Messages in this thread | | | Date | Tue, 28 Feb 2012 15:12:55 +0800 | From | Yong Zhang <> | Subject | Re: [PATCH] Extend mwait idle to optimize away CAL and RES interrupts to an idle CPU -v1 |
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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@infradead.org> 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.
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.
But anyway, it's not an issue anymore since Venki removed __scheduler_ipi() in his latest version.
> 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 :)
Thanks, Yong
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