mlx5/core: Schedule EQ comp tasklet only if necessary

Currently, the mlx5_eq_comp_int() interrupt handler schedules a tasklet
to call mlx5_cq_tasklet_cb() if it processes any completions. For CQs
whose completions don't need to be processed in tasklet context, this
adds unnecessary overhead. In a heavy TCP workload, we see 4% of CPU
time spent on the tasklet_trylock() in tasklet_action_common(), with a
smaller amount spent on the atomic operations in tasklet_schedule(),
tasklet_clear_sched(), and locking the spinlock in mlx5_cq_tasklet_cb().
TCP completions are handled by mlx5e_completion_event(), which schedules
NAPI to poll the queue, so they don't need tasklet processing.

Schedule the tasklet in mlx5_add_cq_to_tasklet() instead to avoid this
overhead. mlx5_add_cq_to_tasklet() is responsible for enqueuing the CQs
to be processed in tasklet context, so it can schedule the tasklet. CQs
that need tasklet processing have their interrupt comp handler set to
mlx5_add_cq_to_tasklet(), so they will schedule the tasklet. CQs that
don't need tasklet processing won't schedule the tasklet. To avoid
scheduling the tasklet multiple times during the same interrupt, only
schedule the tasklet in mlx5_add_cq_to_tasklet() if the tasklet work
queue was empty before the new CQ was pushed to it.

The additional branch in mlx5_add_cq_to_tasklet(), called for each EQE,
may add a small cost for the userspace Infiniband CQs whose completions
are processed in tasklet context. But this seems worth it to avoid the
tasklet overhead for CQs that don't need it.

Note that the mlx4 driver works the same way: it schedules the tasklet
in mlx4_add_cq_to_tasklet() and only if the work queue was empty before.

Signed-off-by: Caleb Sander Mateos <csander@purestorage.com>
Reviewed-by: Parav Pandit <parav@nvidia.com>
Acked-by: Tariq Toukan <tariqt@nvidia.com>
Acked-by: Saeed Mahameed <saeedm@nvidia.com>
Link: https://patch.msgid.link/20241105204000.1807095-1-csander@purestorage.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>