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    SubjectRe: [PATCH tip/core/rcu 4/5] sys_membarrier: Add expedited option
    On Tue, Jul 25, 2017 at 10:17:01AM -0700, Paul E. McKenney wrote:

    > > munmap() TLB invalidate is limited to those CPUs that actually ran
    > > threads of their process, while this is machine wide.
    >
    > Or those CPUs running threads of any process mapping the underlying file
    > or whatever.

    That doesn't sound right. munmap() of a shared file only invalidates
    this process's map of it.

    Swapping a file page otoh will indeed touch the union of cpumasks over
    all processes mapping that page.

    > And in either case, this can span the whole machine. Plus
    > there are a number of other ways for users to do on-demand full-system
    > IPIs, including any number of ways to wake up large numbers of CPUs,
    > including from unrelated processes.

    Which are those? I thought we significantly reduced those with the nohz
    full work. Most IPI uses now first check if a CPU actually needs the IPI
    before sending it IIRC.

    > But I do plan to add another alternative that is limited to threads of
    > the running process. I will be carrying both versions to enable those
    > who have been bugging me about this to do testing.

    Sending IPIs to mm_cpumask() might be better than expedited, but I'm
    still hesitant. Just because people want it doesn't mean its a good
    idea. We need to weight this against the potential for abuse.

    People want userspace preempt disable, no matter how hard they want it,
    they're not getting it because its a completely crap idea.

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