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

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Message-Id: <20180511161746.GX26088@linux.vnet.ibm.com>
Date:   Fri, 11 May 2018 09:17:46 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Byungchul Park <byungchul.park@....com>
Cc:     jiangshanlai@...il.com, josh@...htriplett.org, rostedt@...dmis.org,
        mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org,
        kernel-team@....com, peterz@...radead.org
Subject: Re: [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@....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
>