From: Nicholas Bellinger <nab@linux-iscsi.org>
Date: Fri, 10 Jan 2014 02:06:59 +0000 (+0000)
Subject: iscsi-target: Pre-allocate more tags to avoid ack starvation
X-Git-Tag: v3.14-rc1~17^2~6
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=4a4caa29f1abcb14377e05d57c0793d338fb945d;p=pandora-kernel.git

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>
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