linux-kernel - Re: [PATCH rcu 3/3] rcu: Simplify rcu_init

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Message-ID: <20220901111114.GA103483@lothringen>
Date:   Thu, 1 Sep 2022 13:11:14 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, rostedt@...dmis.org,
        Zhen Lei <thunder.leizhen@...wei.com>,
        Joel Fernandes <joel@...lfernandes.org>
Subject: Re: [PATCH rcu 3/3] rcu: Simplify rcu_init_nohz() cpumask handling

On Thu, Sep 01, 2022 at 03:25:20AM -0700, Paul E. McKenney wrote:
> On Thu, Sep 01, 2022 at 11:15:57AM +0200, Frederic Weisbecker wrote:
> > > +#elif defined(CONFIG_NO_HZ_FULL)
> > > +	if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
> > > +		cpumask = tick_nohz_full_mask;
> > > +#endif
> > 
> > A subtle behaviour difference here too: CONFIG_RCU_NOCB_CPU_DEFAULT_ALL will
> > now override nohz_full=
> > 
> > I don't mind, it's probably what we want in the end, but the changelog should
> > tell about it, or even better, this should be a separate change.
> 
> Good point.  Perhaps the key point is that if there is nohz_full=,
> rcu_nocbs=, and CONFIG_RCU_NOCB_CPU_DEFAULT_ALL, we still need rcu_nocbs=
> to include at least those bits set by nohz_full=.

Not sure I get what you mean. nohz_full= should in any case always force
rcu_nocbs at least on the nohz_full CPUs.

For example assuming the following combination: rcu_nocbs=6, nohz_full=7 AND
CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y, then the result should be:

NOCB CPUs = 6,7
NOHZ_FULL CPUs = 7

(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y is overriden by rcu_nocbs=6).

Now if we have nohz_full=7 AND CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y, then the
result is expected to be either:

NOCB CPUs = 7 (upstream behaviour)
NOHZ_FULL CPUs = 7

or 

NOCB CPUs = all
NOHZ_FULL CPUs = 7

The second makes more sense IMHO but that should be in a separate change.

Thanks.