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

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

[Benjamin LaHaise]

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.

		-ben

diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
index 9663c2a..592f6e4 100644
--- a/arch/x86/kernel/process_32.c
+++ b/arch/x86/kernel/process_32.c
@@ -188,6 +188,9 @@ void cpu_idle(void)
 			rmb();
 			idle = pm_idle;
 
+			if (rcu_pending(cpu))
+				rcu_check_callbacks(cpu, 0);
+
 			if (!idle)
 				idle = default_idle;
 

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

[Andi Kleen]

> And yes, the 
> network stack shouldn't call synchronize_rcu() quite so much, but fixing that 
> is a little more involved.

... but the correct solution.

-Andi

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

[Benjamin LaHaise]

On Sat, Jan 12, 2008 at 03:37:59AM +0100, Andi Kleen wrote:
> > And yes, the 
> > network stack shouldn't call synchronize_rcu() quite so much, but fixing that 
> > is a little more involved.
> 
> ... but the correct solution.

There has to be at least 1 synchronize_rcu() or equivalent in the 
unregister_netdev() path.  I suspect the easiest way to fix it might be to 
use call_rcu() to actually free the network device, as anything else will 
limit performance of single threaded teardown (ie, when an l2tp daemon 
gets terminated via kill -9).  This means an API change that exposes 
rcu for unregister_netdev().

		-ben
-- 
"Time is of no importance, Mr. President, only life is important."
Don't Email: <zyntrop@kvack.org>.

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

[Andi Kleen]

On Saturday 12 January 2008 18:51:35 Benjamin LaHaise wrote:
> On Sat, Jan 12, 2008 at 03:37:59AM +0100, Andi Kleen wrote:
> > > And yes, the 
> > > network stack shouldn't call synchronize_rcu() quite so much, but fixing that 
> > > is a little more involved.
> > 
> > ... but the correct solution.
> 
> There has to be at least 1 synchronize_rcu() or equivalent in the 
> unregister_netdev() path.  I suspect the easiest way to fix it might be to 
> use call_rcu() to actually free the network device, as anything else will 
> limit performance of single threaded teardown (ie, when an l2tp daemon 
> gets terminated via kill -9).  This means an API change that exposes 
> rcu for unregister_netdev().

The call_rcu() could be in free_netdev() couldn't it?

-Andi

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

[Peter Zijlstra]

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.

Even though Andi is right in that its not the proper solution to your
problem, I think its worth doing anyway for the general benefit of RCU.

But lets ask Paul, he is Mr RCU after all :-)

> diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
> index 9663c2a..592f6e4 100644
> --- a/arch/x86/kernel/process_32.c
> +++ b/arch/x86/kernel/process_32.c
> @@ -188,6 +188,9 @@ void cpu_idle(void)
>  			rmb();
>  			idle = pm_idle;
>  
> +			if (rcu_pending(cpu))
> +				rcu_check_callbacks(cpu, 0);
> +
>  			if (!idle)
>  				idle = default_idle;
>  

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

[Paul E. McKenney]

On Sat, Jan 12, 2008 at 10:23:11AM +0100, 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.
>
> Even though Andi is right in that its not the proper solution to your
> problem, I think its worth doing anyway for the general benefit of RCU.
> 
> But lets ask Paul, he is Mr RCU after all :-)

;-)

At first glance, looks workable!

One concern is how often it gets invoked.  If rcu_check_callbacks()
is invoked too often on lots of idle CPUs, it could degrade system
performance due to contention on the RCU internal locks and due to
cacheline bouncing.  Now, my guess is that the rcu_pending() call
should throttle things nicely, but it would be good to test.  All the
testing ideas thus far have been involved and unlikely to test it well,
for example:

CPU 0: lots of synchronize_rcu() calls.

CPU 1: lots of synchronize_rcu() calls.

CPU 2: idle.

CPU 3: CPU-bound workload.

Compare the rate of progress made by CPU 3 with CPUs 0 and 1 active
or not.  But this would not test much -- the load that CPUs 0, 1, and
2 might be placing on the bus/cache/RCU-locks would not be visible to
CPU 3.  One could cache-thrash between CPU 3 and 4, but this requires
a >=5-CPU system.

Will think on it some more.

> > diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c
> > index 9663c2a..592f6e4 100644
> > --- a/arch/x86/kernel/process_32.c
> > +++ b/arch/x86/kernel/process_32.c
> > @@ -188,6 +188,9 @@ void cpu_idle(void)
> >  			rmb();
> >  			idle = pm_idle;
> >  
> > +			if (rcu_pending(cpu))
> > +				rcu_check_callbacks(cpu, 0);

Given that it is not legal to have RCU read-side critical sections in
the idle loop, how about the following?

+				rcu_check_callbacks(cpu, 1);

Perhaps also changing the name of rcu_check_callbacks()'s second
parameter from "user" to something like "in_quiescent_state".  Might
speed up grace-period recognition in some cases -- wouldn't need to
wait for the next trip through the scheduler in some cases.

						Thanx, Paul

> > +
> >  			if (!idle)
> >  				idle = default_idle;
> >  
> 

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

[Andi Kleen]

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