netdev - Re: [PATCH net-next] mlx4: do not fire tasklet unless necessary

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Date:   Thu, 16 Feb 2017 07:49:47 -0800
From:   Eric Dumazet <eric.dumazet@...il.com>
To:     Saeed Mahameed <saeedm@....mellanox.co.il>
Cc:     David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>,
        Matan Barak <matanb@...lanox.com>, jackm@...lanox.com
Subject: Re: [PATCH net-next] mlx4: do not fire tasklet unless necessary

On Thu, 2017-02-16 at 14:44 +0200, Saeed Mahameed wrote:
> On Wed, Feb 15, 2017 at 4:04 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> > On Wed, 2017-02-15 at 05:29 -0800, Eric Dumazet wrote:
> >
> >>
> >> mlx4_eq_int() is a hard irq handler.
> >>
> >> How a tasklet could run in the middle of it ?
> >>
> >> A tasklet is a softirq handler.
> >
> > Speaking of mlx4_eq_int() , 50% of cycles are spent on mb() (mfence)
> > in eq_set_ci()
> >
> 
> I wonder why you have so many interrupts ? don't you have some kind of
> interrupt moderation ?
> what test are you running that got your CPU so busy.


Simply 8 RX queues. About 140,000 irq per second per RX queue,
for a moderate network load on 40Gbit NIC.

Interrupt moderation is a latency killer, we want our usec back.

> 
> > I wonder why this very expensive mb() is required, right before exiting
> > the interrupt handler.
> 
> to make sure the HW knows we handled Completions up to (ci) consumer
> index. so it will generate next irq.


So why a mb() is needed exactly ?

wmb() seems enough.