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Date: Thu, 10 Oct 2013 00:33:15 -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>,
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        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 09:27:57 +0200 Ingo Molnar <mingo@kernel.org> wrote:

> > > 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".
> 
> Even ignoring all the other reasons cited, sys_sched_getaffinity() / 
> sys_sched_setaffinity() are prime time system calls, and as long as the 
> patches are correct, speeding them up is worthwhile.

That I would not have guessed.  What's the use case for calling
get/set_affinity at high frequency?