af_unix.c
> @@ -601,9 +601,15 @@ static void unix_write_space(struct sock *sk)
>   	rcu_read_lock();
>   	if (unix_writable(sk, READ_ONCE(sk->sk_state))) {
>   		wq = rcu_dereference(sk->sk_wq);
> -		if (skwq_has_sleeper(wq))
> -			wake_up_interruptible_sync_poll(&wq->wait,
> -				EPOLLOUT | EPOLLWRNORM | EPOLLWRBAND);
> +		if (skwq_has_sleeper(wq)) {
> +			if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
> +				wake_up_interruptible_sync_poll(&wq->wait,
> +					EPOLLOUT | EPOLLWRNORM | EPOLLWRBAND);
> +			} else {
> +				wake_up_interruptible_poll(&wq->wait,
> +					EPOLLOUT | EPOLLWRNORM | EPOLLWRBAND);
> +			}
> +		}
>   		sk_wake_async_rcu(sk, SOCK_WAKE_SPACE, POLL_OUT);
>   	}
>   	rcu_read_unlock();
> @@ -2603,11 +2609,15 @@ int __unix_dgram_recvmsg(struct sock *sk, struct msghdr *msg, size_t size,
>   		goto out;
>   	}
>   
> -	if (wq_has_sleeper(&u->peer_wait))
> -		wake_up_interruptible_sync_poll(&u->peer_wait,
> -						EPOLLOUT | EPOLLWRNORM |
> -						EPOLLWRBAND);
> -
> +	if (wq_has_sleeper(&u->peer_wait)) {
> +		if (sock_flag(sk, SOCK_SYNC_WAKEUP)) {
> +			wake_up_interruptible_sync_poll(&u->peer_wait,
> +						EPOLLOUT | EPOLLWRNORM | EPOLLWRBAND);
> +		} else {
> +			wake_up_interruptible_poll(&u->peer_wait,
> +						EPOLLOUT | EPOLLWRNORM | EPOLLWRBAND);
> +		}
> +	}
>   	if (msg->msg_name) {
>   		unix_copy_addr(msg, skb->sk);
>   


Would it make sense to write a macro or a wrapper function do the
same instead of sprinkling the same at all the places?

similar comment for patch 1.

IMHO, it would make it easier to read.

Re: [PATCH 0/2] net: Use synchronous wakeups selectively

[Jakub Kicinski]

On Tue, 14 Jul 2026 07:09:41 +0530 Srikar Dronamraju wrote:
> The scheduler assumes in several wakeup paths that a task using WF_SYNC
> is likely to yield the CPU shortly. However several networking wakeup
> paths unconditionally use synchronous wakeups even when the waking task
> continues execution.
> 
> During wakeup, with WF_SYNC flag set, because of the assumption that
> current thread is ready to give up, the wakee thread will be migrated
> from any core within the chip to the current LLC. If these operations
> are frequent, and wakers are actually not going away, then it will lead
> to load imbalance and hurt performance. This is especially true in
> architectures where LLCs are small and number of LLCs per chip are more.
> 
> Running vllm workload was run on Power10 system
>                                   No patch      with patch %diff
> Inference Time (sec)              17.84         16.35     -8.35%
> llm query bandwidth (tokens/sec)  14.78         16.21     +9.68%
> 
> Lower inference time and higher tokens/sec is better.

