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    SubjectRe: [PATCH] cpu hotplug, sched: Introduce cpu_active_map and redoscheddomain managment (take 2)
    Gregory Haskins wrote:
    >>>> On Thu, Jul 17, 2008 at 3:16 AM, in message <>,
    > Max Krasnyansky <> wrote:
    >> Gregory Haskins wrote:
    >>> Well, admittedly I am not entirely clear on what problem is being solved as
    >>> I was not part of the original thread with Linus. My impression of what you
    >>> were trying to solve was to eliminate the need to rebuild the domains for a
    >>> hotplug event (which I think is a good problem to solve), thus eliminating
    >>> some complexity and (iiuc) races there.
    >>> However, based on what you just said, I am not sure I've got that entirely
    >>> right anymore. Can you clarify the intent (or point me at the original
    >> thread)
    >>> so we are on the same page?
    >> Here is the link to the original thread
    >> And here is where Linus explained the idea
    >> I'll reply to the rest of your email tomorrow (can't keep my yes open any
    >> longer :)).
    >> Max
    > Hi Max,
    > Thanks for the pointers. I see that I did indeed misunderstand the intent of the patch.
    > It seems you already solved the rebuild problem, and were just trying to solve the
    > "migrate to a dead cpu" problem that Linus mentions as a solution with cpu_active_map.
    Yes. btw they are definitely related, because the reason we were blowing away
    the domains is to avoid "migration to a dead cpu". ie We were relying on the
    fact that domain masks never contain cpus that are either dying or already dead.

    > In that case, note that rq->rd->online already fits the bill, I believe. In a nutshell,
    > rq->rd->span contains all the cpus within your disjoint cpuset, and rq->rd->online,
    > contains the subset of rq->rd->span that are online. The online bit is cleared at the
    > earliest point in cpu hotplug removal (DYING), and it is set at the very latest point on
    > insertion (ONLINE). Therefore it is redundant with the cpus_active_map concept.
    > I think the simplest solution is to make sure that we cpus_and against rq->rd->online
    > before allowing a migration. This is how I intended the mask to be used, anyway. Its
    > what the RT scheduler does. It sounds like we just need to touch up the few places
    > in the CFS side that were causing those oops.
    > Thoughts?
    None at this point :). I need to run right now and will try to look at this
    later today. My knowledge of the internal sched structs is definitely lacking
    so I need to look at the rq->rd thing to have and opinion.


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