netdev - Re: [PATCH net v4 1/2] net: sched: fix packet stuck problem for lockless qdisc

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Date:   Tue, 20 Apr 2021 10:23:36 +0800
From:   Yunsheng Lin <linyunsheng@...wei.com>
To:     Michal Kubecek <mkubecek@...e.cz>
CC:     <davem@...emloft.net>, <kuba@...nel.org>, <olteanv@...il.com>,
        <ast@...nel.org>, <daniel@...earbox.net>, <andriin@...com>,
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        <cong.wang@...edance.com>, <ap420073@...il.com>,
        <netdev@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        <linuxarm@...neuler.org>, <mkl@...gutronix.de>,
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        <alexander.duyck@...il.com>, <hdanton@...a.com>, <jgross@...e.com>,
        <JKosina@...e.com>
Subject: Re: [PATCH net v4 1/2] net: sched: fix packet stuck problem for
 lockless qdisc

On 2021/4/20 7:55, Michal Kubecek wrote:
> On Mon, Apr 19, 2021 at 05:29:46PM +0200, Michal Kubecek wrote:
>>
>> As pointed out in the discussion on v3, this patch may result in
>> significantly higher CPU consumption with multiple threads competing on
>> a saturated outgoing device. I missed this submission so that I haven't
>> checked it yet but given the description of v3->v4 changes above, it's
>> quite likely that it suffers from the same problem.
> 
> And it indeed does. However, with the additional patch from the v3
> discussion, the numbers are approximately the same as with an unpatched
> mainline kernel.
> 
> As with v4, I tried this patch on top of 5.12-rc7 with real devices.
> I used two machines with 10Gb/s Intel ixgbe NICs, sender has 16 CPUs
> (2 8-core CPUs with HT disabled) and 16 Rx/Tx queues, receiver has
> 48 CPUs (2 12-core CPUs with HT enabled) and 48 Rx/Tx queues.
> 
>   threads    5.12-rc7    5.12-rc7 + v4    5.12-rc7 + v4 + stop
>      1        25.1%          38.1%            22.9%
>      8        66.2%         277.0%            74.1%
>     16        90.1%         150.7%            91.0%
>     32       107.2%         272.6%           108.3%
>     64       116.3%         487.5%           118.1%
>    128       126.1%         946.7%           126.9%
> 
> (The values are normalized to one core, i.e. 100% corresponds to one
> fully used logical CPU.)
> 
> So it seems that repeated scheduling while the queue was stopped is
> indeed the main performance issue and that other cases of the logic
> being too pessimistic do not play significant role. There is an
> exception with 8 connections/threads and the result with just this
> series also looks abnormally high (e.g. much higher than with
> 16 threads). It might be worth investigating what happens there and
> what do the results with other thread counts around 8 look like.

Will try to investigate the 8 connections/threads case.

> 
> I'll run some more tests with other traffic patterns tomorrow and
> I'm also going to take a closer look at the additional patch.

Thanks for taking the detail testing and looking.

> 
> Michal
> 
> .
>