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    Subject[PATCH RFC 1/7] rcu: Move PREEMPT_RCU preemption to switch_to() invocation
    Date
    From: "Paul E. McKenney" <paul.mckenney@linaro.org>

    Currently, PREEMPT_RCU readers are enqueued upon entry to the scheduler.
    This is inefficient because enqueuing is required only if there is a
    context switch, and entry to the scheduler does not guarantee a context
    switch.

    The commit therefore moves the enqueuing to immediately precede the
    call to switch_to() from the scheduler.

    Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    ---
    arch/um/drivers/mconsole_kern.c | 1 +
    include/linux/rcupdate.h | 1 +
    include/linux/rcutiny.h | 6 ------
    include/linux/sched.h | 10 ++++++++++
    kernel/rcutree.c | 1 -
    kernel/rcutree.h | 1 -
    kernel/rcutree_plugin.h | 14 +++-----------
    kernel/sched/core.c | 1 +
    8 files changed, 16 insertions(+), 19 deletions(-)

    diff --git a/arch/um/drivers/mconsole_kern.c b/arch/um/drivers/mconsole_kern.c
    index c70e047..f69ff2d 100644
    --- a/arch/um/drivers/mconsole_kern.c
    +++ b/arch/um/drivers/mconsole_kern.c
    @@ -704,6 +704,7 @@ static void stack_proc(void *arg)
    struct task_struct *from = current, *to = arg;

    to->thread.saved_task = from;
    + rcu_switch_from(from);
    switch_to(from, to, from);
    }

    diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
    index 9372174..aca4ef0 100644
    --- a/include/linux/rcupdate.h
    +++ b/include/linux/rcupdate.h
    @@ -183,6 +183,7 @@ static inline int rcu_preempt_depth(void)
    /* Internal to kernel */
    extern void rcu_sched_qs(int cpu);
    extern void rcu_bh_qs(int cpu);
    +extern void rcu_preempt_note_context_switch(void);
    extern void rcu_check_callbacks(int cpu, int user);
    struct notifier_block;
    extern void rcu_idle_enter(void);
    diff --git a/include/linux/rcutiny.h b/include/linux/rcutiny.h
    index e93df77..080b5bd 100644
    --- a/include/linux/rcutiny.h
    +++ b/include/linux/rcutiny.h
    @@ -87,10 +87,6 @@ static inline void kfree_call_rcu(struct rcu_head *head,

    #ifdef CONFIG_TINY_RCU

    -static inline void rcu_preempt_note_context_switch(void)
    -{
    -}
    -
    static inline void exit_rcu(void)
    {
    }
    @@ -102,7 +98,6 @@ static inline int rcu_needs_cpu(int cpu)

    #else /* #ifdef CONFIG_TINY_RCU */

    -void rcu_preempt_note_context_switch(void);
    extern void exit_rcu(void);
    int rcu_preempt_needs_cpu(void);

    @@ -116,7 +111,6 @@ static inline int rcu_needs_cpu(int cpu)
    static inline void rcu_note_context_switch(int cpu)
    {
    rcu_sched_qs(cpu);
    - rcu_preempt_note_context_switch();
    }

    /*
    diff --git a/include/linux/sched.h b/include/linux/sched.h
    index e692aba..a749c9d 100644
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -1879,12 +1879,22 @@ static inline void rcu_copy_process(struct task_struct *p)
    INIT_LIST_HEAD(&p->rcu_node_entry);
    }

    +static inline void rcu_switch_from(struct task_struct *prev)
    +{
    + if (prev->rcu_read_lock_nesting != 0)
    + rcu_preempt_note_context_switch();
    +}
    +
    #else

    static inline void rcu_copy_process(struct task_struct *p)
    {
    }

    +static inline void rcu_switch_from(struct task_struct *prev)
    +{
    +}
    +
    #endif

    #ifdef CONFIG_SMP
    diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    index 1050d6d..6135150 100644
    --- a/kernel/rcutree.c
    +++ b/kernel/rcutree.c
    @@ -192,7 +192,6 @@ void rcu_note_context_switch(int cpu)
    {
    trace_rcu_utilization("Start context switch");
    rcu_sched_qs(cpu);
    - rcu_preempt_note_context_switch(cpu);
    trace_rcu_utilization("End context switch");
    }
    EXPORT_SYMBOL_GPL(rcu_note_context_switch);
    diff --git a/kernel/rcutree.h b/kernel/rcutree.h
    index cdd1be0..d6b70b08 100644
    --- a/kernel/rcutree.h
    +++ b/kernel/rcutree.h
    @@ -423,7 +423,6 @@ DECLARE_PER_CPU(char, rcu_cpu_has_work);
    /* Forward declarations for rcutree_plugin.h */
    static void rcu_bootup_announce(void);
    long rcu_batches_completed(void);
    -static void rcu_preempt_note_context_switch(int cpu);
    static int rcu_preempt_blocked_readers_cgp(struct rcu_node *rnp);
    #ifdef CONFIG_HOTPLUG_CPU
    static void rcu_report_unblock_qs_rnp(struct rcu_node *rnp,
    diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
    index c023464..b1ac22e 100644
    --- a/kernel/rcutree_plugin.h
    +++ b/kernel/rcutree_plugin.h
    @@ -153,7 +153,7 @@ static void rcu_preempt_qs(int cpu)
    *
    * Caller must disable preemption.
    */
    -static void rcu_preempt_note_context_switch(int cpu)
    +void rcu_preempt_note_context_switch(void)
    {
    struct task_struct *t = current;
    unsigned long flags;
    @@ -164,7 +164,7 @@ static void rcu_preempt_note_context_switch(int cpu)
    (t->rcu_read_unlock_special & RCU_READ_UNLOCK_BLOCKED) == 0) {

    /* Possibly blocking in an RCU read-side critical section. */
    - rdp = per_cpu_ptr(rcu_preempt_state.rda, cpu);
    + rdp = __this_cpu_ptr(rcu_preempt_state.rda);
    rnp = rdp->mynode;
    raw_spin_lock_irqsave(&rnp->lock, flags);
    t->rcu_read_unlock_special |= RCU_READ_UNLOCK_BLOCKED;
    @@ -228,7 +228,7 @@ static void rcu_preempt_note_context_switch(int cpu)
    * means that we continue to block the current grace period.
    */
    local_irq_save(flags);
    - rcu_preempt_qs(cpu);
    + rcu_preempt_qs(smp_processor_id());
    local_irq_restore(flags);
    }

    @@ -1018,14 +1018,6 @@ void rcu_force_quiescent_state(void)
    EXPORT_SYMBOL_GPL(rcu_force_quiescent_state);

    /*
    - * Because preemptible RCU does not exist, we never have to check for
    - * CPUs being in quiescent states.
    - */
    -static void rcu_preempt_note_context_switch(int cpu)
    -{
    -}
    -
    -/*
    * Because preemptible RCU does not exist, there are never any preempted
    * RCU readers.
    */
    diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    index 5255c9d..10689da 100644
    --- a/kernel/sched/core.c
    +++ b/kernel/sched/core.c
    @@ -2051,6 +2051,7 @@ context_switch(struct rq *rq, struct task_struct *prev,
    #endif

    /* Here we just switch the register state and the stack. */
    + rcu_switch_from(prev);
    switch_to(prev, next, prev);

    barrier();
    --
    1.7.8


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