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

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Date:   Thu, 16 Feb 2017 14:34:46 +0200
From:   Saeed Mahameed <saeedm@....mellanox.co.il>
To:     Eric Dumazet <eric.dumazet@...il.com>
Cc:     "Matan Barak (External)" <matanb@...lanox.com>,
        David Miller <davem@...emloft.net>,
        netdev <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        Saeed Mahameed <saeedm@...lanox.com>, jackm@...lanox.com
Subject: Re: [PATCH net-next] mlx4: do not fire tasklet unless necessary

On Wed, Feb 15, 2017 at 5:26 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
> On Wed, 2017-02-15 at 16:52 +0200, Matan Barak (External) wrote:
>
>> So, in case of RDMA CQs, we add some per-CQE overhead of comparing the
>> list pointers and condition upon that. Maybe we could add an
>> invoke_tasklet boolean field on mlx4_cq and return its value from
>> mlx4_cq_completion.
>> That's way we could do invoke_tasklet |= mlx4_cq_completion(....);
>>
>> Outside the while loop we could just
>> if (invoke_tasklet)
>>      tasklet_schedule
>>
>> Anyway, I guess that even with per-CQE overhead, the performance impact
>> here is pretty negligible - so I guess that's fine too :)
>
>
> Real question or suggestion would be to use/fire a tasklet only under
> stress.
>
> Firing a tasklet adds a lot of latencies for user-space CQ completion,
> since softirqs might have to be handled by a kernel thread (ksoftirqd)
>

At least for mlx4_en driver we don't need this tasklet and it is only
adding this overhead. (we have napi)

we must consider removing it for mlx4_en cqs and move the tasklet
handling to mlx4_ib.

I will ack the patch.