On Thu, Jul 02 2026 at 16:07, Paul E. McKenney wrote:
> wouldn't it work better to just leave all CPUs in RCU-callbacks-offloaded
> state? Then you can adjust the nohz_full state of arbitrary CPUs without
> messing with RCU.
[...]
> a continuous stream of race-condition bugs inspired the current state,
> which is to allow this state to change only for offline CPUs.
Thanks Paul. That is appealing, and we would much rather not wade into
the online offload-switching races you describe.
Let me lay out the one tension it creates on our side and ask how you and
Frederic would like it resolved.
DHM's aim is to enable kernel-noise isolation purely at runtime, on
machines that did not pass nohz_full= / rcu_nocbs= at boot. "Leave all
CPUs offloaded" needs the candidate CPUs to be in rcu_nocb_mask, which is
only populated at boot. So the RCU part seems to come down to two options:
(a) Accept a boot hint: require rcu_nocbs= (or nohz_full=) to cover the
set of CPUs that may later be isolated. RCU is then never touched at
runtime, exactly as you suggest. tick / timer / managed_irq /
watchdog stay fully runtime-adjustable, so the "no boot parameter"
property holds for everything except RCU offloading.
(b) Change the offload state at runtime with no boot hint, which is
precisely the online-switching problem you and Frederic hit, and what
Thomas's lightweight-offloaded + CPUHP_AP_RCU_SYNC sketch would need
to make cheap and race-free.
We would lean towards (a) as the pragmatic first step: it keeps RCU out of
the runtime path entirely, per your recommendation, and only asks the admin
who wants runtime RCU-noise isolation to declare the candidate CPUs at boot.
(b) / Thomas's mechanism could be a separate, later effort if a truly
boot-parameter-free RCU story turns out to be wanted.
Does scoping the RCU part to (a) sound acceptable to you and Frederic? If
so, we will drop runtime nocb toggling from DHM entirely and just document
the rcu_nocbs= expectation, leaving the other housekeeping types runtime
adjustable.
Thanks,
Jing Wu
Qiliang Yuan
