Re: [tip:sched/core] sched/core: Avoid _cond_resched() for PREEMPT=y

[Paul E. McKenney]

On Thu, Sep 22, 2016 at 02:25:01PM -0400, Mikulas Patocka wrote:
> I've found a document that says that cond_resched() is needed on
> preemptible kernels to mark RCU quiescent states:
> 
> https://lwn.net/Articles/603252/
> 
> Paul, is cond_resched() still needed on current RCU implementation? Or not?

It can be if you loop in the kernel for a very long time.

But cond_resched_rcu_qs() is instead needed in cases where there is only
one runnable non-idle non-nohz task on a given CPU.

							Thanx, Paul

> Mikulas
> 
> 
> 
> On Thu, 22 Sep 2016, tip-bot for Peter Zijlstra wrote:
> 
> > Commit-ID:  35a773a07926a22bf19d77ee00024522279c4e68
> > Gitweb:     http://git.kernel.org/tip/35a773a07926a22bf19d77ee00024522279c4e68
> > Author:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > AuthorDate: Mon, 19 Sep 2016 12:57:53 +0200
> > Committer:  Ingo Molnar <mingo@xxxxxxxxxx>
> > CommitDate: Thu, 22 Sep 2016 14:53:46 +0200
> > 
> > sched/core: Avoid _cond_resched() for PREEMPT=y
> > 
> > On fully preemptible kernels _cond_resched() is pointless, so avoid
> > emitting any code for it.
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
> > Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> > Cc: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> > Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> > Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> > Cc: linux-kernel@xxxxxxxxxxxxxxx
> > Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> > ---
> >  include/linux/sched.h | 4 ++++
> >  kernel/sched/core.c   | 2 ++
> >  2 files changed, 6 insertions(+)
> > 
> > diff --git a/include/linux/sched.h b/include/linux/sched.h
> > index f00ee8e..b99fcd1 100644
> > --- a/include/linux/sched.h
> > +++ b/include/linux/sched.h
> > @@ -3209,7 +3209,11 @@ 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.
> >   */
> > +#ifndef CONFIG_PREEMPT
> >  extern int _cond_resched(void);
> > +#else
> > +static inline int _cond_resched(void) { return 0; }
> > +#endif
> >  
> >  #define cond_resched() ({			\
> >  	___might_sleep(__FILE__, __LINE__, 0);	\
> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index b2ec53c..d7babcc 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -4883,6 +4883,7 @@ SYSCALL_DEFINE0(sched_yield)
> >  	return 0;
> >  }
> >  
> > +#ifndef CONFIG_PREEMPT
> >  int __sched _cond_resched(void)
> >  {
> >  	if (should_resched(0)) {
> > @@ -4892,6 +4893,7 @@ int __sched _cond_resched(void)
> >  	return 0;
> >  }
> >  EXPORT_SYMBOL(_cond_resched);
> > +#endif
> >  
> >  /*
> >   * __cond_resched_lock() - if a reschedule is pending, drop the given lock,
> > 
>