Re: [PATCH net] net: fix raising a softirq on the current cpu with rps enabled

On Sat, Mar 25, 2023 at 11:57 PM Eric Dumazet [off-list ref] wrote:

On Sat, Mar 25, 2023 at 8:26 AM Jason Xing [off-list ref] wrote:

quoted

From: Jason Xing <kernelxing@tencent.com>

Since we decide to put the skb into a backlog queue of another
cpu, we should not raise the softirq for the current cpu. When
to raise a softirq is based on whether we have more data left to
process later. As to the current cpu, there is no indication of
more data enqueued, so we do not need this action. After enqueuing
to another cpu, net_rx_action() function will call ipi and then
another cpu will raise the softirq as expected.

Also, raising more softirqs which set the corresponding bit field
can make the IRQ mechanism think we probably need to start ksoftirqd
on the current cpu. Actually it shouldn't happen.

Fixes: 0a9627f2649a ("rps: Receive Packet Steering")
Signed-off-by: Jason Xing <kernelxing@tencent.com>
---
 net/core/dev.c | 2 --
 1 file changed, 2 deletions(-)

diff --git a/net/core/dev.c b/net/core/dev.c
index 1518a366783b..bfaaa652f50c 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c

@@ -4594,8 +4594,6 @@ static int napi_schedule_rps(struct softnet_data *sd)
        if (sd != mysd) {
                sd->rps_ipi_next = mysd->rps_ipi_list;
                mysd->rps_ipi_list = sd;
-
-               __raise_softirq_irqoff(NET_RX_SOFTIRQ);
                return 1;
        }
 #endif /* CONFIG_RPS */
--

2.37.3

This is not going to work in some cases. Please take a deeper look.

I have to run, if you (or others) do not find the reason, I will give
more details when I am done traveling.

I'm wondering whether we could use @mysd instead of @sd like this:

if (!__test_and_set_bit(NAPI_STATE_SCHED, &mysd->backlog.state))
    __raise_softirq_irqoff(NET_RX_SOFTIRQ);

I traced back to some historical changes and saw some relations with
this commit ("net: solve a NAPI race"):
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=39e6c8208d7b6fb9d2047850fb3327db567b564b

Thanks,
Jason