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

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Date:	Thu, 10 Oct 2013 09:27:57 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Paul McKenney <paulmck@...ux.vnet.ibm.com>,
	Mel Gorman <mgorman@...e.de>, Rik van Riel <riel@...hat.com>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Andrea Arcangeli <aarcange@...hat.com>,
	Johannes Weiner <hannes@...xchg.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/6] Optimize the cpu hotplug locking -v2


* Andrew Morton <akpm@...ux-foundation.org> wrote:

> On Thu, 10 Oct 2013 08:27:41 +0200 Ingo Molnar <mingo@...nel.org> wrote:
> 
> > * Andrew Morton <akpm@...ux-foundation.org> wrote:
> > 
> > > On Tue, 08 Oct 2013 12:25:05 +0200 Peter Zijlstra <peterz@...radead.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".

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.

Thanks,

	Ingo
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