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    SubjectRe: [PATCH v2 1/2] sched: Add cond_resched_rcu_lock() helper
    On Wed, May 01, 2013 at 05:22:05PM +0300, Julian Anastasov wrote:

    > > static void inline cond_resched_rcu_lock(void)
    > > {
    > > #ifdef CONFIG_PREEMPT_RCU
    >
    > You mean '#ifndef' here, right? But in the non-preempt
    > case is using the need_resched() needed? rcu_read_unlock
    > and rcu_read_lock do not generate code.

    Uhm... yes!

    > > if (need_resched()) {
    > > rcu_read_unlock();
    > > cond_resched();
    > > rcu_read_lock();
    > > }
    > > #endif /* CONFIG_PREEMPT_RCU */
    > > }
    > >
    > > That would have an rcu_read_lock() break and voluntary preemption point for
    > > non-preemptible RCU and not bother with the stuff for preemptible RCU.
    >
    > I see. So, can we choose one of both variants:
    >
    > 1. Your variant but with ifndef:
    >
    > static void inline cond_resched_rcu_lock(void)
    > {
    > #ifndef CONFIG_PREEMPT_RCU
    > if (need_resched()) {
    > rcu_read_unlock();
    > cond_resched();
    > rcu_read_lock();
    > }
    > #endif
    > }
    >
    > 2. Same without need_resched because cond_resched already
    > performs the same checks:
    >
    > static void inline cond_resched_rcu_lock(void)
    > {
    > #ifndef CONFIG_PREEMPT_RCU
    > rcu_read_unlock();
    > cond_resched();
    > rcu_read_lock();
    > #endif
    > }

    Ah so the 'problem' with this last version is that it does an unconditional /
    unnessecary rcu_read_unlock().

    The below would be in line with all the other cond_resched*() implementations.

    ---
    include/linux/sched.h | 7 +++++++
    kernel/sched/core.c | 14 ++++++++++++++
    2 files changed, 21 insertions(+)

    diff --git a/include/linux/sched.h b/include/linux/sched.h
    index 802a751..fd2c77f 100644
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -2449,6 +2449,13 @@ extern int __cond_resched_softirq(void);
    __cond_resched_softirq(); \
    })

    +extern int __cond_resched_rcu(void);
    +
    +#define cond_resched_rcu() ({ \
    + __might_sleep(__FILE__, __LINE__, 0); \
    + __cond_resched_rcu(); \
    +})
    +
    /*
    * Does a critical section need to be broken due to another
    * task waiting?: (technically does not depend on CONFIG_PREEMPT,
    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index 7d7901a..2b3b4e6 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -4358,6 +4358,20 @@ int __sched __cond_resched_softirq(void)
    }
    EXPORT_SYMBOL(__cond_resched_softirq);

    +int __sched __cond_resched_rcu(void)
    +{
    +#ifndef CONFIG_PREEMPT_RCU
    + if (should_resched()) {
    + rcu_read_unlock();
    + __cond_resched();
    + rcu_read_lock();
    + return 1;
    + }
    +#endif
    + return 0;
    +}
    +EXPORT_SYMBOL(__cond_resched_rcu);
    +
    /**
    * yield - yield the current processor to other threads.
    *


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