Re: Folllowing up on LSF/MM RCU/idle discussion

[Peter Zijlstra <peterz@infradead.org>]

On Tue, May 10, 2022 at 10:43:51AM +0100, Mark Rutland wrote:
> On Tue, May 10, 2022 at 08:54:57AM +0200, Peter Zijlstra wrote:
> > On Mon, May 09, 2022 at 08:56:33AM -0700, Paul E. McKenney wrote:
> > > Hello, Jiri!
> > > 
> > > It was good chatting with you last week, and I hope that travels went
> > > well!
> > > 
> > > Just wanted to follow up on the non-noinstr code between the call
> > > to rcu_idle_enter() and rcu_idle_exit().  Although the most correct
> > > approach is to never have non-noinstr code in arch_cpu_idle(), for all I
> > > know there might well be architectures for which this is not feasible.
> > > If so, one workaround would be to supply a flag set by each arch (or
> > > subarch) that says that rcu_idle_enter() and rcu_idle_exit() are invoked
> > > within arch_cpu_idle().
> > > 
> > > CCing Peter, who just might have an opinion on this.  ;-)
> > 
> > Definitely have an opinion; just lack the tools to enforce these rules.
> > I cleaned up the worst of it for x86 but it's a shit-show for most
> > others. ARM in particular has some 'issues'.
> 
> Probably worth pointing out that arch_cpu_idle() is the simple case (and I
> fixed that for arm64 to be correct for RCU and noinstr). I think the same
> applies for most architectures.
> 
> The real beast is the cpuidle framework, which is what I think you're referring
> to below, and IIRC that does the rcu_idle_enter() ... rcu_idle_exit() itself?
> Maybe that was just for suspend.

The whole group idle nonsense on arm32 was the worse I think.