Hello Ankur and Paul,
On Mon, Feb 12, 2024 at 09:55:39PM -0800, Ankur Arora wrote:
> With PREEMPT_RCU=n, cond_resched() provides urgently needed quiescent
> states for read-side critical sections via rcu_all_qs().
> One reason why this was necessary: lacking preempt-count, the tick
> handler has no way of knowing whether it is executing in a read-side
> critical section or not.
>
> With PREEMPT_AUTO=y, there can be configurations with (PREEMPT_COUNT=y,
> PREEMPT_RCU=n). This means that cond_resched() is a stub which does
> not provide for quiescent states via rcu_all_qs().
>
> So, use the availability of preempt_count() to report quiescent states
> in rcu_flavor_sched_clock_irq().
>
> Suggested-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Signed-off-by: Ankur Arora <ankur.a.arora@xxxxxxxxxx>
> ---
> kernel/rcu/tree_plugin.h | 11 +++++++----
> 1 file changed, 7 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
> index 26c79246873a..9b72e9d2b6fe 100644
> --- a/kernel/rcu/tree_plugin.h
> +++ b/kernel/rcu/tree_plugin.h
> @@ -963,13 +963,16 @@ static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
> */
> static void rcu_flavor_sched_clock_irq(int user)
> {
> - if (user || rcu_is_cpu_rrupt_from_idle()) {
> + if (user || rcu_is_cpu_rrupt_from_idle() ||
> + (IS_ENABLED(CONFIG_PREEMPT_COUNT) &&
> + !(preempt_count() & (PREEMPT_MASK | SOFTIRQ_MASK)))) {
I was wondering if it makes sense to even support !PREEMPT_RCU under
CONFIG_PREEMPT_AUTO.
AFAIU, this CONFIG_PREEMPT_AUTO series preempts the kernel on
the next tick boundary in the worst case, with all preempt modes including
the preempt=none mode.
Considering this, does it makes sense for RCU to be non-preemptible in
CONFIG_PREEMPT_AUTO=y? Because if that were the case, and a read-side critical
section extended beyond the tick, then it prevents the PREEMPT_AUTO preemption
from happening, because rcu_read_lock() would preempt_disable().
To that end, I wonder if CONFIG_PREEMPT_AUTO should select CONFIG_PREEMPTION
(or CONFIG_PREEMPT_BUILD, not sure which) as well because it does cause
kernel preemption. That then forces selection of CONFIG_PREEMPT_RCU as well.
thanks,
- Joel
>
> /*
> * Get here if this CPU took its interrupt from user
> - * mode or from the idle loop, and if this is not a
> - * nested interrupt. In this case, the CPU is in
> - * a quiescent state, so note it.
> + * mode, from the idle loop without this being a nested
> + * interrupt, or while not holding a preempt count.
> + * In this case, the CPU is in a quiescent state, so note
> + * it.
> *
> * No memory barrier is required here because rcu_qs()
> * references only CPU-local variables that other CPUs
> --
> 2.31.1
>
