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

[Andrew Morton]

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?
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