netdev - Re: [Linuxarm] Re: [PATCH net v6 3/3] net: sched: fix tx action reschedule issue with stopped queue

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Message-ID: <20210512124731.2993dac7@kicinski-fedora-PC1C0HJN>
Date:   Wed, 12 May 2021 12:47:31 -0700
From:   Jakub Kicinski <kuba@...nel.org>
To:     Yunsheng Lin <linyunsheng@...wei.com>
Cc:     <davem@...emloft.net>, <olteanv@...il.com>, <ast@...nel.org>,
        <daniel@...earbox.net>, <andriin@...com>, <edumazet@...gle.com>,
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        <linux-kernel@...r.kernel.org>, <linuxarm@...neuler.org>,
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Subject: Re: [Linuxarm] Re: [PATCH net v6 3/3] net: sched: fix tx action
 reschedule issue with stopped queue

On Wed, 12 May 2021 11:34:55 +0800 Yunsheng Lin wrote:
> > This is indeed the idiomatic way of dealing with Tx queue stopping race,
> > but it's a bit of code to sprinkle around. My vote would be option 1.  
> 
> I had done some performance testing to see which is better, tested using
> pktgen and dummy netdev with pfifo_fast qdisc attached:
> 
> unit: Mpps
> 
> threads    V6         V6 + option 1     V6 + option 3
>   1       2.60          2.54               2.60
>   2       3.86          3.84               3.84
>   4       5.56          5.50               5.51
>   8       2.79          2.77               2.77
>   16      2.23          2.24               2.22
> 
> So it seems the netif_xmit_frozen_or_stopped checking overhead for non-stopped queue
> is noticable for 1 pktgen thread.
> 
> And the performance increase for V6 + option 1 with 16 pktgen threads is because of
> "clear_bit(__QDISC_STATE_MISSED, &qdisc->state)" at the end of qdisc_run_end(), which
> may avoid the another round of dequeuing in the pfifo_fast_dequeue(). And adding the
> "clear_bit(__QDISC_STATE_MISSED, &qdisc->state)"  for V6 + option 3, the data for
> 16 pktgen thread also go up to 2.24Mpps.
> 
> 
> So it seems V6 + option 3 with "clear_bit(__QDISC_STATE_MISSED, &qdisc->state)" at
> the end of qdisc_run_end() is better?

Alright, sounds good.