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Date: Wed, 9 Oct 2013 23:34:19 -0700
From: Andrew Morton <akpm@linux-foundation.org>
To: Ingo Molnar <mingo@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>, Oleg Nesterov <oleg@redhat.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Mel Gorman <mgorman@suse.de>, Rik van Riel <riel@redhat.com>,
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        Andrea Arcangeli <aarcange@redhat.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2
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On Thu, 10 Oct 2013 08:27:41 +0200 Ingo Molnar <mingo@kernel.org> wrote:

> * Andrew Morton <akpm@linux-foundation.org> wrote:
> 
> > On Tue, 08 Oct 2013 12:25:05 +0200 Peter Zijlstra <peterz@infradead.org> 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?
> 
> Should be fairly straightforward to test: the sys_sched_getaffinity() and 
> sys_sched_setaffinity() syscalls both make use of 
> get_online_cpus()/put_online_cpus(), so a testcase frobbing affinities on 
> N CPUs in parallel ought to demonstrate scalability improvements pretty 
> nicely.

Well, an in-kernel microbenchmark which camps in a loop doing get/put
would measure this as well.

But neither approach answers the question "how useful is this patchset".