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

[Peter Zijlstra]

On Wed, Oct 09, 2013 at 10:50:06PM -0700, Andrew Morton wrote:
> On Tue, 08 Oct 2013 12:25:05 +0200 Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> > The current cpu hotplug lock is a single global lock; therefore excluding
> > hotplug is a very expensive proposition even though it is rare occurrence under
> > normal operation.
> > 
> > There is a desire for a more light weight implementation of
> > {get,put}_online_cpus() from both the NUMA scheduling as well as the -RT side.
> > 
> > The current hotplug lock is a full reader preference lock -- and thus supports
> > reader recursion. However since we're making the read side lock much cheaper it
> > is the expectation that it will also be used far more. Which in turn would lead
> > to writer starvation.
> > 
> > Therefore the new lock proposed is completely fair; albeit somewhat expensive
> > on the write side. This in turn means that we need a per-task nesting count to
> > support reader recursion.
> 
> This is a lot of code and a lot of new complexity.  It needs some pretty
> convincing performance numbers to justify its inclusion, no?

And here I thought it was generally understood to be unwise to bash
global state on anything like a regular manner from every cpu.

The NUMA bits really ought to use get_online_cpus()/put_online_cpus() on
every balance pass; which is about once a second on every cpu.

RT -- which has some quite horrible hotplug hacks due to this --
basically takes get_online_cpus() for every spin_lock/spin_unlock in the
kernel.

But the thing is; our sense of NR_CPUS has shifted, where it used to be
ok to do something like:

  for_each_cpu()

With preemption disabled; it gets to be less and less sane to do so,
simply because 'common' hardware has 256+ CPUs these days. If we cannot
rely on preempt disable to exclude hotplug, we must use
get_online_cpus(), but get_online_cpus() is global state and thus cannot
be used at any sort of frequency.


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