Re: [PATCH] Extend mwait idle to optimize away CAL and RESinterrupts to an idle CPU -v1

[Yong Zhang]

On Tue, Feb 28, 2012 at 05:05:52AM -0800, Paul E. McKenney wrote:
> On Tue, Feb 28, 2012 at 03:12:55PM +0800, Yong Zhang wrote:
> > On Mon, Feb 27, 2012 at 09:05:27AM -0800, Paul E. McKenney wrote:
> > > On Mon, Feb 27, 2012 at 05:06:46PM +0800, Yong Zhang wrote:
> > > > On Sat, Feb 25, 2012 at 05:32:53PM -0800, Paul E. McKenney wrote:
> > > > > On Fri, Feb 24, 2012 at 01:41:50PM +0800, Yong Zhang wrote:
> > > > > > On Thu, Feb 23, 2012 at 11:34:11AM -0800, Venki Pallipadi wrote:
> > > > > > > On Thu, Feb 23, 2012 at 1:30 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> > > > > > > > Why not scheduler_ipi()?
> > > > > > > 
> > > > > > > Was trying to avoid irq_enter/exit. As the work here is done in idle
> > > > > > > thread context, I though we could avoid enter/exit. 
> > > > > > 
> > > > > > It seems we could not.
> > > > > > At least RCU need it, see commit c5d753a55, otherwise we will get
> > > > > > warning like 'RCU used illegally from extended quiescent state!'
> > > > > 
> > > > > If the use is tracing, then Steven Rostedt's patchset plus use of his
> > > > > _rcuidle() tracing variants handles this:
> > > > > 
> > > > > 	https://lkml.org/lkml/2012/2/7/231
> > > > > 
> > > > > If this is instead algorithmic use of RCU, a set of patches I have queued
> > > > > up for 3.4 will be required.
> > > > 
> > > > scheduler_ipi() doing more than tracing. Will look at your patches :)
> > > 
> > > Ah!  The key question is whether or not the code in question is called
> > > both from idle and from non-idle.
> > 
> > In fact before this patch from Venki, the only call site of scheduler_ipi()
> > is resched irq handler. Then Venki introduce __scheduler_ipi()(which avoid
> > irq_enter()/irq_exit()) into cpu_idle(). So the answer is yes.
> 
> Ah, that explains why I didn't see it in my testing.  ;-)
> 
> > But when I was testing this patch, I didn't see explicit warning on
> > illegal rcu usage. The reason maybe 1) there are no much rcu dereference
> > in scheduler_ipi(), but we indeed do tracing in it; 2) rq->lock provide
> > some kind of protection.
> > Maybe I'm overstraining, but it is potential danger.
> 
> Did you have CONFIG_PROVE_RCU=y when testing?

Yeah.

> zgrep PROVE /proc/config.gz 
CONFIG_PROVE_LOCKING=y
CONFIG_PROVE_RCU=y
CONFIG_PROVE_RCU_REPEATEDLY=y

> 
> > But anyway, it's not an issue anymore since Venki removed __scheduler_ipi()
> > in his latest version.
> 
> OK.
> 
> > > This will be easiest if the code is
> > > called only from idle, in which case you should only need this one:
> > > 
> > > 	https://lkml.org/lkml/2012/2/3/498
> > 
> > Hmm... Yeah, RCU_NONIDLE() could survive IMHO :)
> 
> Seems like it might be needed sooner rather than later...

;-)

Thanks,
Yong
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