Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2

[Steven Rostedt]

On Thu, 10 Oct 2013 18:53:37 +0200
Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:

> On Thu, Oct 10, 2013 at 09:43:55AM -0700, Andrew Morton wrote:
> > > But we would like to remove stomp machine from
> > > CPU hotplug.
> > 
> > We do?  That's news.  It wasn't mentioned in the changelog and should
> > have been.  Why?  
> 
> It would be an unrelated change to this and unrelated to the reasons as
> to why I want a faster get_online_cpus().

Yeah, sorry for the confusion. My comment wasn't really about this
change set but about stop machine and hotplug in general. Needing stop
machine for hotplug has been a complaint by many, but off topic for
this particular change set.

> 
> > > Why prevent all CPUs from running when we want to remove
> > > one?
> > 
> > So get_online_cpus() goes away.  Nothing is more scalable than nothing!
> 
> Very much agreed; now stop_machine() wouldn't actually work for hotplug
> because it will instantly preempt everybody, including someone who might
> be in the middle of using per-cpu state of the cpu we're about to
> remove.

Well, stop machine doesn't instantly preempt everybody. Only those that
don't have preemption disabled. Using per_cpu without preemption
disabled can be dangerous. Except for the migrate disable we want to
add for -rt. Then we can't rely on migrate disable and stop machine
making sure per_cpu data isn't being used. Oh, and for threads that use
per_cpu that are bound to a CPU. But then they need to be taken care of
too for their CPU going off line.

But this would also require the "get_online_cpu()s" to disable
preemption as well. Not quite a "nothing" we are looking for.


-- Steve
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