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SubjectRe: [PATCH RFC 2/8] rcu: Clarify usage of cond_resched for tasks-RCU
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()
>

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