linux-kernel - Re: [RFC v2] net/core: add optional threading for rps backlog processing

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Date:   Fri, 17 Feb 2023 16:54:11 +0100
From:   Eric Dumazet <edumazet@...gle.com>
To:     Felix Fietkau <nbd@....name>
Cc:     netdev@...r.kernel.org, "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Paolo Abeni <pabeni@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC v2] net/core: add optional threading for rps backlog processing

On Fri, Feb 17, 2023 at 4:26 PM Felix Fietkau <nbd@....name> wrote:

>
> > Then, process_backlog() has been designed to run only from the cpu
> > tied to the per-cpu data (softnet_data)
> > There are multiple comments about this assumption, and various things
> > that would need to be changed
> > (eg sd_has_rps_ipi_waiting() would be wrong in its current implementation)
> That's why I added the NAPI_STATE_THREADED check in napi_schedule_rps,
> so that sd_has_rps_ipi_waiting would always return false.
> Or are you worried about a race when enabling threading?
>

Please look at all uses of sd->process_queue, without locking. They do
not care about NAPI_STATE_THREADED

flush_backlog() is one instance, but process_backlog() is also using
__skb_dequeue(&sd->process_queue)

I suspect the following patch would work today, and would show
process_queue lock is not used.

diff --git a/net/core/dev.c b/net/core/dev.c
index 5687b528d4c18ef2960edb6bf3161bbad666aece..bed540b417a1b4cd3e384611a4681b8e2a43fd30
100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -11396,7 +11396,7 @@ static int __init net_dev_init(void)
                INIT_WORK(flush, flush_backlog);

                skb_queue_head_init(&sd->input_pkt_queue);
-               skb_queue_head_init(&sd->process_queue);
+               __skb_queue_head_init(&sd->process_queue);
 #ifdef CONFIG_XFRM_OFFLOAD
                skb_queue_head_init(&sd->xfrm_backlog);
 #endif