On Sun, 13 May 2018 20:15:35 -0700 "Joel Fernandes (Google)" <[email protected]> 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/[email protected] > > Signed-off-by: Joel Fernandes (Google) <[email protected]> > --- > 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. */ > */ > #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()
