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

[Eric Dumazet]

From: Eric Dumazet <eduma...@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.

Signed-off-by: Eric Dumazet <eduma...@google.com>
Cc: Tariq Toukan <tar...@mellanox.com>
Cc: Saeed Mahameed <sae...@mellanox.com>
---
 drivers/net/ethernet/mellanox/mlx4/cq.c |    6 +++++-
 drivers/net/ethernet/mellanox/mlx4/eq.c |    9 +--------
 2 files changed, 6 insertions(+), 9 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx4/cq.c 
b/drivers/net/ethernet/mellanox/mlx4/cq.c
index 
6b8635378f1fcb2aae4e8ac390bcd09d552c2256..fa6d2354a0e910ee160863e3cbe21a512d77bf03
 100644
--- a/drivers/net/ethernet/mellanox/mlx4/cq.c
+++ b/drivers/net/ethernet/mellanox/mlx4/cq.c
@@ -81,8 +81,9 @@ void mlx4_cq_tasklet_cb(unsigned long data)
 
 static void mlx4_add_cq_to_tasklet(struct mlx4_cq *cq)
 {
-       unsigned long flags;
        struct mlx4_eq_tasklet *tasklet_ctx = cq->tasklet_ctx.priv;
+       unsigned long flags;
+       bool kick;
 
        spin_lock_irqsave(&tasklet_ctx->lock, flags);
        /* When migrating CQs between EQs will be implemented, please note
@@ -92,7 +93,10 @@ static void mlx4_add_cq_to_tasklet(struct mlx4_cq *cq)
         */
        if (list_empty_careful(&cq->tasklet_ctx.list)) {
                atomic_inc(&cq->refcount);
+               kick = list_empty(&tasklet_ctx->list);
                list_add_tail(&cq->tasklet_ctx.list, &tasklet_ctx->list);
+               if (kick)
+                       tasklet_schedule(&tasklet_ctx->task);
        }
        spin_unlock_irqrestore(&tasklet_ctx->lock, flags);
 }
diff --git a/drivers/net/ethernet/mellanox/mlx4/eq.c 
b/drivers/net/ethernet/mellanox/mlx4/eq.c
index 
0509996957d9664b612358dd805359f4bc67b8dc..39232b6a974f4b4b961d3b0b8634f04e6b9d0caa
 100644
--- a/drivers/net/ethernet/mellanox/mlx4/eq.c
+++ b/drivers/net/ethernet/mellanox/mlx4/eq.c
@@ -494,7 +494,7 @@ static int mlx4_eq_int(struct mlx4_dev *dev, struct mlx4_eq 
*eq)
 {
        struct mlx4_priv *priv = mlx4_priv(dev);
        struct mlx4_eqe *eqe;
-       int cqn = -1;
+       int cqn;
        int eqes_found = 0;
        int set_ci = 0;
        int port;
@@ -840,13 +840,6 @@ static int mlx4_eq_int(struct mlx4_dev *dev, struct 
mlx4_eq *eq)
 
        eq_set_ci(eq, 1);
 
-       /* cqn is 24bit wide but is initialized such that its higher bits
-        * are ones too. Thus, if we got any event, cqn's high bits should be 
off
-        * and we need to schedule the tasklet.
-        */
-       if (!(cqn & ~0xffffff))
-               tasklet_schedule(&eq->tasklet_ctx.task);
-
        return eqes_found;
 }