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

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Date:   Mon, 4 Oct 2021 15:47:48 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Uladzislau Rezki <urezki@...il.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Boqun Feng <boqun.feng@...il.com>,
        Neeraj Upadhyay <neeraju@...eaurora.org>,
        Josh Triplett <josh@...htriplett.org>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: Re: [PATCH 10/11] rcu: Apply callbacks processing time limit only on
 softirq

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@...nel.org>
> > Cc: Valentin Schneider <valentin.schneider@....com>
> > Cc: Peter Zijlstra <peterz@...radead.org>
> > Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> > Cc: Josh Triplett <josh@...htriplett.org>
> > Cc: Joel Fernandes <joel@...lfernandes.org>
> > Cc: Boqun Feng <boqun.feng@...il.com>
> > Cc: Neeraj Upadhyay <neeraju@...eaurora.org>
> > Cc: Uladzislau Rezki <urezki@...il.com>
> > Cc: Thomas Gleixner <tglx@...utronix.de>
> 
> Reviewed-by: Valentin Schneider <valentin.schneider@....com>