Re: [PATCH RFC 2/8] rcu: Clarify usage of cond_resched for tasks-RCU

From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: "Joel Fernandes (Google)" <joel@joelfernandes.org>,
	linux-kernel@vger.kernel.org,
	Josh Triplett <josh@joshtriplett.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	byungchul.park@lge.com, kernel-team@android.com
Subject: Re: [PATCH RFC 2/8] rcu: Clarify usage of cond_resched for tasks-RCU
Date: Mon, 14 May 2018 10:22:05 -0700	[thread overview]
Message-ID: <20180514172205.GZ26088@linux.vnet.ibm.com> (raw)
In-Reply-To: <20180514105454.45946ad3@gandalf.local.home>

On Mon, May 14, 2018 at 10:54:54AM -0400, Steven Rostedt wrote:
> On Sun, 13 May 2018 20:15:35 -0700
> "Joel Fernandes (Google)" <joel@joelfernandes.org> wrote:
> 
> > Recently we had a discussion about cond_resched unconditionally
> > recording a voluntary context switch [1].
> > 
> > Lets add a comment clarifying that how this API is to be used.
> > 
> > [1] https://lkml.kernel.org/r/1526027434-21237-1-git-send-email-byungchul.park@lge.com
> > 
> > Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> > ---
> >  include/linux/rcupdate.h | 11 ++++++++---
> >  1 file changed, 8 insertions(+), 3 deletions(-)
> > 
> > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > index 743226176350..a9881007ece6 100644
> > --- a/include/linux/rcupdate.h
> > +++ b/include/linux/rcupdate.h
> > @@ -159,8 +159,12 @@ static inline void rcu_init_nohz(void) { }
> >  	} while (0)
> >  
> >  /*
> > - * Note a voluntary context switch for RCU-tasks benefit.  This is a
> > - * macro rather than an inline function to avoid #include hell.
> > + * Note an attempt to perform a voluntary context switch for RCU-tasks benefit.
> > + *
> > + * This is called even in situations where a context switch didn't really
> > + * happen even though it was requested. The caller uses it to indicate
> > + * traversal of an RCU-tasks quiescent state. This is a macro rather than an
> > + * inline function to avoid #include hell.
> 
> I don't know. I just don't like the wording. It sounds too much like
> it was written by someone that was confused for it being called when a
> context switch didn't occur ;-)
> 
> What about something more like:
> 
> /*
>  * This is called to denote a RCU-task quiescent state. It is placed at
>  * voluntary preemption points, as RCU-task critical sections may not
>  * perform voluntary preemption or scheduling calls. It does not matter
>  * if the task is scheduled out or not, just that a voluntary preemption
>  * may be done.
>  */

s/RCU-task/RCU-tasks/ and I am good with this.

							Thanx, Paul

> >   */
> >  #ifdef CONFIG_TASKS_RCU
> >  #define rcu_note_voluntary_context_switch_lite(t) \
> > @@ -187,7 +191,8 @@ static inline void exit_tasks_rcu_finish(void) { }
> >  #endif /* #else #ifdef CONFIG_TASKS_RCU */
> >  
> >  /**
> > - * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU
> > + * cond_resched_tasks_rcu_qs - Report potential quiescent states to RCU.
> > + * The quiescent state report is made even if cond_resched() did nothing.
> 
> Same thing here.
> 
>  * The quiescent state report does not depend on cond_resched() scheduling.
> 
> -- Steve
> 
> 
> >   *
> >   * This macro resembles cond_resched(), except that it is defined to
> >   * report potential quiescent states to RCU-tasks even if the cond_resched()
> 