netdev - Re: [WIP] net+mlx4: auto doorbell

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Date:   Fri, 02 Dec 2016 10:16:30 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Tom Herbert <tom@...bertland.com>
Cc:     Jesper Dangaard Brouer <brouer@...hat.com>,
        Willem de Bruijn <willemb@...gle.com>,
        Rick Jones <rick.jones2@....com>,
        Linux Kernel Network Developers <netdev@...r.kernel.org>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Tariq Toukan <tariqt@...lanox.com>,
        Achiad Shochat <achiad@...lanox.com>
Subject: Re: [WIP] net+mlx4: auto doorbell

On Wed, 2016-11-30 at 18:50 -0800, Eric Dumazet wrote:
> On Wed, 2016-11-30 at 18:32 -0800, Eric Dumazet wrote:
> 
> > I simply suggest we try to queue the qdisc for further servicing as we
> > do today, from net_tx_action(), but we might use a different bit, so
> > that we leave the opportunity for another cpu to get __QDISC_STATE_SCHED
> > before we grab it from net_tx_action(), maybe 100 usec later (time to
> > flush all skbs queued in napi_consume_skb() and maybe RX processing,
> > since most NIC handle TX completion before doing RX processing from thei
> > napi poll handler.
> > 
> > Should be doable with few changes in __netif_schedule() and
> > net_tx_action(), plus some control paths that will need to take care of
> > the new bit at dismantle time, right ?
> 
> Hmm... this is silly. Code already implements a different bit.
> 
> qdisc_run() seems to run more often from net_tx_action(), I have to find
> out why.

After more analysis I believe TSQ was one of the bottlenecks.

I prepared a patch series that helped my use cases.