[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]
Date: Wed, 9 Oct 2013 23:34:19 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Ingo Molnar <mingo@...nel.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
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".
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
Powered by blists - more mailing lists