Re: [PATCH] rcu: Report a quiescent state when it's exactly in the state

[Byungchul Park]

On 2018-05-12 ìì 7:41, Joel Fernandes wrote:
> On Fri, May 11, 2018 at 09:17:46AM -0700, Paul E. McKenney wrote:
> > On Fri, May 11, 2018 at 09:57:54PM +0900, Byungchul Park wrote:
> > > Hello folks,
> > >
> > > I think I wrote the title in a misleading way.
> > >
> > > Please change the title to something else such as,
> > > "rcu: Report a quiescent state when it's in the state" or,
> > > "rcu: Add points reporting quiescent states where proper" or so on.
> > >
> > > On 2018-05-11 ìí 5:30, Byungchul Park wrote:
> > > > We expect a quiescent state of TASKS_RCU when cond_resched_tasks_rcu_qs()
> > > > is called, no matter whether it actually be scheduled or not. However,
> > > > it currently doesn't report the quiescent state when the task enters
> > > > into __schedule() as it's called with preempt = true. So make it report
> > > > the quiescent state unconditionally when cond_resched_tasks_rcu_qs() is
> > > > called.
> > > >
> > > > And in TINY_RCU, even though the quiescent state of rcu_bh also should
> > > > be reported when the tick interrupt comes from user, it doesn't. So make
> > > > it reported.
> > > >
> > > > Lastly in TREE_RCU, rcu_note_voluntary_context_switch() should be
> > > > reported when the tick interrupt comes from not only user but also idle,
> > > > as an extended quiescent state.
> > > >
> > > > Signed-off-by: Byungchul Park <byungchul.park@xxxxxxx>
> > > > ---
> > > >   include/linux/rcupdate.h | 4 ++--
> > > >   kernel/rcu/tiny.c        | 6 +++---
> > > >   kernel/rcu/tree.c        | 4 ++--
> > > >   3 files changed, 7 insertions(+), 7 deletions(-)
> > > >
> > > > diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> > > > index ee8cf5fc..7432261 100644
> > > > --- a/include/linux/rcupdate.h
> > > > +++ b/include/linux/rcupdate.h
> > > > @@ -195,8 +195,8 @@ static inline void exit_tasks_rcu_finish(void) { }
> > > >    */
> > > >   #define cond_resched_tasks_rcu_qs() \
> > > >   do { \
> > > > -	if (!cond_resched()) \
> > > > -		rcu_note_voluntary_context_switch_lite(current); \
> > > > +	rcu_note_voluntary_context_switch_lite(current); \
> > > > +	cond_resched(); \
> >
> > Ah, good point.
> >
> > Peter, I have to ask...  Why is "cond_resched()" considered a preemption
> > while "schedule()" is not?
>
> Infact something interesting I inferred from the __schedule loop related to
> your question:
>
> switch_count can either be set to prev->invcsw or prev->nvcsw. If we can
> assume that switch_count reflects whether the context switch is involuntary
> or voluntary,
>                        
> task-running-state	preempt		switch_count
> 0 (running)		1		involuntary
> 0			0		involuntary
> 1			0		voluntary
> 1			1		involuntary
>
> According to the above table, both the task's running state and the preempt
> parameter to __schedule should be used together to determine if the switch is
> a voluntary one or not.
>
> So this code in rcu_note_context_switch should really be:
> if (!preempt && !(current->state & TASK_RUNNING))
> 	rcu_note_voluntary_context_switch_lite(current);
>
> According to the above table, cond_resched always classifies as an
> involuntary switch which makes sense to me. Even though cond_resched is

Hello guys,

The classification for nivcsw/nvcsw used in scheduler core, Joel, you
showed us is different from that used in when we distinguish between
non preemption/voluntary preemption/preemption/full and so on, even
they use the same word, "voluntary" though.

The name, rcu_note_voluntary_context_switch_lite() used in RCU has
a lot to do with the latter, the term of preemption. Furthermore, I
think the function should be called even when calling schedule() for
sleep as well. I think it would be better to change the function
name to something else to prevent confusing, it's up to Paul tho. :)

> explicitly called, its still sort of involuntary in the sense its not called
> into the scheduler for sleeping, but rather for seeing if something else can
> run instead (a preemption point). Infact none of the task deactivation in the
> __schedule loop will run if cond_resched is used.
>
> I agree that if schedule was called directly but with TASK_RUNNING=1, then
> that could probably be classified an involuntary switch too...
>
> Also since we're deciding to call rcu_note_voluntary_context_switch_lite
> unconditionally, then IMO this comment on that macro:
>
> /*
>   * Note a voluntary context switch for RCU-tasks benefit.  This is a
>   * macro rather than an inline function to avoid #include hell.
>   */
>   #ifdef CONFIG_TASKS_RCU
>   #define rcu_note_voluntary_context_switch_lite(t)
>
> Should be changed to:
>
> /*
>   * Note a 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. This is a
>   * macro rather than an inline function to avoid #include hell.
>   */
>   #ifdef CONFIG_TASKS_RCU
>   #define rcu_note_voluntary_context_switch_lite(t)
>
> Right?
>
> Correct me if I'm wrong about anything, thanks,
>
> - Joel

--
Thanks,
Byungchul