Re: CPU Hotplug rework

[Rusty Russell <rusty@rustcorp.com.au>]

On Wed, 21 Mar 2012 10:01:59 +0100, Peter Zijlstra <a.p.zijlstra@chello.nl> wrote:
> On Wed, 2012-03-21 at 09:30 +1030, Rusty Russell wrote:
> > > > (2) Do something more efficient with userspace threads than migrating
> > > >     them one at a time.
> > > 
> > > Sadly that can't really be done. We need to pick up every task
> > > (userspace, but also running kernel threads) and update their state.
> > 
> > What if we had an "orphan" runqueue which everyone pulled from?  Then we
> > could grab the lock, move them all to the fake rq, then let stuff happen
> > normally.
> 
> Well, we could simply let them sit where they are and fudge load-balance
> to consider it a source but not a destination until its empty, but it
> might be somewhat tricky to make it fast enough to not introduce
> noticable latencies. Also, you really don't want everyone to pull,
> that's a serialization/scalability problem.
> 
> Also, since we really only move the currently runnable tasks it
> shouldn't be too many in the first place. Is it really that expensive?

Good question, requires measurement to answer.

> > Maybe that's crap, but at least we could move the migration out of the
> > hotplug callback somehow. 
> 
> Thing is, if its really too much for some people, they can orchestrate
> it such that its not. Just move everybody in a cpuset, clear the to be
> offlined cpu from the cpuset's mask -- this will migrate everybody away.
> Then hotplug will find an empty runqueue and its fast, no?

I like this solution better.

Thanks,
Rusty.
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