Re: [PATCH] net: use sync wakeups for socket error reports

[Eric Dumazet]

On Wed, Jul 8, 2026 at 6:09 PM Usama Arif <usama.arif@xxxxxxxxx> wrote:
>
>
>
> On 08/07/2026 16:25, Breno Leitao wrote:
> > On Wed, Jul 08, 2026 at 06:38:15AM -0700, Usama Arif wrote:
> >> Measured on a 176-core EPYC 9D64 host running a Meta production
> >> workload, bpftrace on tracepoint:ipi:ipi_send_cpu with a kstack filter
> >> attributed the sock_def_error_report -> ep_poll_callback ->
> >> try_to_wake_up -> ttwu_queue_wakelist -> __smp_call_single_queue
> >> chain to 16,326 IPIs/min.
> >
> > I am interested in why so many sock_def_error_report().
> >
> > That's seems a lot for genuine socket errors (RST/ICMP) on a healthy
> > host, so I suspect these aren't errors at all?
> >
> > Can you share the full stack above sock_def_error_report()?
>
>
> I ran this bpftrace script the host now (results added at the end):
>
> sudo bpftrace -e '
>   kprobe:sock_def_error_report
>   {
>     @wake_src[kstack()] = count();
>   }
>   interval:s:60
>   {
>     print(@wake_src, 5);
>     exit();
>   }'
>
>
> The biggest source is tcp_sendmsg -> __skb_tstamp_tx, which as you said
> is not an actual error. __skb_tstamp_tx clones the outgoing skb, tags it
> with ee_origin = SO_EE_ORIGIN_TIMESTAMPING and ee_errno = ENOMSG, enqueues
> it on sk->sk_error_queue via sock_queue_err_skb, and calls sk_error_report
> so epoll raises EPOLLERR. Userspace then reads it with recvmsg(MSG_ERRQUEUE)
> to get the SND/ACK timestamp.
>
> So the workload has SO_TIMESTAMPING enabled on its TCP sockets, and every
> packet completion and every ACK triggers a timestamp delivery through the
> error-queue path, which is why sock_def_error_report fires.
>

It seems we can not please everyone.

https://lore.kernel.org/netdev/20260526063650.952-1-xuewen.yan@xxxxxxxxxx/

Perhaps this SYNC heuristic should be a per-socket choice so that
applications can decide what is best for them.