netdev - Re: [PATCH v3 net-next 3/3] net: use napi_skb

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Message-ID: <CANn89iJb8hLw7Mx1+Td_BK7gGm5guRaUe6zdhqRqtfdw_0gLzA@mail.gmail.com>
Date: Mon, 17 Nov 2025 00:57:04 -0800
From: Eric Dumazet <edumazet@...gle.com>
To: Jason Xing <kerneljasonxing@...il.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	Kuniyuki Iwashima <kuniyu@...gle.com>, netdev@...r.kernel.org, eric.dumazet@...il.com, 
	Maciej Fijalkowski <maciej.fijalkowski@...el.com>
Subject: Re: [PATCH v3 net-next 3/3] net: use napi_skb_cache even in process context

On Mon, Nov 17, 2025 at 12:41 AM Jason Xing <kerneljasonxing@...il.com> wrote:
>
> On Mon, Nov 17, 2025 at 9:07 AM Jason Xing <kerneljasonxing@...il.com> wrote:
> >
> > On Mon, Nov 17, 2025 at 4:27 AM Eric Dumazet <edumazet@...gle.com> wrote:
> > >
> > > This is a followup of commit e20dfbad8aab ("net: fix napi_consume_skb()
> > > with alien skbs").
> > >
> > > Now the per-cpu napi_skb_cache is populated from TX completion path,
> > > we can make use of this cache, especially for cpus not used
> > > from a driver NAPI poll (primary user of napi_cache).
> > >
> > > We can use the napi_skb_cache only if current context is not from hard irq.
> > >
> > > With this patch, I consistently reach 130 Mpps on my UDP tx stress test
> > > and reduce SLUB spinlock contention to smaller values.
> > >
> > > Note there is still some SLUB contention for skb->head allocations.
> > >
> > > I had to tune /sys/kernel/slab/skbuff_small_head/cpu_partial
> > > and /sys/kernel/slab/skbuff_small_head/min_partial depending
> > > on the platform taxonomy.
> > >
> > > Signed-off-by: Eric Dumazet <edumazet@...gle.com>
> >
> > Reviewed-by: Jason Xing <kerneljasonxing@...il.com>
> >
> > Thanks for working on this. Previously I was thinking about this as
> > well since it affects the hot path for xsk (please see
> > __xsk_generic_xmit()->xsk_build_skb()->sock_alloc_send_pskb()). But I
> > wasn't aware of the benefits between disabling irq and allocating
> > memory. AFAIK, I once removed an enabling/disabling irq pair and saw a
> > minor improvement as this commit[1] says. Would you share your
> > invaluable experience with us in this case?
> >
> > In the meantime, I will do more rounds of experiments to see how they perform.
>
> Tested-by: Jason Xing <kerneljasonxing@...il.com>
> Done! I managed to see an improvement. The pps number goes from
> 1,458,644 to 1,647,235 by running [2].
>
> But sadly the news is that the previous commit [3] leads to a huge
> decrease in af_xdp from 1,980,000 to 1,458,644. With commit [3]
> applied, I observed and found xdpsock always allocated the skb on cpu
> 0 but the napi poll triggered skb_attempt_defer_free() on another
> call[4], which affected the final results.
>
> [2]
> taskset -c 0 ./xdpsock -i enp2s0f1 -q 1 -t -S -s 64
>
> [3]
> commit e20dfbad8aab2b7c72571ae3c3e2e646d6b04cb7
> Author: Eric Dumazet <edumazet@...gle.com>
> Date:   Thu Nov 6 20:29:34 2025 +0000
>
>     net: fix napi_consume_skb() with alien skbs
>
>     There is a lack of NUMA awareness and more generally lack
>     of slab caches affinity on TX completion path.
>
> [4]
> @c[
>     skb_attempt_defer_free+1
>     ixgbe_clean_tx_irq+723
>     ixgbe_poll+119
>     __napi_poll+48
> , ksoftirqd/24]: 1964731
>
> @c[
>     kick_defer_list_purge+1
>     napi_consume_skb+333
>     ixgbe_clean_tx_irq+723
>     ixgbe_poll+119
> , 34, swapper/34]: 123779
>
> Thanks,
> Jason

Hi Jason.

It is a bit hard to guess without more details (cpu you are using),
and perhaps perf profiles.
In particular which cpu is the bottleneck ?

1) There is still the missing part about tuning NAPI_SKB_CACHE_SIZE /
NAPI_SKB_CACHE_BULK, I was hoping you could send the patch we
discussed earlier ?

2) I am also working on allowing batches of skbs for skb_attempt_defer_free().

Another item I am working on is to let the qdisc being serviced
preferably not by the cpu performing TX completion,
I mentioned about making qdisc->running a sequence that we can latch
in __netif_schedule().
(Idea is to be able to not spin on qdisc spinlock from net_tx_action()
if another cpu was able to call qdisc_run())