Re: [PATCH bpf v2 02/12] bpf: sockmap, convert schedule_work into delayed_work

Jakub Sitnicki wrote:

On Mon, Mar 27, 2023 at 10:54 AM -07, John Fastabend wrote:

quoted

Sk_buffs are fed into sockmap verdict programs either from a strparser
(when the user might want to decide how framing of skb is done by attaching
another parser program) or directly through tcp_read_sock. The
tcp_read_sock is the preferred method for performance when the BPF logic is
a stream parser.

The flow for Cilium's common use case with a stream parser is,

 tcp_read_sock()
  sk_psock_verdict_recv
    ret = bpf_prog_run_pin_on_cpu()
    sk_psock_verdict_apply(sock, skb, ret)
     // if system is under memory pressure or app is slow we may
     // need to queue skb. Do this queuing through ingress_skb and
     // then kick timer to wake up handler
     skb_queue_tail(ingress_skb, skb)
     schedule_work(work);


The work queue is wired up to sk_psock_backlog(). This will then walk the
ingress_skb skb list that holds our sk_buffs that could not be handled,
but should be OK to run at some later point. However, its possible that
the workqueue doing this work still hits an error when sending the skb.
When this happens the skbuff is requeued on a temporary 'state' struct
kept with the workqueue. This is necessary because its possible to
partially send an skbuff before hitting an error and we need to know how
and where to restart when the workqueue runs next.

Now for the trouble, we don't rekick the workqueue. This can cause a
stall where the skbuff we just cached on the state variable might never
be sent. This happens when its the last packet in a flow and no further
packets come along that would cause the system to kick the workqueue from
that side.

To fix we could do simple schedule_work(), but while under memory pressure
it makes sense to back off some instead of continue to retry repeatedly. So
instead to fix convert schedule_work to schedule_delayed_work and add
backoff logic to reschedule from backlog queue on errors. Its not obvious
though what a good backoff is so use '1'.

To test we observed some flakes whil running NGINX compliance test with
sockmap we attributed these failed test to this bug and subsequent issue.

Fixes: 04919bed948dc ("tcp: Introduce tcp_read_skb()")
Tested-by: William Findlay <redacted>
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
---

[...]

quoted

--- a/net/core/skmsg.c
+++ b/net/core/skmsg.c

@@ -481,7 +481,7 @@ int sk_msg_recvmsg(struct sock *sk, struct sk_psock *psock, struct msghdr *msg,
 	}
 out:
 	if (psock->work_state.skb && copied > 0)
-		schedule_work(&psock->work);
+		schedule_delayed_work(&psock->work, 0);
 	return copied;
 }
 EXPORT_SYMBOL_GPL(sk_msg_recvmsg);

@@ -639,7 +639,8 @@ static void sk_psock_skb_state(struct sk_psock *psock,
 
 static void sk_psock_backlog(struct work_struct *work)
 {
-	struct sk_psock *psock = container_of(work, struct sk_psock, work);
+	struct delayed_work *dwork = to_delayed_work(work);
+	struct sk_psock *psock = container_of(dwork, struct sk_psock, work);
 	struct sk_psock_work_state *state = &psock->work_state;
 	struct sk_buff *skb = NULL;
 	bool ingress;

@@ -679,6 +680,10 @@ static void sk_psock_backlog(struct work_struct *work)
 				if (ret == -EAGAIN) {
 					sk_psock_skb_state(psock, state, skb,
 							   len, off);
+
+					// Delay slightly to prioritize any
+					// other work that might be here.
+					schedule_delayed_work(&psock->work, 1);

Do IIUC that this means we can back out changes from commit bec217197b41
("skmsg: Schedule psock work if the cached skb exists on the psock")?

Yeah I think so this is a more direct way to get the same result. I'm also
thinking this check,

       if (psock->work_state.skb && copied > 0)
               schedule_work(&psock->work)

is not correct copied=0 which could happen on empty queue could be the
result of a skb stuck from this eagain error in backlog.

I think its OK to revert that patch in a separate patch. And ideally we
could get some way to load up the stack to hit these corner cases without
running long stress tests.

WDYT?

Nit: Comment syntax.

Yep happy to fix.