linux-kernel - Re: [PATCH 07/10] rcu: Temporarily assume that nohz full CPUs might not be NOCB

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Date:   Thu, 14 May 2020 01:08:28 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        Josh Triplett <josh@...htriplett.org>
Subject: Re: [PATCH 07/10] rcu: Temporarily assume that nohz full CPUs might
 not be NOCB

On Wed, May 13, 2020 at 11:25:27AM -0700, Paul E. McKenney wrote:
> On Wed, May 13, 2020 at 06:47:11PM +0200, Frederic Weisbecker wrote:
> > So far nohz_full CPUs had to be nocb. This requirement may change
> > temporarily as we are working on preparing RCU to be able to toggle the
> > nocb state of a CPU. Once that is done and nohz_full can be toggled as
> > well dynamically, we'll restore that initial requirement.
> 
> Would it simplify anything to make the CPU exit nohz_full first and
> then exit rcu_nocb and vice versa in the other direction?  That way the
> assumption about nohz_full CPUs always being rcu_nocb could remain while
> still allowing runtime changes to both states.

That's the future plan but for now nohz_full can't even be exited yet.
RCU is unlucky enough to be chosen as the starting point of this whole work :-)

> Of course, given that setup, it would not be possible to cause a CPU to
> exit rcu_nocb state if it was still in nohz_full state.

Right.

> My fear is that allowing a CPU to be in nohz_full state without also
> being in rcu_nocb state will cause needless confusion and bug reports.

Well, it should only be visible to those who work on it since there
won't be a proper interface before we achieve the whole.

Thanks.