linux-kernel - [PATCH 3/3] iscsi-target: Pre-allocate more tags to avoid ack starvation

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Date:	Fri, 10 Jan 2014 03:00:47 +0000
From:	"Nicholas A. Bellinger" <nab@...erainc.com>
To:	target-devel <target-devel@...r.kernel.org>
Cc:	linux-kernel <linux-kernel@...r.kernel.org>,
	Nicholas Bellinger <nab@...ux-iscsi.org>
Subject: [PATCH 3/3] iscsi-target: Pre-allocate more tags to avoid ack starvation

From: Nicholas Bellinger <nab@...ux-iscsi.org>

This patch addresses an traditional iscsi-target fabric ack starvation
issue where iscsit_allocate_cmd() -> percpu_ida_alloc_state() ends up
hitting slow path percpu-ida code, because iscsit_ack_from_expstatsn()
is expected to free ack'ed tags after tag allocation.

This is done to take into account the tags waiting to be acknowledged
and released in iscsit_ack_from_expstatsn(), but who's number are not
directly limited by the CmdSN Window queue_depth being enforced by
the target.

So that said, this patch bumps up the pre-allocated number of
per session tags to:

  min(queue_depth, ISCSIT_MIN_TAGS * 2) + ISCSIT_EXTRA_TAGS

for good measure to avoid the percpu_ida_alloc_state() slow path.

Signed-off-by: Nicholas Bellinger <nab@...ux-iscsi.org>
---
 drivers/target/iscsi/iscsi_target_nego.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/drivers/target/iscsi/iscsi_target_nego.c b/drivers/target/iscsi/iscsi_target_nego.c
index 83c965c..582ba84 100644
--- a/drivers/target/iscsi/iscsi_target_nego.c
+++ b/drivers/target/iscsi/iscsi_target_nego.c
@@ -1192,7 +1192,7 @@ get_target:
 	 */
 alloc_tags:
 	tag_num = max_t(u32, ISCSIT_MIN_TAGS, queue_depth);
-	tag_num += (tag_num / 2) + ISCSIT_EXTRA_TAGS;
+	tag_num = (tag_num * 2) + ISCSIT_EXTRA_TAGS;
 	tag_size = sizeof(struct iscsi_cmd) + conn->conn_transport->priv_size;

 	ret = transport_alloc_session_tags(sess->se_sess, tag_num, tag_size);
-- 
1.7.10.4

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