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    SubjectRe: New NUMA scheduler and hotplug CPU


    Andrew Theurer wrote:

    >>>To me, it'd make more sense to add the CPUs to the scheduler structures
    >>>as they get brought online. I can also imagine machines where you have
    >>>a massive (infinite?) variety of possible CPUs that could appear -
    >>>like an NUMA box where you could just plug arbitrary numbers of new
    >>>nodes in as you wanted.
    >>>
    >>I guess so, but you'd still need NR_CPUS to be >= that arbitrary
    >>number.
    >>
    >>
    >>>Moreover, as the CPUs aren't fixed numbers in advance, how are you going
    >>>to know which node to put them in, etc? Setting up every possible thing
    >>>in advance seems like an infeasible way to do hotplug to me.
    >>>
    >>Well this would be the problem. I guess its quite possible that
    >>one doesn't know the topology of newly added CPUs before hand.
    >>
    >>Well OK, this would require a per architecture function to handle
    >>CPU hotplug. It could possibly just default to arch_init_sched_domains,
    >>and just completely reinitialise everything which would be the simplest.
    >>
    >
    >Call me crazy, but why not let the topology be determined via userspace at a
    >more appropriate time? When you hotplug, you tell it where in the scheduler
    >to plug it. Have structures in the scheduler which represent the
    >nodes-runqueues-cpus topology (in the past I tried a node/rq/cpu structs with
    >simple pointers), but let the topology be built based on user's desires thru
    >hotplug.
    >

    Well isn't userspace's idea of topology just what the kernel tells it?
    I'm not sure what it would buy you... but I guess it wouldn't be too
    much harder than doing it in kernel, just a matter of making the userspace
    API.

    BTW. I guess you haven't seen my sched domains code. It can describe
    arbitrary topologies.


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