Re: CPU Hotplug rework

[Peter Zijlstra]

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?

> 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?
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