linux-kernel - [PATCH] mlx5: only schedule EQ comp tasklet if necessary

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Message-ID: <20241027040700.1616307-1-csander@purestorage.com>
Date: Sat, 26 Oct 2024 22:06:55 -0600
From: Caleb Sander Mateos <csander@...estorage.com>
To: Saeed Mahameed <saeedm@...dia.com>,
	Leon Romanovsky <leon@...nel.org>,
	Tariq Toukan <tariqt@...dia.com>,
	Andrew Lunn <andrew+netdev@...n.ch>,
	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>,
	Jakub Kicinski <kuba@...nel.org>,
	Paolo Abeni <pabeni@...hat.com>
Cc: Caleb Sander Mateos <csander@...estorage.com>,
	netdev@...r.kernel.org,
	linux-rdma@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: [PATCH] mlx5: only schedule EQ comp tasklet 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
overhead is unnecessary. Atomic operations are needed to schedule, lock,
and clear the tasklet. And when mlx5_cq_tasklet_cb() runs, it acquires a
spin lock to access the list of CQs enqueued for 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.

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@...estorage.com>
---
 drivers/net/ethernet/mellanox/mlx5/core/cq.c | 5 +++++
 drivers/net/ethernet/mellanox/mlx5/core/eq.c | 5 +----
 2 files changed, 6 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/mellanox/mlx5/core/cq.c b/drivers/net/ethernet/mellanox/mlx5/core/cq.c
index 4caa1b6f40ba..25f3b26db729 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/cq.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/cq.c
@@ -69,22 +69,27 @@ void mlx5_cq_tasklet_cb(struct tasklet_struct *t)
 static void mlx5_add_cq_to_tasklet(struct mlx5_core_cq *cq,
 				   struct mlx5_eqe *eqe)
 {
 	unsigned long flags;
 	struct mlx5_eq_tasklet *tasklet_ctx = cq->tasklet_ctx.priv;
+	bool schedule_tasklet = false;
 
 	spin_lock_irqsave(&tasklet_ctx->lock, flags);
 	/* When migrating CQs between EQs will be implemented, please note
 	 * that you need to sync this point. It is possible that
 	 * while migrating a CQ, completions on the old EQs could
 	 * still arrive.
 	 */
 	if (list_empty_careful(&cq->tasklet_ctx.list)) {
 		mlx5_cq_hold(cq);
+		schedule_tasklet = list_empty(&tasklet_ctx->list);
 		list_add_tail(&cq->tasklet_ctx.list, &tasklet_ctx->list);
 	}
 	spin_unlock_irqrestore(&tasklet_ctx->lock, flags);
+
+	if (schedule_tasklet)
+		tasklet_schedule(&tasklet_ctx->task);
 }
 
 /* Callers must verify outbox status in case of err */
 int mlx5_create_cq(struct mlx5_core_dev *dev, struct mlx5_core_cq *cq,
 		   u32 *in, int inlen, u32 *out, int outlen)
diff --git a/drivers/net/ethernet/mellanox/mlx5/core/eq.c b/drivers/net/ethernet/mellanox/mlx5/core/eq.c
index 68cb86b37e56..66fc17d9c949 100644
--- a/drivers/net/ethernet/mellanox/mlx5/core/eq.c
+++ b/drivers/net/ethernet/mellanox/mlx5/core/eq.c
@@ -112,17 +112,17 @@ static int mlx5_eq_comp_int(struct notifier_block *nb,
 	struct mlx5_eq_comp *eq_comp =
 		container_of(nb, struct mlx5_eq_comp, irq_nb);
 	struct mlx5_eq *eq = &eq_comp->core;
 	struct mlx5_eqe *eqe;
 	int num_eqes = 0;
-	u32 cqn = -1;
 
 	eqe = next_eqe_sw(eq);
 	if (!eqe)
 		goto out;
 
 	do {
+		u32 cqn;
 		struct mlx5_core_cq *cq;
 
 		/* Make sure we read EQ entry contents after we've
 		 * checked the ownership bit.
 		 */
@@ -145,13 +145,10 @@ static int mlx5_eq_comp_int(struct notifier_block *nb,
 	} while ((++num_eqes < MLX5_EQ_POLLING_BUDGET) && (eqe = next_eqe_sw(eq)));
 
 out:
 	eq_update_ci(eq, 1);
 
-	if (cqn != -1)
-		tasklet_schedule(&eq_comp->tasklet_ctx.task);
-
 	return 0;
 }
 
 /* Some architectures don't latch interrupts when they are disabled, so using
  * mlx5_eq_poll_irq_disabled could end up losing interrupts while trying to
-- 
2.45.2