iscsi-target: Pre-allocate more tags to avoid ack starvation
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: <4fbacc2fa0ffdbb11bf1ad6925b886ebd08dd15f@vger.kernel.org> #3.12+
Signed-off-by: Nicholas Bellinger <978acd1567d5598152161fdf8bf3ca568f950c9b@linux-iscsi.org>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs