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 [off-list ref] wrote:

quoted

From: Eric Dumazet <edumazet@google.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 :)