On Thu, Sep 16, 2021 at 04:27:15PM +0200, Peter Zijlstra wrote: > On Thu, Sep 16, 2021 at 06:57:12AM -0700, Paul E. McKenney wrote: > > RT systems they avoid expedited grace periods by booting with either > > the rcupdate.rcu_normal or the rcupdate.rcu_normal_after_boot kernel > > boot parameters. And here is the definition for the latter: > > > > static int rcu_normal_after_boot = IS_ENABLED(CONFIG_PREEMPT_RT); > > > > In other words, RT systems shut off expedited grace periods by default, > > and are thus free to use nohz_full CPU or not, as they choose. When using > > nohz_full, they can also enable expedited grace periods by booting with > > rcupdate.rcu_normal_after_boot=0. Or not, sysadm's choice. > > > > So I am not seeing a problem here. What am I missing? > > That wasn't at all clear to me from the Changelog. I thought it was > enabling expedited crud for RT. This paragraph of the current commit log covers that point: Make rcupdate.rcu_normal_after_boot= again writeable on RT (if NO_HZ_ FULL is defined), but keep its default value to 1 (enabled) to avoid regressions. Users who need synchronize_rcu_expedited() will boot with rcupdate.rcu_normal_after_ boot=0 in the kernel cmdline. Does that cover it from your viewpoint? If not, any suggested changes for clarification? Thanx, Paul
