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

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)

I would be surprised if no customer was hit by your commit,
( net/mlx4_core: Use tasklet for user-space CQ completion events )
especially when using specific (RT) scheduler classes.