netdev - [PATCH] qed: avoid spin loops in _qed_mcp_cmd_and

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Message-Id: <20211027214519.606096-1-csander@purestorage.com>
Date:   Wed, 27 Oct 2021 15:45:19 -0600
From:   Caleb Sander <csander@...estorage.com>
To:     netdev@...r.kernel.org
Cc:     Ariel Elior <aelior@...vell.com>, GR-everest-linux-l2@...vell.com,
        Caleb Sander <csander@...estorage.com>,
        Joern Engel <joern@...estorage.com>
Subject: [PATCH] qed: avoid spin loops in _qed_mcp_cmd_and_union()

By default, qed_mcp_cmd_and_union() sets max_retries to 500K and
usecs to 10, so these loops can together delay up to 5s.
We observed thread scheduling delays of over 700ms in production,
with stacktraces pointing to this code as the culprit.

Add calls to cond_resched() in both loops to yield the CPU if necessary.

Signed-off-by: Caleb Sander <csander@...estorage.com>
Reviewed-by: Joern Engel <joern@...estorage.com>
---
 drivers/net/ethernet/qlogic/qed/qed_mcp.c | 12 ++++++++----
 1 file changed, 8 insertions(+), 4 deletions(-)

diff --git a/drivers/net/ethernet/qlogic/qed/qed_mcp.c b/drivers/net/ethernet/qlogic/qed/qed_mcp.c
index 24cd41567..d6944f020 100644
--- a/drivers/net/ethernet/qlogic/qed/qed_mcp.c
+++ b/drivers/net/ethernet/qlogic/qed/qed_mcp.c
@@ -485,10 +485,12 @@ _qed_mcp_cmd_and_union(struct qed_hwfn *p_hwfn,
 
 		spin_unlock_bh(&p_hwfn->mcp_info->cmd_lock);
 
-		if (QED_MB_FLAGS_IS_SET(p_mb_params, CAN_SLEEP))
+		if (QED_MB_FLAGS_IS_SET(p_mb_params, CAN_SLEEP)) {
 			msleep(msecs);
-		else
+		} else {
+			cond_resched();
 			udelay(usecs);
+		}
 	} while (++cnt < max_retries);
 
 	if (cnt >= max_retries) {
@@ -517,10 +519,12 @@ _qed_mcp_cmd_and_union(struct qed_hwfn *p_hwfn,
 		 * The spinlock stays locked until the list element is removed.
 		 */
 
-		if (QED_MB_FLAGS_IS_SET(p_mb_params, CAN_SLEEP))
+		if (QED_MB_FLAGS_IS_SET(p_mb_params, CAN_SLEEP)) {
 			msleep(msecs);
-		else
+		} else {
+			cond_resched();
 			udelay(usecs);
+		}
 
 		spin_lock_bh(&p_hwfn->mcp_info->cmd_lock);
 
-- 
2.25.1