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iscsi-target: Pre-allocate more tags to avoid ack starvation 

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:

  (max(queue_depth, ISCSIT_MIN_TAGS) * 2) + ISCSIT_EXTRA_TAGS

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

Cc: <stable@vger.kernel.org> #3.12+
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>
This commit is contained in:
Nicholas Bellinger 2014-01-10 02:06:59 +00:00
parent 49a47f2caf
commit 4a4caa29f1
1 changed files with 1 additions and 1 deletions
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2
drivers/target/iscsi/iscsi_target_nego.c
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@ -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);