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    SubjectRe: CPU affinity & IPI latency
    On Fri, Jul 13, 2001 at 03:43:05PM -0700, Mike Kravetz wrote:
    > - Define a new field in the task structure 'saved_cpus_allowed'.
    > With a little collapsing of existing fields, there is room to put
    > this on the same cache line as 'cpus_allowed'.
    > - In reschedule_idle() if we determine that the best CPU for a task
    > is the CPU it is associated with (p->processor), then temporarily
    > bind the task to that CPU. The task is temporarily bound to the
    > CPU by overwriting the 'cpus_allowed' field such that the task can
    > only be scheduled on the target CPU. Of course, the original
    > value of 'cpus_allowed' is saved in 'saved_cpus_allowed'.
    > - In schedule(), the loop which examines all tasks on the runqueue
    > will restore the value of 'cpus_allowed'.

    This sounds racy, at least with your simple description. Even other CPUs
    could walk their run queue while the IPI is being processed and reset the
    cpus_allowed flag too early. I guess it would need a check in the
    schedule loop that restores cpus_allowed that the current CPU is the only
    on set in that task's cpus_allowed. This unfortunately would hang the
    process if the CPU for some reason cannot process schedules timely, so
    it may be needed to add a timestamp also to the task_struct that allows
    the restoration of cpus_allowed even from non target CPUs after some

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