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

[Frederic Weisbecker]

On Fri, Oct 01, 2021 at 06:51:32PM +0100, Valentin Schneider wrote:
> 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()")

But if cond_resched_tasks_rcu_qs() was called and then on the next iteration
tlimit is checked, the time spent scheduling out is included, right?

Thanks.

> , 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>