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[PATCH tip/core/rcu 11/20] sched,rcu: Make cond_resched() provide RCU quiescent state

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  [PATCH tip/core/rcu 11/20] sched,rcu: Make cond_resched() provide RCU quiescent state "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-14 10:20 +0100
    Re: [PATCH tip/core/rcu 11/20] sched,rcu: Make cond_resched()  provide RCU quiescent state Josh Triplett <josh@joshtriplett.org> - 2017-01-16 09:10 +0100
      Re: [PATCH tip/core/rcu 11/20] sched,rcu: Make cond_resched()  provide RCU quiescent state "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> - 2017-01-16 12:40 +0100
    Re: [PATCH tip/core/rcu 11/20] sched,rcu: Make cond_resched()  provide RCU quiescent state Peter Zijlstra <peterz@infradead.org> - 2017-01-16 18:20 +0100

#1558892 — [PATCH tip/core/rcu 11/20] sched,rcu: Make cond_resched() provide RCU quiescent state

There is some confusion as to which of cond_resched() or
cond_resched_rcu_qs() should be added to long in-kernel loops.
This commit therefore eliminates the decision by adding RCU
quiescent states to cond_resched().

Warning: This is a prototype.  For example, it does not correctly
handle Tasks RCU.  Which is OK for the moment, given that no one
actually uses Tasks RCU yet.

Reported-by: Michal Hocko <mhocko@kernel.org>
Not-yet-signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
---
 include/linux/sched.h | 3 ++-
 kernel/sched/core.c   | 1 +
 2 files changed, 3 insertions(+), 1 deletion(-)

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4d1905245c7a..1531c48f56e2 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -3352,10 +3352,11 @@ static inline int signal_pending_state(long state, struct task_struct *p)
  * cond_resched_lock() will drop the spinlock before scheduling,
  * cond_resched_softirq() will enable bhs before scheduling.
  */
+void rcu_all_qs(void);
 #ifndef CONFIG_PREEMPT
 extern int _cond_resched(void);
 #else
-static inline int _cond_resched(void) { return 0; }
+static inline int _cond_resched(void) { rcu_all_qs(); return 0; }
 #endif
 
 #define cond_resched() ({			\
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c56fb57f2991..b442f8918eb9 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4907,6 +4907,7 @@ int __sched _cond_resched(void)
 		preempt_schedule_common();
 		return 1;
 	}
+	rcu_all_qs();
 	return 0;
 }
 EXPORT_SYMBOL(_cond_resched);
-- 
2.5.2

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#1559507 — Re: [PATCH tip/core/rcu 11/20] sched,rcu: Make cond_resched() provide RCU quiescent state

On Sat, Jan 14, 2017 at 01:13:12AM -0800, Paul E. McKenney wrote:
> There is some confusion as to which of cond_resched() or
> cond_resched_rcu_qs() should be added to long in-kernel loops.
> This commit therefore eliminates the decision by adding RCU
> quiescent states to cond_resched().
> 
> Warning: This is a prototype.  For example, it does not correctly
> handle Tasks RCU.  Which is OK for the moment, given that no one
> actually uses Tasks RCU yet.
> 
> Reported-by: Michal Hocko <mhocko@kernel.org>
> Not-yet-signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> Cc: Peter Zijlstra <peterz@infradead.org>

Do you intend to merge this patch, or have you just posted it for
review?

You may want to remove it from this 20-patch series and post it as a
separate RFC PATCH, to ensure that it doesn't get merged while still a
prototype.

