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