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

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Date:   Thu, 16 Feb 2017 14:44:56 +0200
From:   Saeed Mahameed <saeedm@....mellanox.co.il>
To:     Eric Dumazet <eric.dumazet@...il.com>
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 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.

> 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.