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    Subject[PATCH] configure HAVE_UNSTABLE_SCHED_CLOCK for SGI_SN systems
    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"

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