Re: [GIT pull] locking/urgent for v5.10-rc6

["Paul E. McKenney" <paulmck@kernel.org>]

On Sun, Nov 29, 2020 at 11:31:41AM -0800, Linus Torvalds wrote:
> On Sun, Nov 29, 2020 at 5:38 AM Thomas Gleixner <tglx@linutronix.de> wrote:
> >
> > Yet two more places which invoke tracing from RCU disabled regions in the
> > idle path. Similar to the entry path the low level idle functions have to
> > be non-instrumentable.
> 
> This really seems less than optimal.
> 
> In particular, lookie here:
> 
> > @@ -94,9 +94,35 @@ void __cpuidle default_idle_call(void)
> >
> >                 trace_cpu_idle(1, smp_processor_id());
> >                 stop_critical_timings();
> > +
> > +               /*
> > +                * arch_cpu_idle() is supposed to enable IRQs, however
> > +                * we can't do that because of RCU and tracing.
> > +                *
> > +                * Trace IRQs enable here, then switch off RCU, and have
> > +                * arch_cpu_idle() use raw_local_irq_enable(). Note that
> > +                * rcu_idle_enter() relies on lockdep IRQ state, so switch that
> > +                * last -- this is very similar to the entry code.
> > +                */
> > +               trace_hardirqs_on_prepare();
> > +               lockdep_hardirqs_on_prepare(_THIS_IP_);
> >                 rcu_idle_enter();
> > +               lockdep_hardirqs_on(_THIS_IP_);
> > +
> >                 arch_cpu_idle();
> > +
> > +               /*
> > +                * OK, so IRQs are enabled here, but RCU needs them disabled to
> > +                * turn itself back on.. funny thing is that disabling IRQs
> > +                * will cause tracing, which needs RCU. Jump through hoops to
> > +                * make it 'work'.
> > +                */
> > +               raw_local_irq_disable();
> > +               lockdep_hardirqs_off(_THIS_IP_);
> >                 rcu_idle_exit();
> > +               lockdep_hardirqs_on(_THIS_IP_);
> > +               raw_local_irq_enable();
> > +
> >                 start_critical_timings();
> >                 trace_cpu_idle(PWR_EVENT_EXIT, smp_processor_id());
> >         }
> 
> And look at what the code generation for the idle exit path is when
> lockdep isn't even on.
> 
> It's *literally*
> 
>         cli
>         call rcu_idle_exit
>         sti
> 
> and guess what rcu_idle_exit does?
> 
> Yeah, that one does "pushf; cli; call rcu_eqs_exit; popf".
> 
> So here we are, in the somewhat critical "an interrupt woke us up"
> section, and we're doing just ridiculously stupid things.
> 
> I've pulled this, because it solves a problem, but there's a deeper
> problem here in how all this is done.
> 
> The idle path is actually quite important. I can point to real loads
> where this is a big part of the CPU profile, because you end up having
> lots of "go to sleep for very short times, because the thing we were
> waiting for takes almost no time at all".

This is because of the noinline implied by the noinstr on rcu_eqs_exit().
If I replace that with inline, it does get inlined.  Except that, if
I remember correctly, making that change messes up the tooling that
enforces the no-instrumentation regions.

I -think- that a combination of instrumentation_end() and s/noinstr/inline/
might work, but I will need to consult with the experts on this.

							Thanx, Paul