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Subject: Re: [PATCH net-next] net: sched: remove unnecessay lock protection
 for skb_bad_txq/gso_skb
To:     Cong Wang <xiyou.wangcong@gmail.com>
CC:     David Miller <davem@davemloft.net>,
        Jakub Kicinski <kuba@kernel.org>,
        Jamal Hadi Salim <jhs@mojatatu.com>,
        Jiri Pirko <jiri@resnulli.us>,
        "Linux Kernel Network Developers" <netdev@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>, <linuxarm@openeuler.org>
References: <1615800610-34700-1-git-send-email-linyunsheng@huawei.com>
 <CAM_iQpXT+tS1NdpiF2M0hAocWJ90mxd5Wp8HoxkEhp4k9QM4hw@mail.gmail.com>
From:   Yunsheng Lin <linyunsheng@huawei.com>
Message-ID: <e6fd29fe-ccc1-d360-9840-0314cd77d7e6@huawei.com>
Date:   Wed, 17 Mar 2021 09:01:05 +0800
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On 2021/3/17 2:41, Cong Wang wrote:
> On Mon, Mar 15, 2021 at 2:29 AM Yunsheng Lin <linyunsheng@huawei.com> wrote:
>>
>> Currently qdisc_lock(q) is taken before enqueuing and dequeuing
>> for lockless qdisc's skb_bad_txq/gso_skb queue, qdisc->seqlock is
>> also taken, which can provide the same protection as qdisc_lock(q).
>>
>> This patch removes the unnecessay qdisc_lock(q) protection for
>> lockless qdisc' skb_bad_txq/gso_skb queue.
>>
>> And dev_reset_queue() takes the qdisc->seqlock for lockless qdisc
>> besides taking the qdisc_lock(q) when doing the qdisc reset,
>> some_qdisc_is_busy() takes both qdisc->seqlock and qdisc_lock(q)
>> when checking qdisc status. It is unnecessary to take both lock
>> while the fast path only take one lock, so this patch also changes
>> it to only take qdisc_lock(q) for locked qdisc, and only take
>> qdisc->seqlock for lockless qdisc.
>>
>> Since qdisc->seqlock is taken for lockless qdisc when calling
>> qdisc_is_running() in some_qdisc_is_busy(), use qdisc->running
>> to decide if the lockless qdisc is running.
> 
> What's the benefit here? Since qdisc->q.lock is also per-qdisc,
> so there is no actual contention to take it when we already acquire
> q->seqlock, right?

Yes, there is no actual contention to take qdisc->q.lock while
q->seqlock is acquired, but a cleanup or minor optimization.

> 
> Also, is ->seqlock supposed to be used for protecting skb_bad_txq
> etc.? From my understanding, it was introduced merely for replacing
> __QDISC_STATE_RUNNING. If you want to extend it, you probably
> have to rename it too.

How about just using qdisc->q.lock for lockless qdisc too and remove
dqisc->seqlock completely?

> 
> Thanks.
> 
> .
>