From: Yunsheng Lin <linyunsheng@huawei.com>
To: David Miller <davem@redhat.com>
Cc: <kuba@kernel.org>, <jhs@mojatatu.com>, <xiyou.wangcong@gmail.com>,
<jiri@resnulli.us>, <netdev@vger.kernel.org>,
<linux-kernel@vger.kernel.org>, <linuxarm@openeuler.org>
Subject: Re: [PATCH net-next] net: sched: remove unnecessay lock protection for skb_bad_txq/gso_skb
Date: Tue, 16 Mar 2021 10:40:56 +0800 [thread overview]
Message-ID: <1fea8225-69b0-5a73-0e9d-f5bfdecdc840@huawei.com> (raw)
In-Reply-To: <20210315.164151.1093629330365238718.davem@redhat.com>
On 2021/3/16 7:41, David Miller wrote:
> From: Yunsheng Lin <linyunsheng@huawei.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@huawei.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.
>
>
> .
>
next prev parent reply other threads:[~2021-03-16 2:42 UTC|newest]
Thread overview: 9+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-03-15 9:30 [PATCH net-next] net: sched: remove unnecessay lock protection for skb_bad_txq/gso_skb Yunsheng Lin
2021-03-15 23:41 ` David Miller
2021-03-16 2:40 ` Yunsheng Lin [this message]
2021-03-16 21:45 ` David Miller
2021-03-17 0:45 ` Yunsheng Lin
2021-03-16 18:43 ` Cong Wang
2021-03-17 0:50 ` Yunsheng Lin
2021-03-16 18:41 ` Cong Wang
2021-03-17 1:01 ` Yunsheng Lin
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