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

[Andi Kleen <ak@suse.de>]

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