[PATCH RFC 1/2] net/mlx5: Submit blocking commands from the caller's context

[Breno Leitao <leitao@debian.org>]

mlx5_cmd_invoke() queues every command that is not a page-queue command
on the ordered cmd->wq, and a blocking caller then sleeps in wait_func()
until the command completes. The work item only takes a command slot,
writes the descriptor and rings the doorbell, so each blocking command
pays for a queue_work(), a worker wakeup and a context switch while its
caller sits idle.

All ordered workqueues share one unbound worker pool, so the command
work items of every mlx5 function also contend on a single pool->lock.
On an arm64 host with a dozen mlx5 functions in switchdev mode, where
monitoring daemons read /proc/net/dev, lock contention profiling put
36% of the pool->lock wait in queue_work() called from cmd_exec(), and
the rest in the workers running the command and stats work items.

Page-queue commands already call cmd_work_handler() directly. Do the
same for every command without a callback. Callback commands keep using
the workqueue, since mlx5_cmd_exec_cb() must not block.

Firmware runs up to max_reg_cmds commands concurrently, so callers could
not rely on the submission order, and the slot semaphore still serves
its waiters in FIFO order.

With 32 parallel ethtool -S loops on a ConnectX-4 Lx, this removes all
46.7k mlx5_cmd work items of the run and cuts kworker context switches
from 50.7k to 3.9k.

Signed-off-by: Breno Leitao <leitao@debian.org>
---
 drivers/net/ethernet/mellanox/mlx5/core/cmd.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/cmd.c b/drivers/net/ethernet/mellanox/mlx5/core/cmd.c
index 84583dc5eb1c05..988518155582a4 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/cmd.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/cmd.c
@@ -1301,7 +1301,8 @@ static int mlx5_cmd_invoke(struct mlx5_core_dev *dev, struct mlx5_cmd_msg *in,
 
 	INIT_DELAYED_WORK(&ent->cb_timeout_work, cb_timeout_handler);
 	INIT_WORK(&ent->work, cmd_work_handler);
-	if (page_queue) {
+	/* A blocking caller waits for the command anyway, submit it inline */
+	if (page_queue || !callback) {
 		cmd_work_handler(&ent->work);
 	} else if (!queue_work(cmd->wq, &ent->work)) {
 		mlx5_core_warn(dev, "failed to queue work\n");

-- 
2.53.0-Meta