Re: [RFC PATCH] net/core: use wake_up_interruptible_poll() in sock_def_readable()

[Xuewen Yan]

Hello everyone,
Any comments about this?

Or we only ignore sync for thread:
---
diff --git a/net/core/sock.c b/net/core/sock.c
index b37b664b6eb9..a46334266e86 100644
--- a/net/core/sock.c
+++ b/net/core/sock.c
@@ -3610,9 +3610,14 @@ 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 (in_interrupt())
+                       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();
 }
@@ -3628,9 +3633,14 @@ 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 (in_interrupt())
+
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);

On Tue, May 26, 2026 at 2:37 PM Xuewen Yan <xuewen.yan@xxxxxxxxxx> wrote:
>
> sock_def_readable() currently uses wake_up_interruptible_sync_poll() to
> wake up tasks waiting for readable data on a socket. The _sync variant
> sets the WF_SYNC flag, which tells the scheduler that the waker will
> schedule away soon, so the wakee should stay on the same CPU to avoid
> needless cache bouncing.
>
> However, we found that the following stack:
>  -vfs_write
>  -sock_write_iter
>  -unix_stream_sendmsg
>  -sock_def_readable
>  -__wake_up_sync_key
>
> In this process-context scenario, the waker does NOT go to sleep
> after the wakeup. With WF_SYNC, the scheduler is misled into placing
> the wakee on the waker's CPU (via wake_affine_idle()'s sync path when
> nr_running == 1), causing both the sender and receiver to contend for
> the same CPU. This may hurt throughput for IPC workloads on multi-core
> systems where the sender and receiver could otherwise run in parallel
> on different CPUs.
>
> Switch to wake_up_interruptible_poll() which does not set WF_SYNC.
> This allows the scheduler to freely migrate the wakee to an idle CPU,
> enabling true parallelism between the sending and receiving processes.
>
> Co-developed-by: Guohua Yan <guohua.yan@xxxxxxxxxx>
> Signed-off-by: Guohua Yan <guohua.yan@xxxxxxxxxx>
> Signed-off-by: Xuewen Yan <xuewen.yan@xxxxxxxxxx>
> ---
> Note:
> The possible cost is that for softirq callers (TCP/UDP receive path), the wakee
> may be migrated away from the current CPU where the received data is
> cache-hot. However, this is mitigated by:
>   - The scheduler's existing wake_affine logic which already considers
>     cache affinity regardless of WF_SYNC.
>
> We are not very familiar with the networking code here,
> so we would greatly appreciate any suggestions or advice from the community.
>
> Thanks！
> ---
>  net/core/sock.c | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
>
> diff --git a/net/core/sock.c b/net/core/sock.c
> index b37b664b6eb9..42ab9373194f 100644
> --- a/net/core/sock.c
> +++ b/net/core/sock.c
> @@ -3611,7 +3611,7 @@ 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 |
> +               wake_up_interruptible_poll(&wq->wait, EPOLLIN | EPOLLPRI |
>                                                 EPOLLRDNORM | EPOLLRDBAND);
>         sk_wake_async_rcu(sk, SOCK_WAKE_WAITD, POLL_IN);
>         rcu_read_unlock();
> --
> 2.25.1
>