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

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Date:   Mon, 14 Mar 2022 14:53:48 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Marco Elver <elver@...gle.com>
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, Mar 14, 2022 at 09:34:26PM +0100, Marco Elver wrote:
> 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).

I will be marking 2/3 "EXP" to prevent myself from accidentally sending
it upstream.

But longer term, maybe I should pick up Valentin's series.

This stuff is v5.19 at the earliest, so not (yet) urgent.

							Thanx, Paul