linux-kernel - Re: [PATCH] tun: use skb_attempt_defer_free in tun_do

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Message-ID: <CANn89iK_v3CWvf7=QakbB3dwvJEOxuVjEn14rjmONaa1rKVWKw@mail.gmail.com>
Date: Fri, 7 Nov 2025 01:11:07 -0800
From: Eric Dumazet <edumazet@...gle.com>
To: "Hudson, Nick" <nhudson@...mai.com>
Cc: Jason Wang <jasowang@...hat.com>, Willem de Bruijn <willemdebruijn.kernel@...il.com>, 
	Andrew Lunn <andrew+netdev@...n.ch>, "David S. Miller" <davem@...emloft.net>, 
	Jakub Kicinski <kuba@...nel.org>, Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	"netdev@...r.kernel.org" <netdev@...r.kernel.org>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] tun: use skb_attempt_defer_free in tun_do_read

On Fri, Nov 7, 2025 at 12:41 AM Hudson, Nick <nhudson@...mai.com> wrote:
>
>
>
> > On 7 Nov 2025, at 02:21, Jason Wang <jasowang@...hat.com> wrote:
> >
> > !-------------------------------------------------------------------|
> >  This Message Is From an External Sender
> >  This message came from outside your organization.
> > |-------------------------------------------------------------------!
> >
> > On Thu, Nov 6, 2025 at 11:51 PM Nick Hudson <nhudson@...mai.com> wrote:
> >>
> >> On a 640 CPU system running virtio-net VMs with the vhost-net driver, and
> >> multiqueue (64) tap devices testing has shown contention on the zone lock
> >> of the page allocator.
> >>
> >> A 'perf record -F99 -g sleep 5' of the CPUs where the vhost worker threads run shows
> >>
> >>    # perf report -i perf.data.vhost --stdio --sort overhead  --no-children | head -22
> >>    ...
> >>    #
> >>       100.00%
> >>                |
> >>                |--9.47%--queued_spin_lock_slowpath
> >>                |          |
> >>                |           --9.37%--_raw_spin_lock_irqsave
> >>                |                     |
> >>                |                     |--5.00%--__rmqueue_pcplist
> >>                |                     |          get_page_from_freelist
> >>                |                     |          __alloc_pages_noprof
> >>                |                     |          |
> >>                |                     |          |--3.34%--napi_alloc_skb
> >>    #
> >>
> >> That is, for Rx packets
> >> - ksoftirqd threads pinned 1:1 to CPUs do SKB allocation.
> >> - vhost-net threads float across CPUs do SKB free.
> >>
> >> One method to avoid this contention is to free SKB allocations on the same
> >> CPU as they were allocated on. This allows freed pages to be placed on the
> >> per-cpu page (PCP) lists so that any new allocations can be taken directly
> >> from the PCP list rather than having to request new pages from the page
> >> allocator (and taking the zone lock).
> >>
> >> Fortunately, previous work has provided all the infrastructure to do this
> >> via the skb_attempt_defer_free call which this change uses instead of
> >> consume_skb in tun_do_read.
> >>
> >> Testing done with a 6.12 based kernel and the patch ported forward.
> >>
> >> Server is Dual Socket AMD SP5 - 2x AMD SP5 9845 (Turin) with 2 VMs
> >> Load generator: iPerf2 x 1200 clients MSS=400
> >>
> >> Before:
> >> Maximum traffic rate: 55Gbps
> >>
> >> After:
> >> Maximum traffic rate 110Gbps
> >> ---
> >> drivers/net/tun.c | 2 +-
> >> net/core/skbuff.c | 2 ++
> >> 2 files changed, 3 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/drivers/net/tun.c b/drivers/net/tun.c
> >> index 8192740357a0..388f3ffc6657 100644
> >> --- a/drivers/net/tun.c
> >> +++ b/drivers/net/tun.c
> >> @@ -2185,7 +2185,7 @@ static ssize_t tun_do_read(struct tun_struct *tun, struct tun_file *tfile,
> >>                if (unlikely(ret < 0))
> >>                        kfree_skb(skb);
> >>                else
> >> -                       consume_skb(skb);
> >> +                       skb_attempt_defer_free(skb);
> >>        }
> >>
> >>        return ret;
> >> diff --git a/net/core/skbuff.c b/net/core/skbuff.c
> >> index 6be01454f262..89217c43c639 100644
> >> --- a/net/core/skbuff.c
> >> +++ b/net/core/skbuff.c
> >> @@ -7201,6 +7201,7 @@ nodefer:  kfree_skb_napi_cache(skb);
> >>        DEBUG_NET_WARN_ON_ONCE(skb_dst(skb));
> >>        DEBUG_NET_WARN_ON_ONCE(skb->destructor);
> >>        DEBUG_NET_WARN_ON_ONCE(skb_nfct(skb));
> >> +       DEBUG_NET_WARN_ON_ONCE(skb_shared(skb));
> >
> > I may miss something but it looks there's no guarantee that the packet
> > sent to TAP is not shared.
>
> Yes, I did wonder.
>
> How about something like
>
> /**
> * consume_skb_attempt_defer - free an skbuff
> * @skb: buffer to free
> *
> * Drop a ref to the buffer and attempt to defer free it if the usage count
> * has hit zero.
> */
> void consume_skb_attempt_defer(struct sk_buff *skb)
> {
> if (!skb_unref(skb))
> return;
>
> trace_consume_skb(skb, __builtin_return_address(0));
>
> skb_attempt_defer_free(skb);
> }
> EXPORT_SYMBOL(consume_skb_attempt_defer);
>
> and an inline version for the !CONFIG_TRACEPOINTS case

I will take care of the changes, have you seen my recent series ?

https://lore.kernel.org/netdev/20251106202935.1776179-1-edumazet@google.com/T/#m94e853a732f3cf1bb6a8f613f7d9d6f150f87f6f

I think you are missing a few points....