linux-kernel - Re: [PATCH 2/3] preempt/dynamic: Introduce preempt mode accessors

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Message-ID: <CANpmjNMsyb9aOqcvUUMLbkyHiE9ZieBigU1XqBXgtYz_O00y3g@mail.gmail.com>
Date:   Mon, 14 Mar 2022 21:34:26 +0100
From:   Marco Elver <elver@...gle.com>
To:     paulmck@...nel.org
Cc:     Frederic Weisbecker <frederic@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Uladzislau Rezki <uladzislau.rezki@...y.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH 2/3] preempt/dynamic: Introduce preempt mode accessors

On Mon, 14 Mar 2022 at 21:06, Paul E. McKenney <paulmck@...nel.org> wrote:
>
> On Mon, Mar 14, 2022 at 03:44:39PM +0100, Marco Elver wrote:
> > On Mon, 14 Mar 2022 at 14:37, Frederic Weisbecker <frederic@...nel.org> wrote:
> > >
> > > From: Valentin Schneider <valentin.schneider@....com>
> > >
> > > CONFIG_PREEMPT{_NONE, _VOLUNTARY} designate either:
> > > o The build-time preemption model when !PREEMPT_DYNAMIC
> > > o The default boot-time preemption model when PREEMPT_DYNAMIC
> > >
> > > IOW, using those on PREEMPT_DYNAMIC kernels is meaningless - the actual
> > > model could have been set to something else by the "preempt=foo" cmdline
> > > parameter.
> > >
> > > Introduce a set of helpers to determine the actual preemption mode used by
> > > the live kernel.
> > >
> > > Suggested-by: Marco Elver <elver@...gle.com>
> > > Signed-off-by: Valentin Schneider <valentin.schneider@....com>
> > > Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> > > Cc: Uladzislau Rezki <uladzislau.rezki@...y.com>
> > > Cc: Joel Fernandes <joel@...lfernandes.org>
> > > Cc: Boqun Feng <boqun.feng@...il.com>
> > > Cc: Peter Zijlstra <peterz@...radead.org>
> > > Cc: Neeraj Upadhyay <quic_neeraju@...cinc.com>
> > > ---
> > >  include/linux/sched.h | 16 ++++++++++++++++
> > >  kernel/sched/core.c   | 11 +++++++++++
> > >  2 files changed, 27 insertions(+)
> > >
> > > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > > index 508b91d57470..d348e886e4d0 100644
> > > --- a/include/linux/sched.h
> > > +++ b/include/linux/sched.h
> > > @@ -2096,6 +2096,22 @@ static inline void cond_resched_rcu(void)
> > >  #endif
> > >  }
> > >
> > > +#ifdef CONFIG_PREEMPT_DYNAMIC
> > > +
> > > +extern bool preempt_mode_none(void);
> > > +extern bool preempt_mode_voluntary(void);
> > > +extern bool preempt_mode_full(void);
> > > +
> > > +#else
> > > +
> > > +#define preempt_mode_none() IS_ENABLED(CONFIG_PREEMPT_NONE)
> > > +#define preempt_mode_voluntary() IS_ENABLED(CONFIG_PREEMPT_VOLUNTARY)
> > > +#define preempt_mode_full() IS_ENABLED(CONFIG_PREEMPT)
> > > +
> >
> > Shame this was somehow forgotten.
> > There was a v3 of this patch that fixed a bunch of things (e.g. making
> > these proper functions so all builds error if accidentally used in
> > #if).
> >
> > https://lore.kernel.org/lkml/20211112185203.280040-3-valentin.schneider@arm.com/
> >
> > Is it also possible to take all the rest of that series (all 4
> > patches) from Valentin?
>
> Me, I am assuming that #2/3 is an experimental test so that I am able
> to easily whack this series over the head with rcutorture.  ;-)

I might be out of the loop here. All I can add is that any issues that
are a consequence of the preempt mode accessors are only testable if
the preemption model is actually changed at runtime and AFAIK
rcutorture doesn't do that. But as noted, this patch wasn't the latest
version and there were issues with it fixed by Valentin's latest v3
(from November, but had never been picked up anywhere).

Thanks,
-- Marco