netdev - Re: [PATCH RFC net-next 5/5] net: dev_queue

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Message-ID: <30754f20-b556-4790-b978-f27df5657583@redhat.com>
Date: Wed, 8 Oct 2025 09:44:57 +0200
From: Paolo Abeni <pabeni@...hat.com>
To: Eric Dumazet <edumazet@...gle.com>
Cc: "David S . Miller" <davem@...emloft.net>, Jakub Kicinski
 <kuba@...nel.org>, Simon Horman <horms@...nel.org>,
 Jamal Hadi Salim <jhs@...atatu.com>, Cong Wang <xiyou.wangcong@...il.com>,
 Jiri Pirko <jiri@...nulli.us>, Kuniyuki Iwashima <kuniyu@...gle.com>,
 Willem de Bruijn <willemb@...gle.com>, netdev@...r.kernel.org,
 eric.dumazet@...il.com
Subject: Re: [PATCH RFC net-next 5/5] net: dev_queue_xmit() llist adoption

On 10/8/25 9:32 AM, Eric Dumazet wrote:
> On Tue, Oct 7, 2025 at 11:37 PM Paolo Abeni <pabeni@...hat.com> wrote:
>>
>> Hi,
>>
>> On 10/6/25 9:31 PM, Eric Dumazet wrote:
>>> diff --git a/include/net/sch_generic.h b/include/net/sch_generic.h
>>> index 31561291bc92fd70d4d3ca8f5f7dbc4c94c895a0..94966692ccdf51db085c236319705aecba8c30cf 100644
>>> --- a/include/net/sch_generic.h
>>> +++ b/include/net/sch_generic.h
>>> @@ -115,7 +115,9 @@ struct Qdisc {
>>>       struct Qdisc            *next_sched;
>>>       struct sk_buff_head     skb_bad_txq;
>>>
>>> -     spinlock_t              busylock ____cacheline_aligned_in_smp;
>>> +     atomic_long_t           defer_count ____cacheline_aligned_in_smp;
>>> +     struct llist_head       defer_list;
>>> +
>>
>> Dumb question: I guess the above brings back Qdisc to the original size
>> ? (pre patch 4/5?) If so, perhaps is worth mentioning somewhere in the
>> commit message.
> 
> 
> Hmm, I do not think this changes the size.
> The cache line that was starting at busylock had holes.
> You can see that even adding the long and the pointer, we still have room in it.

Ah, nice! I did not take in account the ____cacheline_aligned_in_smp on
a later field (privdata).

Thanks for the clarifying pahole output!

Paolo