Re: [PATCH 08/11] rcu/nocb: Limit number of softirq callbacks only on softirq

[Valentin Schneider]

On 30/09/21 00:10, Frederic Weisbecker wrote:
> The current condition to limit the number of callbacks executed in a
> row checks the offloaded state of the rdp. Not only is it volatile
> but it is also misleading: the rcu_core() may well be executing
> callbacks concurrently with NOCB kthreads, and the offloaded state
> would then be verified on both cases. As a result the limit would
> spuriously not apply anymore on softirq while in the middle of
> (de-)offloading process.
>
> Another issue with the condition is that rcu_is_callbacks_kthread()
> doesn't check if we are actually running callbacks from rcuc itself or
> from a softirq interrupting rcuc.
>

Doesn't rcutree.use_softirq imply rcuc is never woken, in which case
RCU_SOFTIRQ can't interrupt rcuc (e.g. while run atop an IRQ exit)?
I suppose during the (de)offload sequence we could have RCU_SOFTIRQ running
atop the NOCB CB kthread, but that's not something
rcu_is_callbacks_kthread() detects.

Also, why is rcu_is_callbacks_kthread() hardcoded to false for
!CONFIG_RCU_BOOST? Isn't it relevant for do_rcu_batch() ratelimiting
regardless (at least before your patches)?