Somewhat related recent patch:

https://lore.kernel.org/all/20260708133815.3419465-1-usama.arif@linux.dev/

Maybe other maintainers will chime in, but I'm not convinced 
by the heuristics you're adding. The behavior will be workload
specific. The extent of networking involvement should be letting
the scheduler know that we're waking from IO, not leaking scheduler
heuristics into networking.
-- 
pw-bot: cr

Re: [PATCH 0/2] net: Use synchronous wakeups selectively

[Eric Dumazet]

On Wed, Jul 22, 2026 at 7:08 PM Jakub Kicinski <kuba@kernel.org> wrote:
>
> On Tue, 14 Jul 2026 07:09:41 +0530 Srikar Dronamraju wrote:
> > The scheduler assumes in several wakeup paths that a task using WF_SYNC
> > is likely to yield the CPU shortly. However several networking wakeup
> > paths unconditionally use synchronous wakeups even when the waking task
> > continues execution.
> >
> > During wakeup, with WF_SYNC flag set, because of the assumption that
> > current thread is ready to give up, the wakee thread will be migrated
> > from any core within the chip to the current LLC. If these operations
> > are frequent, and wakers are actually not going away, then it will lead
> > to load imbalance and hurt performance. This is especially true in
> > architectures where LLCs are small and number of LLCs per chip are more.
> >
> > Running vllm workload was run on Power10 system
> >                                   No patch      with patch %diff
> > Inference Time (sec)              17.84         16.35     -8.35%
> > llm query bandwidth (tokens/sec)  14.78         16.21     +9.68%
> >
> > Lower inference time and higher tokens/sec is better.
>
> Somewhat related recent patch:
>
> https://lore.kernel.org/all/20260708133815.3419465-1-usama.arif@linux.dev/
>
> Maybe other maintainers will chime in, but I'm not convinced
> by the heuristics you're adding. The behavior will be workload
> specific. The extent of networking involvement should be letting
> the scheduler know that we're waking from IO, not leaking scheduler
> heuristics into networking.
> --
> pw-bot: cr

In any case, adding code to the fast path in every network system call
is undesirable.

Re: [PATCH 0/2] net: Use synchronous wakeups selectively

[Srikar Dronamraju]

* Eric Dumazet <edumazet@google.com> [2026-07-23 05:17:08]:

> On Wed, Jul 22, 2026 at 7:08 PM Jakub Kicinski <kuba@kernel.org> wrote:
> >
> > On Tue, 14 Jul 2026 07:09:41 +0530 Srikar Dronamraju wrote:
> > > The scheduler assumes in several wakeup paths that a task using WF_SYNC
> > > is likely to yield the CPU shortly. However several networking wakeup
> > > paths unconditionally use synchronous wakeups even when the waking task
> > > continues execution.
> >
> > Somewhat related recent patch:
> >
> > https://lore.kernel.org/all/20260708133815.3419465-1-usama.arif@linux.dev/
> >
> > Maybe other maintainers will chime in, but I'm not convinced
> > by the heuristics you're adding. The behavior will be workload
> > specific. The extent of networking involvement should be letting
> > the scheduler know that we're waking from IO, not leaking scheduler
> > heuristics into networking.
> > --
> > pw-bot: cr
> 
> In any case, adding code to the fast path in every network system call
> is undesirable.

Yes, I understand adding code in fast path is undesirable. However as
mentioned previous reply, we need to differentiate between blocking and
non-blocking API. Doing unconditional nonblocking, is probably even worse.

-- 
Thanks and Regards
Srikar Dronamraju

Re: [PATCH 0/2] net: Use synchronous wakeups selectively

[Eric Dumazet]

On Thu, Jul 23, 2026 at 10:50 AM Srikar Dronamraju <srikar@linux.ibm.com> wrote:
>
> * Eric Dumazet <edumazet@google.com> [2026-07-23 05:17:08]:
>
> > On Wed, Jul 22, 2026 at 7:08 PM Jakub Kicinski <kuba@kernel.org> wrote:
> > >
> > > On Tue, 14 Jul 2026 07:09:41 +0530 Srikar Dronamraju wrote:
> > > > The scheduler assumes in several wakeup paths that a task using WF_SYNC
> > > > is likely to yield the CPU shortly. However several networking wakeup
> > > > paths unconditionally use synchronous wakeups even when the waking task
> > > > continues execution.
> > >
> > > Somewhat related recent patch:
> > >
> > > https://lore.kernel.org/all/20260708133815.3419465-1-usama.arif@linux.dev/
> > >
> > > Maybe other maintainers will chime in, but I'm not convinced
> > > by the heuristics you're adding. The behavior will be workload
> > > specific. The extent of networking involvement should be letting
> > > the scheduler know that we're waking from IO, not leaking scheduler
> > > heuristics into networking.
> > > --
> > > pw-bot: cr
> >
> > In any case, adding code to the fast path in every network system call
> > is undesirable.
>
> Yes, I understand adding code in fast path is undesirable. However as
> mentioned previous reply, we need to differentiate between blocking and
> non-blocking API. Doing unconditional nonblocking, is probably even worse.

