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

From: Steven Rostedt <rostedt@goodmis.org>
To: Joel Fernandes <joel@joelfernandes.org>
Cc: "Paul E. McKenney" <paulmck@linux.ibm.com>,
	Nikolay Borisov <nborisov@suse.com>,
	linux-kernel@vger.kernel.org, Jonathan Corbet <corbet@lwn.net>,
	Josh Triplett <josh@joshtriplett.org>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	linux-doc@vger.kernel.org,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Subject: Re: [PATCH RFC] doc: rcu: remove obsolete (non-)requirement about disabling preemption
Date: Thu, 18 Oct 2018 21:50:35 -0400	[thread overview]
Message-ID: <20181018215035.5e8ff553@vmware.local.home> (raw)
In-Reply-To: <20181019012645.GC89903@joelaf.mtv.corp.google.com>

On Thu, 18 Oct 2018 18:26:45 -0700
Joel Fernandes <joel@joelfernandes.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?