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

[Byungchul Park]

On 2018-05-15 06:04, Paul E. McKenney wrote:
> On Mon, May 14, 2018 at 11:59:41AM +0900, Byungchul Park wrote:
> > 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. :)
>
> Given what it currently does, the name should be rcu_tasks_qs() to go
> along with rcu_bh_qs(), rcu_preempt_qs(), and rcu_sched_qs().  Much as
> I would like cond_resched() to be an RCU-tasks quiescent state, it is
> nothingness for PREEMPT=y kernels, and Peter has indicated a strong
> interest in having it remain so.  But I did update a few comments.
>
> I left rcu_note_voluntary_context_switch() alone because it should be
> disappearing entirely Real Soon Now.
>
> Please see patch below.
>
> 							Thanx, Paul
>
> PS.  Oddly enough, the recent patch removing the "if" from
>       cond_resched_tasks_rcu_qs() is (technically speaking) pointless.
>       If the kernel contains RCU-tasks, it must be preemptible, which
>       means that cond_resched() unconditionally returns false, which
>       in turn means that rcu_note_voluntary_context_switch_lite() was
>       unconditionally invoked.
>
>       Simiarly, in non-preemptible kernels, where cond_resched()
>       might well return true, rcu_note_voluntary_context_switch_lite()
>       is a no-op.

Interesting. Right. Thanks for your explanation. :)

--
Thanks,
Byungchul