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    SubjectRe: [PATCH 07/10] rcu: Temporarily assume that nohz full CPUs might not be NOCB
    On Thu, May 14, 2020 at 01:08:28AM +0200, Frederic Weisbecker wrote:
    > 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 :-)

    But testing could still start with CPUs marked rcu_nocb but not marked
    nohz_full, right? I must confess that I am a bit concerned about the
    increase in state space.

    > > 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.

    Fair point, but I am also concerned about the welfare of the people
    working on it. ;-)

    Thanx, Paul

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