[PATCH] configure HAVE_UNSTABLE_SCHED_CLOCK for SGI_SN systems

[Dimitri Sivanich <sivanich@sgi.com>]

Turn on CONFIG_HAVE_UNSTABLE_SCHED_CLOCK for SGI_SN.

SGI Altix has unsynchronized itc clocks.  This results in rq->clock
occasionally being set to a time in the past by a remote cpu.

Note that it is possible that this problem may exist for other ia64
machines as well, based on the following comment for sched_clock() in
arch/ia64/kernel/head.S:

 * Return a CPU-local timestamp in nano-seconds.  This timestamp is
 * NOT synchronized across CPUs its return value must never be
 * compared against the values returned on another CPU.  The usage in
 * kernel/sched.c ensures that.


Signed-off-by: Dimitri Sivanich <sivanich@sgi.com>

---

Greg, if everyone is OK with this patch, this should also be applied
to all stable trees starting with 2.6.26.

 arch/ia64/Kconfig |    1 +
 1 file changed, 1 insertion(+)

Index: linux-2.6/arch/ia64/Kconfig
=================================--- linux-2.6.orig/arch/ia64/Kconfig	2009-01-06 10:13:13.051918923 -0600
+++ linux-2.6/arch/ia64/Kconfig	2009-01-06 10:13:44.547856328 -0600
@@ -536,6 +536,7 @@ config IA64_MC_ERR_INJECT
 
 config SGI_SN
 	def_bool y if (IA64_SGI_SN2 || IA64_GENERIC)
+	select HAVE_UNSTABLE_SCHED_CLOCK
 
 config IA64_ESI
 	bool "ESI (Extensible SAL Interface) support"