netdev - Re: Packet gets stuck in NOLOCK pfifo

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Message-ID: <9dbb08f05890cc4130b54c80e4f4072b49d9f0ed.camel@redhat.com>
Date:   Thu, 09 Jul 2020 11:20:58 +0200
From:   Paolo Abeni <pabeni@...hat.com>
To:     Cong Wang <xiyou.wangcong@...il.com>
Cc:     Josh Hunt <johunt@...mai.com>,
        Jonas Bonn <jonas.bonn@...rounds.com>,
        Michael Zhivich <mzhivich@...mai.com>,
        David Miller <davem@...emloft.net>,
        John Fastabend <john.fastabend@...il.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>
Subject: Re: Packet gets stuck in NOLOCK pfifo_fast qdisc

On Wed, 2020-07-08 at 13:16 -0700, Cong Wang wrote:
> On Tue, Jul 7, 2020 at 7:18 AM Paolo Abeni <pabeni@...hat.com> wrote:
> > So the regression with 2 pktgen threads is still relevant. 'perf' shows
> > relevant time spent into net_tx_action() and __netif_schedule().
> 
> So, touching the __QDISC_STATE_SCHED bit in __dev_xmit_skb() is
> not a good idea.
> 
> Let me see if there is any other way to fix this.

Thank you very much for the effort! I'm personally out of ideas for a
real fix that would avoid regressions. 

To be more exaustive this are the sources of overhead, as far as I can
observe them with perf:

- contention on q->state, in __netif_schedule()
- execution of net_tx_action() when there are no packet to be served

Cheers,

Paolo