Re: [PATCH] rcu: Fix missing TICK_DEP_MASK_RCU_EXP dependency check

[Frederic Weisbecker <frederic@kernel.org>]

On Fri, Jan 06, 2023 at 07:01:28PM -0500, Joel Fernandes wrote:
> (lost html content)

I can't find a place where the exp grace period sends an IPI to
CPUs slow to report a QS. But anyway you really need the tick to poll
periodically on the CPU to chase a quiescent state.

Now arguably it's probably only useful when CONFIG_PREEMPT_COUNT=y
and rcu_exp_handler() has interrupted a preempt-disabled or bh-disabled
section. Although rcu_exp_handler() sets TIF_RESCHED, which is handled
by preempt_enable() and local_bh_enable() when CONFIG_PREEMPT=y.
So probably it's only useful when CONFIG_PREEMPT_COUNT=y and CONFIG_PREEMPT=n
(and there is also PREEMPT_DYNAMIC to consider).

If CONFIG_PREEMPT_COUNT=n, the tick can only report idle and user
as QS, but those are already reported explicitly on ct_kernel_exit() ->
rcu_preempt_deferred_qs().

Thanks.