From: Ivo Sieben <meltedpianoman@gmail.com>
Date: Thu, 27 Sep 2012 12:02:05 +0000 (+0200)
Subject: tty: prevent unnecessary work queue lock checking on flip buffer copy
X-Git-Tag: omap-for-v3.8/cleanup-headers-serial-take2-signed~29
X-Git-Url: http://git.openpandora.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=cee4ad1ed90a0959fc29f9d30a2526e5e9522cfa;p=pandora-kernel.git

tty: prevent unnecessary work queue lock checking on flip buffer copy

When low_latency flag is set the TTY receive flip buffer is copied to the
line discipline directly instead of using a work queue in the background.
Therefor only in case a workqueue is actually used for copying data to the
line discipline we'll have to flush the workqueue.

This prevents unnecessary spin lock/unlock on the workqueue spin lock that
can cause additional scheduling overhead on a PREEMPT_RT system. On a 200
MHz AT91SAM9261 processor setup this fixes about 100us of scheduling
overhead on the TTY read call.

Signed-off-by: Ivo Sieben <meltedpianoman@gmail.com>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
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