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SubjectRe: [PATCH] rcu: Report a quiescent state when it's exactly in the state
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 오후 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@lge.com>
> >---
> > 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?

> > } while (0)
> > /*
> >diff --git a/kernel/rcu/tiny.c b/kernel/rcu/tiny.c
> >index a64eee0..68d2332 100644
> >--- a/kernel/rcu/tiny.c
> >+++ b/kernel/rcu/tiny.c
> >@@ -120,12 +120,12 @@ void rcu_bh_qs(void)
> > */
> > void rcu_check_callbacks(int user)
> > {
> >- if (user)
> >+ if (user) {
> > rcu_sched_qs();
> >- else if (!in_softirq())
> > rcu_bh_qs();
> >- if (user)
> > rcu_note_voluntary_context_switch(current);
> >+ } else if (!in_softirq())
> >+ rcu_bh_qs();
> > }
> > /*
> >diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> >index 36075dd..1abe29a 100644
> >--- a/kernel/rcu/tree.c
> >+++ b/kernel/rcu/tree.c
> >@@ -2595,6 +2595,7 @@ void rcu_check_callbacks(int user)
> > rcu_sched_qs();
> > rcu_bh_qs();
> >+ rcu_note_voluntary_context_switch(current);
> > } else if (!in_softirq()) {
> >@@ -2610,8 +2611,7 @@ void rcu_check_callbacks(int user)
> > rcu_preempt_check_callbacks();
> > if (rcu_pending())
> > invoke_rcu_core();
> >- if (user)
> >- rcu_note_voluntary_context_switch(current);
> >+

I recall that I had some reason for wanting this down here, but
do not recall the reason itself. I will try testing this patch
to see if rcutorture reminds me.

Thanx, Paul

> > trace_rcu_utilization(TPS("End scheduler-tick"));
> > }
> >
>
> --
> Thanks,
> Byungchul
>

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