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 <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,
>
--
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