linux-kernel - Re: [PATCH/RFC] synchronize_rcu(): high latency on idle system

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Date:	Sat, 12 Jan 2008 18:33:09 +0100
From:	Andi Kleen <ak@...e.de>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	Benjamin LaHaise <bcrl@...ck.org>, dipankar@...ibm.com,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	rostedt <rostedt@...dmis.org>, Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH/RFC] synchronize_rcu(): high latency on idle system

On Saturday 12 January 2008 10:23:11 Peter Zijlstra wrote:
> 
> On Fri, 2008-01-11 at 20:26 -0500, Benjamin LaHaise wrote:
> > Hello folks,
> > 
> > I'd like to put the patch below out for comments to see if folks think the 
> > approach is a valid fix to reduce the latency of synchronize_rcu().  The 
> > motivation is that an otherwise idle system takes about 3 ticks per network 
> > interface in unregister_netdev() due to multiple calls to synchronize_rcu(), 
> > which adds up to quite a few seconds for tearing down thousands of 
> > interfaces.  By flushing pending rcu callbacks in the idle loop, the system 
> > makes progress hundreds of times faster.  If this is indeed a sane thing to, 
> > it probably needs to be done for other architectures than x86.  And yes, the 
> > network stack shouldn't call synchronize_rcu() quite so much, but fixing that 
> > is a little more involved.
> 
> So, instead of only relying on the tick to drive the RCU state machine,
> you add the idle loop to it. This seems to make sense, esp because nohz
> is held off until rcu is idle too.

For NOHZ I agree it would be probably better to just force a quiescent
cycle than to schedule a one jiffie tick like it is currently done.

For non NOHZ I'm not so sure.

-Andi
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