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?

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 <pet...@infradead.org>
> AuthorDate: Mon, 19 Sep 2016 12:57:53 +0200
> Committer:  Ingo Molnar <mi...@kernel.org>
> 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) <pet...@infradead.org>
> Cc: Linus Torvalds <torva...@linux-foundation.org>
> Cc: Mikulas Patocka <mpato...@redhat.com>
> Cc: Oleg Nesterov <o...@redhat.com>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Cc: linux-kernel@vger.kernel.org
> Signed-off-by: Ingo Molnar <mi...@kernel.org>
> ---
>  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,
> 

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