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    SubjectRe: [RFC PATCH] kernel: sched: Provide a pointer to the valid CPU mask
    On Thu, 6 Apr 2017, Peter Zijlstra wrote:
    > On Thu, Apr 06, 2017 at 01:03:16PM +0200, Thomas Gleixner wrote:
    >
    > > Fair enough, but I prefer to have the ability for a temporary restriction
    > > of the user space visible cpus allowed mask in the core code, which can be
    > > used for both migrate_disable() and things like that MIPS FPU stuff rather
    > > than all those home brewn hackeries which are prone to bitrot, security
    > > issues and subtle wreckage. cpus_allowed should be solely under sched core
    > > control and not accessible from anything outside.
    >
    > So the big difference between what MIPS does and what migrate_disable()
    > needs here is the amount of storage.
    >
    > For migrate_disable() we don't need a whole second cpumask bitmap. We
    > can simply use one of the static cpumask_of() things.
    >
    > Given that cpumask bitmaps can be quite large, adding another one to
    > task_struct just for $very_rare_occasion, doesn't seem like a good
    > thing.

    A pointer to the effective mask is definitely enough. And that's what we
    need for migrate_disable() as well. That still leaves the storage
    requirement to MIPS.

    Thanks,

    tglx


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