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

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Date:   Fri, 01 Oct 2021 18:51:32 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Frederic Weisbecker <frederic@...nel.org>,
        "Paul E . McKenney" <paulmck@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Frederic Weisbecker <frederic@...nel.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 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@...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>