linux-kernel - Re: [PATCH v2 2/5] preempt/dynamic: Introduce preempt mode accessors

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Date:   Thu, 11 Nov 2021 11:32:40 +0100
From:   Mike Galbraith <efault@....de>
To:     Marco Elver <elver@...gle.com>
Cc:     Valentin Schneider <valentin.schneider@....com>,
        linux-kernel@...r.kernel.org, kasan-dev@...glegroups.com,
        linuxppc-dev@...ts.ozlabs.org, linux-kbuild@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Frederic Weisbecker <frederic@...nel.org>,
        Dmitry Vyukov <dvyukov@...gle.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Nick Desaulniers <ndesaulniers@...gle.com>
Subject: Re: [PATCH v2 2/5] preempt/dynamic: Introduce preempt mode accessors

On Thu, 2021-11-11 at 10:36 +0100, Marco Elver wrote:
> On Thu, 11 Nov 2021 at 04:47, Mike Galbraith <efault@....de> wrote:
> >
> > On Thu, 2021-11-11 at 04:35 +0100, Mike Galbraith wrote:
> > > On Thu, 2021-11-11 at 04:16 +0100, Mike Galbraith wrote:
> > > > On Wed, 2021-11-10 at 20:24 +0000, Valentin Schneider wrote:
> > > > >
> > > > > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > > > > index 5f8db54226af..0640d5622496 100644
> > > > > --- a/include/linux/sched.h
> > > > > +++ b/include/linux/sched.h
> > > > > @@ -2073,6 +2073,22 @@ static inline void cond_resched_rcu(void)
> > > > >  #endif
> > > > >  }
> > > > >
> > > > > +#ifdef CONFIG_PREEMPT_DYNAMIC
> > > > > +
> > > > > +extern bool is_preempt_none(void);
> > > > > +extern bool is_preempt_voluntary(void);
> > > > > +extern bool is_preempt_full(void);
> > > > > +
> > > > > +#else
> > > > > +
> > > > > +#define is_preempt_none() IS_ENABLED(CONFIG_PREEMPT_NONE)
> > > > > +#define is_preempt_voluntary()
> > > > > IS_ENABLED(CONFIG_PREEMPT_VOLUNTARY)
> > > > > +#define is_preempt_full() IS_ENABLED(CONFIG_PREEMPT)
> > > >
> > > > I think that should be IS_ENABLED(CONFIG_PREEMPTION), see
> > > > c1a280b68d4e.
> > > >
> > > > Noticed while applying the series to an RT tree, where tglx
> > > > has done that replacement to the powerpc spot your next patch
> > > > diddles.
> > >
> > > Damn, then comes patch 5 properly differentiating PREEMPT/PREEMPT_RT.
> >
> > So I suppose the powerpc spot should remain CONFIG_PREEMPT and become
> > CONFIG_PREEMPTION when the RT change gets merged, because that spot is
> > about full preemptibility, not a distinct preemption model.
> >
> > That's rather annoying :-/
>
> I guess the question is if is_preempt_full() should be true also if
> is_preempt_rt() is true?

That's what CONFIG_PREEMPTION is.  More could follow, but it was added
to allow multiple models to say "preemptible".

> Not sure all cases are happy with that, e.g. the kernel/trace/trace.c
> case, which wants to print the precise preemption level.

Yeah, that's the "annoying" bit, needing one oddball model accessor
that isn't about a particular model.

> To avoid confusion, I'd introduce another helper that says true if the
> preemption level is "at least full", currently that'd be "full or rt".
> Something like is_preempt_full_or_rt() (but might as well write
> "is_preempt_full() || is_preempt_rt()"), or is_preemption() (to match
> that Kconfig variable, although it's slightly confusing). The
> implementation of that helper can just be a static inline function
> returning "is_preempt_full() || is_preempt_rt()".
>
> Would that help?

Yeah, as it sits two accessors are needed, one that says PREEMPT the
other PREEMPTION, spelling optional.

	-Mike