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

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Message-Id: <AD5D3F27-9E32-4B18-97D8-762F0C3A9285@icloud.com>
Date: Wed, 19 Nov 2025 20:49:08 -0500
From: Jon Kohler <jonmkohler@...oud.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "Hudson, Nick" <nhudson@...mai.com>,
 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 Nov 7, 2025, at 4:19 AM, Eric Dumazet <edumazet@...gle.com> wrote:
> 
> On Fri, Nov 7, 2025 at 1:16 AM Hudson, Nick <nhudson@...mai.com> wrote:
>> 
>> 
>> 
>>> On 7 Nov 2025, at 09:11, Eric Dumazet <edumazet@...gle.com> wrote:
>>> 
>>> !-------------------------------------------------------------------|
>>> This Message Is From an External Sender
>>> This message came from outside your organization.
>>> |-------------------------------------------------------------------!
>>> 
>>> 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 ?
>> 
>> Great, thanks. I did see your series and will evaluate the improvement in our test setup.
>> 
>>> 
>>> 
>>> I think you are missing a few points….
>> 
>> Sure, still learning.
> 
> Sure !
> 
> Make sure to add in your dev .config : CONFIG_DEBUG_NET=y
> 

Hey Nick,
Thanks for sending this out, and funny enough, I had almost this
exact same series of thoughts back in May, but ended up getting
sucked into a rabbit hole the size of Texas and never circled
back to finish up the series.

Check out my series here: 
https://patchwork.kernel.org/project/netdevbpf/patch/20250506145530.2877229-5-jon@nutanix.com/

I was also monkeying around with defer free in this exact spot,
but it too got lost in the rabbit hole, so I’m glad I stumbled
upon this again tonight.

Let me dust this baby off and send a v2 on top of Eric’s
napi_consume_skb() series, as the combination of the two
of them should net out positively for you

Jon