>  include/linux/sched.h | 3 ++-
>  kernel/sched/core.c   | 1 +
>  2 files changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 4d1905245c7a..1531c48f56e2 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -3352,10 +3352,11 @@ static inline int signal_pending_state(long state, struct task_struct *p)
>   * cond_resched_lock() will drop the spinlock before scheduling,
>   * cond_resched_softirq() will enable bhs before scheduling.
>   */
> +void rcu_all_qs(void);
>  #ifndef CONFIG_PREEMPT
>  extern int _cond_resched(void);
>  #else
> -static inline int _cond_resched(void) { return 0; }
> +static inline int _cond_resched(void) { rcu_all_qs(); return 0; }
>  #endif
>  
>  #define cond_resched() ({			\
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c56fb57f2991..b442f8918eb9 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4907,6 +4907,7 @@ int __sched _cond_resched(void)
>  		preempt_schedule_common();
>  		return 1;
>  	}
> +	rcu_all_qs();
>  	return 0;
>  }
>  EXPORT_SYMBOL(_cond_resched);
> -- 
> 2.5.2
> 

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#1559659 — Re: [PATCH tip/core/rcu 11/20] sched,rcu: Make cond_resched() provide RCU quiescent state

On Mon, Jan 16, 2017 at 12:08:43AM -0800, Josh Triplett wrote:
> On Sat, Jan 14, 2017 at 01:13:12AM -0800, Paul E. McKenney wrote:
> > There is some confusion as to which of cond_resched() or
> > cond_resched_rcu_qs() should be added to long in-kernel loops.
> > This commit therefore eliminates the decision by adding RCU
> > quiescent states to cond_resched().
> > 
> > Warning: This is a prototype.  For example, it does not correctly
> > handle Tasks RCU.  Which is OK for the moment, given that no one
> > actually uses Tasks RCU yet.
> > 
> > Reported-by: Michal Hocko <mhocko@kernel.org>
> > Not-yet-signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> 
> Do you intend to merge this patch, or have you just posted it for
> review?
> 
> You may want to remove it from this 20-patch series and post it as a
> separate RFC PATCH, to ensure that it doesn't get merged while still a
> prototype.

It does look like I should delay this one until I get my story straight.

Good catch!

							Thanx, Paul

> >  include/linux/sched.h | 3 ++-
> >  kernel/sched/core.c   | 1 +
> >  2 files changed, 3 insertions(+), 1 deletion(-)
> > 
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index 4d1905245c7a..1531c48f56e2 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -3352,10 +3352,11 @@ static inline int signal_pending_state(long state, struct task_struct *p)
> >   * cond_resched_lock() will drop the spinlock before scheduling,
> >   * cond_resched_softirq() will enable bhs before scheduling.
> >   */
> > +void rcu_all_qs(void);
> >  #ifndef CONFIG_PREEMPT
> >  extern int _cond_resched(void);
> >  #else
> > -static inline int _cond_resched(void) { return 0; }
> > +static inline int _cond_resched(void) { rcu_all_qs(); return 0; }
> >  #endif
> >  
> >  #define cond_resched() ({			\
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index c56fb57f2991..b442f8918eb9 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4907,6 +4907,7 @@ int __sched _cond_resched(void)
> >  		preempt_schedule_common();
> >  		return 1;
> >  	}
> > +	rcu_all_qs();
> >  	return 0;
> >  }
> >  EXPORT_SYMBOL(_cond_resched);
> > -- 
> > 2.5.2
> > 
> 

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#1559929 — Re: [PATCH tip/core/rcu 11/20] sched,rcu: Make cond_resched() provide RCU quiescent state

On Sat, Jan 14, 2017 at 01:13:12AM -0800, Paul E. McKenney wrote:
> There is some confusion as to which of cond_resched() or
> cond_resched_rcu_qs() should be added to long in-kernel loops.
> This commit therefore eliminates the decision by adding RCU
> quiescent states to cond_resched().

Which would make: rcu_read_lock(); cond_resched(); rcu_read_unlock();
invalid under preemptible RCU. Is it already?

> Warning: This is a prototype.  For example, it does not correctly
> handle Tasks RCU.  Which is OK for the moment, given that no one
> actually uses Tasks RCU yet.

> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -4907,6 +4907,7 @@ int __sched _cond_resched(void)
>  		preempt_schedule_common();
>  		return 1;
>  	}
> +	rcu_all_qs();
>  	return 0;
>  }

Still not a real fan of this, it does make cond_resched() touch a bunch
more cachelines, also, I suppose that if we're going to do this, we
should make __cond_resched_lock() and __cond_resched_softirq() act
similarly.

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