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

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Date:   Wed, 15 Feb 2017 16:52:43 +0200
From:   "Matan Barak (External)" <matanb@...lanox.com>
To:     Saeed Mahameed <saeedm@....mellanox.co.il>,
        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>, <jackm@...lanox.com>
Subject: Re: [PATCH net-next] mlx4: do not fire tasklet unless necessary

On 15/02/2017 13:10, Saeed Mahameed wrote:
> On Fri, Feb 10, 2017 at 2:27 PM, Eric Dumazet <eric.dumazet@...il.com> wrote:
>> From: Eric Dumazet <edumazet@...gle.com>
>>
>> All rx and rx netdev interrupts are handled by respectively
>> by mlx4_en_rx_irq() and mlx4_en_tx_irq() which simply schedule a NAPI.
>>
>> But mlx4_eq_int() also fires a tasklet to service all items that were
>> queued via mlx4_add_cq_to_tasklet(), but this handler was not called
>> unless user cqe was handled.
>>
>> This is very confusing, as "mpstat -I SCPU ..." show huge number of
>> tasklet invocations.
>>
>> This patch saves this overhead, by carefully firing the tasklet directly
>> from mlx4_add_cq_to_tasklet(), removing four atomic operations per IRQ.
>>

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 :)