linux-kernel - Re: [RFC][PATCH 5/5] sched: Reduce ttwu rq->lock contention

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Date:	Thu, 16 Dec 2010 16:31:03 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Chris Mason <chris.mason@...cle.com>,
	Frank Rowand <frank.rowand@...sony.com>,
	Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	Mike Galbraith <efault@....de>,
	Oleg Nesterov <oleg@...hat.com>, Paul Turner <pjt@...gle.com>,
	Jens Axboe <axboe@...nel.dk>, linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCH 5/5] sched: Reduce ttwu rq->lock contention

On Thu, Dec 16, 2010 at 03:56:07PM +0100, Peter Zijlstra wrote:
> Reduce rq->lock contention on try_to_wake_up() by changing the task
> state using a cmpxchg loop.
> 
> Once the task is set to TASK_WAKING we're guaranteed the only one
> poking at it, then proceed to pick a new cpu without holding the
> rq->lock (XXX this opens some races).
> 
> Then instead of locking the remote rq and activating the task, place
> the task on a remote queue, again using cmpxchg, and notify the remote
> cpu per IPI if this queue was empty to start processing its wakeups.
> 
> This avoids (in most cases) having to lock the remote runqueue (and
> therefore the exclusive cacheline transfer thereof) but also touching
> all the remote runqueue data structures needed for the actual
> activation.
> 
> As measured using: http://oss.oracle.com/~mason/sembench.c
> 
> $ echo 4096 32000 64 128 > /proc/sys/kernel/sem
> $ ./sembench -t 2048 -w 1900 -o 0
> 
> unpatched: run time 30 seconds 537953 worker burns per second
> patched:   run time 30 seconds 657336 worker burns per second
> 
> Still need to sort out all the races marked XXX (non-trivial), and its
> x86 only for the moment.
> 
> Signed-off-by: Peter Zijlstra <a.p.zijlstra@...llo.nl>
> ---
>  arch/x86/kernel/smp.c   |    1 
>  include/linux/sched.h   |    7 -
>  kernel/sched.c          |  241 ++++++++++++++++++++++++++++++++++--------------
>  kernel/sched_fair.c     |    5 
>  kernel/sched_features.h |    3 
>  kernel/sched_idletask.c |    2 
>  kernel/sched_rt.c       |    4 
>  kernel/sched_stoptask.c |    3 
>  8 files changed, 190 insertions(+), 76 deletions(-)
> 
> Index: linux-2.6/arch/x86/kernel/smp.c
> ===================================================================
> --- linux-2.6.orig/arch/x86/kernel/smp.c
> +++ linux-2.6/arch/x86/kernel/smp.c
> @@ -205,6 +205,7 @@ void smp_reschedule_interrupt(struct pt_
>  	/*
>  	 * KVM uses this interrupt to force a cpu out of guest mode
>  	 */
> +	sched_ttwu_pending();
>  }

Great, that's going to greatly simplify and lower the overhead of
the remote tick restart I'm doing on wake up for the nohz task thing.
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