linux-kernel - Re: [PATCH RFC] doc: rcu: remove obsolete (non-)requirement about disabling preemption

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Date:   Thu, 18 Oct 2018 21:50:35 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Joel Fernandes <joel@...lfernandes.org>
Cc:     "Paul E. McKenney" <paulmck@...ux.ibm.com>,
        Nikolay Borisov <nborisov@...e.com>,
        linux-kernel@...r.kernel.org, Jonathan Corbet <corbet@....net>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-doc@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH RFC] doc: rcu: remove obsolete (non-)requirement about
 disabling preemption

On Thu, 18 Oct 2018 18:26:45 -0700
Joel Fernandes <joel@...lfernandes.org> wrote:

> Yes, local_irq_restore is light weight, and does not check for reschedules.
> 
> I was thinking of case where ksoftirqd is woken up, but does not run unless
> we set the NEED_RESCHED flag. But that should get set anyway since probably
> ksoftirqd is of high enough priority than the currently running task..
> 
> Roughly speaking the scenario could be something like:
> 
> rcu_read_lock();
>                  <-- IPI comes in for the expedited GP, sets exp_hint
> local_irq_disable();
> // do a bunch of stuff
> rcu_read_unlock();   <-- This calls the rcu_read_unlock_special which raises
>                          the soft irq, and wakesup softirqd.

If softirqd is of higher priority than the current running task, then
the try_to_wake_up() will set NEED_RESCHED of the current task here.

-- Steve

> local_irq_enable();
> 
> // Now ksoftirqd is ready to run but we don't switch into the
> // scheduler for sometime because tif_need_resched() returns false and
> // any cond_resched calls do nothing. So we potentially spend lots of
> // time before the next scheduling event.
> 
> You think this should not be an issue?