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SubjectRe: Question about usage of RCU in the input layer
On Sat, 21 Mar 2009 22:18:22 -0700
"Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:

> > I'm measuring the time that the following code takes:
> >
> > init_completion(&rcu.completion);
> > /* Will wake me after RCU finished. */
> > call_rcu(&rcu.head, wakeme_after_rcu);
> > /* Wait for it. */
> > wait_for_completion(&rcu.completion);
> >

>
> No, my confusion -- I misread as 2700 milliseconds rather than 2700
> -microseconds-. 2700 microseconds (or 2.7 milliseconds) is in the
> expected range for synchronize_rcu() on an HZ=1000 system. 2.7
> seconds would of course be way out of line.

> > > If the former, exactly which kernel are you using? The single-CPU
> > > optimization was added in 2.6.29-rc7, commit ID a682604838.
> >
> > a bit after -rc8, specifically commit
> > 5bee17f18b595937e6beafeee5197868a3f74a06
>
> How many synchronize_rcu() calls are you seeing on the boot path?

I see 20 that hit the above code path (eg ones that wait) until
userspace starts.


> Also, are you running with NO_HZ=y?

of course... is there any other way ? ;-)

--
Arjan van de Ven Intel Open Source Technology Centre
For development, discussion and tips for power savings,
visit http://www.lesswatts.org


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