linux-kernel - Re: [PATCH net-next] net: sched: remove unnecessay lock protection for skb_bad_txq/gso

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Date:   Tue, 16 Mar 2021 10:40:56 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     David Miller <davem@...hat.com>
CC:     <kuba@...nel.org>, <jhs@...atatu.com>, <xiyou.wangcong@...il.com>,
        <jiri@...nulli.us>, <netdev@...r.kernel.org>,
        <linux-kernel@...r.kernel.org>, <linuxarm@...neuler.org>
Subject: Re: [PATCH net-next] net: sched: remove unnecessay lock protection
 for skb_bad_txq/gso_skb

On 2021/3/16 7:41, David Miller wrote:
> From: Yunsheng Lin <linyunsheng@...wei.com>
> Date: Mon, 15 Mar 2021 17:30:10 +0800
> 
>> 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.
>>
>> Signed-off-by: Yunsheng Lin <linyunsheng@...wei.com>
> 
> What about other things protected by this lock, such as statistics and qlen?
> 
> This change looks too risky to me.

Ok, If that is the case, maybe we just remove qdisc->seqlock and use
qdisc_lock(q) for lockless qdisc too, so that we do not need to worry
about "lockless qdisc' other things protected by qdisc_lock(q)".

At least for the fast path, taking two locks for lockless qdisc hurts
performance when handling requeued skb, especially if the lockless
qdisc supports TCQ_F_CAN_BYPASS.

> 
> 
> .
>