Re: [PATCH 10/11] rcu: Apply callbacks processing time limit only on softirq

[Valentin Schneider]

On 30/09/21 00:10, Frederic Weisbecker wrote:
> Time limit only makes sense when callbacks are serviced in softirq mode
> because:
>
> _ In case we need to get back to the scheduler,
>   cond_resched_tasks_rcu_qs() is called after each callback.
>
> _ In case some other softirq vector needs the CPU, the call to
>   local_bh_enable() before cond_resched_tasks_rcu_qs() takes care about
>   them via a call to do_softirq().
>
> _ The time spent on other tasks after scheduling out, or on softirqs
>   processing, is spuriously accounted to the time limit.
>

That wasn't the case before ("rcu: Fix callbacks processing time limit
retaining cond_resched()"), though under PREEMPT_RT that *was* true (since
bh-off remains preemptible). So I'd say that's a change we want.

> Therefore, make sure the time limit only applies to softirq mode.
>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Valentin Schneider <valentin.schneider@xxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Neeraj Upadhyay <neeraju@xxxxxxxxxxxxxx>
> Cc: Uladzislau Rezki <urezki@xxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>

Reviewed-by: Valentin Schneider <valentin.schneider@xxxxxxx>