I suggested that applications choose what they prefer for each socket,
once and for all.
(This can also be a cgroup/BPF setting)

Flipping a socket bit at each recvmsg()/sendmsg() is a bad idea, you
might break old application expectations.

Re: [PATCH 0/2] net: Use synchronous wakeups selectively

[Srikar Dronamraju]

* Jakub Kicinski <kuba@kernel.org> [2026-07-22 10:08:42]:

Hi Jakub, Thanks for taking a look.

> On Tue, 14 Jul 2026 07:09:41 +0530 Srikar Dronamraju wrote:
> > The scheduler assumes in several wakeup paths that a task using WF_SYNC
> > is likely to yield the CPU shortly. However several networking wakeup
> > paths unconditionally use synchronous wakeups even when the waking task
> > continues execution.
> > 
> > During wakeup, with WF_SYNC flag set, because of the assumption that
> > current thread is ready to give up, the wakee thread will be migrated
> > from any core within the chip to the current LLC. If these operations
> > are frequent, and wakers are actually not going away, then it will lead
> > to load imbalance and hurt performance. This is especially true in
> > architectures where LLCs are small and number of LLCs per chip are more.
> > 
> > Running vllm workload was run on Power10 system
> >                                   No patch      with patch %diff
> > Inference Time (sec)              17.84         16.35     -8.35%
> > llm query bandwidth (tokens/sec)  14.78         16.21     +9.68%
> > 
> > Lower inference time and higher tokens/sec is better.
> 
> Somewhat related recent patch:
> 
> https://lore.kernel.org/all/20260708133815.3419465-1-usama.arif@linux.dev/
> 
> Maybe other maintainers will chime in, but I'm not convinced 
> by the heuristics you're adding. The behavior will be workload
> specific.

Currently scheduler provides 2 APIs because it supports 2 subtle behaviours.
1st API being wake_up_interruptible_poll and 2nd API being
wake_up_interruptible_sync_poll. The reason scheduler provides 2 APIs is
because the scheduler cant decide which behaviour the callee wants.

If the scheduler could have figured out which behaviour then there would
have been only 1 API. So the onus of using the right API is on the
callee.

Now the behaviour between the 2 APIs depends on fact that the callee is
going to give up the CPU now or if the callee will continue to run. If the
callee is going to continue to run, its better to use the more generic API
of the two which is wake_up_interruptible_poll. However if the callee is
giving up the CPU, its better to use wake_up_interruptible_sync_poll.

@peterz @ingom can you please confirm this behaviour is inline with
scheduler maintainers view.

Currently the networking code as I see is unconditionally (or blindly)
asking for sync behaviour i.e the socket API is saying the callees are going
to give up CPU soon. However the problem I am seeing is the callees are
actually not giving up CPU.

> The extent of networking involvement should be letting
> the scheduler know that we're waking from IO, not leaking scheduler
> heuristics into networking.

In this change, what we are depending on is file's flag that the socket
request is blocking or non-blocking. So why do we say we are leaking
scheduler hueristics? Would using an API or another API mean leaking
scheduler hueristics?  If we are blocking, this change will use
wake_up_interruptible_sync_poll and if non-blocking, it will use
wake_up_interruptible_poll.

-- 
Thanks and Regards
Srikar Dronamraju