linux-kernel - Re: [PATCH 14/36] cpuidle: Fix rcu_idle

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Message-ID: <Yqi+gg+p0sv0F7Di@FVFF77S0Q05N>
Date:   Tue, 14 Jun 2022 17:59:46 +0100
From:   Mark Rutland <mark.rutland@....com>
To:     Peter Zijlstra <peterz@...radead.org>
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Subject: Re: [PATCH 14/36] cpuidle: Fix rcu_idle_*() usage

On Tue, Jun 14, 2022 at 06:40:53PM +0200, Peter Zijlstra wrote:
> On Tue, Jun 14, 2022 at 01:41:13PM +0100, Mark Rutland wrote:
> > On Wed, Jun 08, 2022 at 04:27:37PM +0200, Peter Zijlstra wrote:
> > > --- a/kernel/time/tick-broadcast.c
> > > +++ b/kernel/time/tick-broadcast.c
> > > @@ -622,9 +622,13 @@ struct cpumask *tick_get_broadcast_onesh
> > >   * to avoid a deep idle transition as we are about to get the
> > >   * broadcast IPI right away.
> > >   */
> > > -int tick_check_broadcast_expired(void)
> > > +noinstr int tick_check_broadcast_expired(void)
> > >  {
> > > +#ifdef _ASM_GENERIC_BITOPS_INSTRUMENTED_NON_ATOMIC_H
> > > +	return arch_test_bit(smp_processor_id(), cpumask_bits(tick_broadcast_force_mask));
> > > +#else
> > >  	return cpumask_test_cpu(smp_processor_id(), tick_broadcast_force_mask);
> > > +#endif
> > >  }
> > 
> > This is somewhat not-ideal. :/
> 
> I'll say.
> 
> > Could we unconditionally do the arch_test_bit() variant, with a comment, or
> > does that not exist in some cases?
> 
> Loads of build errors ensued, which is how I ended up with this mess ...

Yaey :(

I see the same is true for the thread flag manipulation too.

I'll take a look and see if we can layer things so that we can use the arch_*()
helpers and wrap those consistently so that we don't have to check the CPP
guard.

Ideally we'd have a a better language that allows us to make some
context-senstive decisions, then we could hide all this gunk in the lower
levels with somethin like:

	if (!THIS_IS_A_NOINSTR_FUNCTION()) {
		explicit_instrumentation(...);
	}

... ho hum.

Mark.