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    Subject[PATCH RFC tip/core/rcu 08/13] rcu: Eliminate RCU_FAST_NO_HZ grace-period hang
    Date
    From: Paul E. McKenney <paul.mckenney@linaro.org>

    With the new implementation of RCU_FAST_NO_HZ, it was possible to hang
    RCU grace periods as follows:

    o CPU 0 attempts to go idle, cycles several times through the
    rcu_prepare_for_idle() loop, then goes dyntick-idle when
    RCU needs nothing more from it, while still having at least
    on RCU callback pending.

    o CPU 1 goes idle with no callbacks.

    Both CPUs can then stay in dyntick-idle mode indefinitely, preventing
    the RCU grace period from ever completing, possibly hanging the system.

    This commit therefore prevents CPUs that have RCU callbacks from entering
    dyntick-idle mode. This approach also eliminates the need for the
    end-of-grace-period IPIs used previously.

    Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
    ---
    include/trace/events/rcu.h | 1 -
    kernel/rcutree.c | 2 -
    kernel/rcutree.h | 3 --
    kernel/rcutree_plugin.h | 78 +------------------------------------------
    4 files changed, 2 insertions(+), 82 deletions(-)

    diff --git a/include/trace/events/rcu.h b/include/trace/events/rcu.h
    index debe453..8dd6fcb 100644
    --- a/include/trace/events/rcu.h
    +++ b/include/trace/events/rcu.h
    @@ -287,7 +287,6 @@ TRACE_EVENT(rcu_dyntick,
    *
    * "No callbacks": Nothing to do, no callbacks on this CPU.
    * "In holdoff": Nothing to do, holding off after unsuccessful attempt.
    - * "Dyntick with callbacks": Callbacks remain, but RCU doesn't need CPU.
    * "Begin holdoff": Attempt failed, don't retry until next jiffy.
    * "More callbacks": Still more callbacks, try again to clear them out.
    * "Callbacks drained": All callbacks processed, off to dyntick idle!
    diff --git a/kernel/rcutree.c b/kernel/rcutree.c
    index 7fb8b0e..13fab4a 100644
    --- a/kernel/rcutree.c
    +++ b/kernel/rcutree.c
    @@ -1086,7 +1086,6 @@ static void rcu_report_qs_rsp(struct rcu_state *rsp, unsigned long flags)
    * callbacks are waiting on the grace period that just now
    * completed.
    */
    - rcu_schedule_wake_gp_end();
    if (*rdp->nxttail[RCU_WAIT_TAIL] == NULL) {
    raw_spin_unlock(&rnp->lock); /* irqs remain disabled. */

    @@ -1672,7 +1671,6 @@ static void rcu_process_callbacks(struct softirq_action *unused)
    &__get_cpu_var(rcu_sched_data));
    __rcu_process_callbacks(&rcu_bh_state, &__get_cpu_var(rcu_bh_data));
    rcu_preempt_process_callbacks();
    - rcu_wake_cpus_for_gp_end();
    trace_rcu_utilization("End RCU core");
    }

    diff --git a/kernel/rcutree.h b/kernel/rcutree.h
    index ea32405..70d8a55 100644
    --- a/kernel/rcutree.h
    +++ b/kernel/rcutree.h
    @@ -88,7 +88,6 @@ struct rcu_dynticks {
    /* Process level is worth LLONG_MAX/2. */
    int dynticks_nmi_nesting; /* Track NMI nesting level. */
    atomic_t dynticks; /* Even value for idle, else odd. */
    - int wake_gp_end; /* A GP ended, need to wake up CPUs. */
    };

    /* RCU's kthread states for tracing. */
    @@ -469,7 +468,5 @@ static void rcu_yield(void (*f)(unsigned long), unsigned long arg);
    static void rcu_cpu_kthread_setrt(int cpu, int to_rt);
    static void __cpuinit rcu_prepare_kthreads(int cpu);
    static void rcu_prepare_for_idle(int cpu);
    -static void rcu_wake_cpus_for_gp_end(void);
    -static void rcu_schedule_wake_gp_end(void);

    #endif /* #ifndef RCU_TREE_NONCORE */
    diff --git a/kernel/rcutree_plugin.h b/kernel/rcutree_plugin.h
    index c4daf1e..3d84dbc 100644
    --- a/kernel/rcutree_plugin.h
    +++ b/kernel/rcutree_plugin.h
    @@ -1964,28 +1964,11 @@ static void rcu_prepare_for_idle(int cpu)
    {
    }

    -/*
    - * CPUs are never putting themselves to sleep with callbacks pending,
    - * so there is no need to awaken them.
    - */
    -static void rcu_wake_cpus_for_gp_end(void)
    -{
    -}
    -
    -/*
    - * CPUs are never putting themselves to sleep with callbacks pending,
    - * so there is no need to schedule the act of awakening them.
    - */
    -static void rcu_schedule_wake_gp_end(void)
    -{
    -}
    -
    #else /* #if !defined(CONFIG_RCU_FAST_NO_HZ) */

    #define RCU_NEEDS_CPU_FLUSHES 5
    static DEFINE_PER_CPU(int, rcu_dyntick_drain);
    static DEFINE_PER_CPU(unsigned long, rcu_dyntick_holdoff);
    -static DEFINE_PER_CPU(bool, rcu_awake_at_gp_end);

    /*
    * Allow the CPU to enter dyntick-idle mode if either: (1) There are no
    @@ -2032,26 +2015,16 @@ static void rcu_prepare_for_idle(int cpu)
    local_irq_save(flags);

    /*
    - * If there are no callbacks on this CPU or if RCU has no further
    - * need for this CPU at the moment, enter dyntick-idle mode.
    - * Also reset state so as to not prejudice later attempts.
    + * If there are no callbacks on this CPU, enter dyntick-idle mode.
    + * Also reset state to avoid prejudicing later attempts.
    */
    if (!rcu_cpu_has_callbacks(cpu)) {
    per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
    per_cpu(rcu_dyntick_drain, cpu) = 0;
    - per_cpu(rcu_awake_at_gp_end, cpu) = 0;
    local_irq_restore(flags);
    trace_rcu_prep_idle("No callbacks");
    return;
    }
    - if (!rcu_pending(cpu)) {
    - per_cpu(rcu_dyntick_holdoff, cpu) = jiffies - 1;
    - per_cpu(rcu_dyntick_drain, cpu) = 0;
    - per_cpu(rcu_awake_at_gp_end, cpu) = 1;
    - local_irq_restore(flags);
    - trace_rcu_prep_idle("Dyntick with callbacks");
    - return; /* Nothing to do immediately. */
    - }

    /*
    * If in holdoff mode, just return. We will presumably have
    @@ -2067,7 +2040,6 @@ static void rcu_prepare_for_idle(int cpu)
    if (per_cpu(rcu_dyntick_drain, cpu) <= 0) {
    /* First time through, initialize the counter. */
    per_cpu(rcu_dyntick_drain, cpu) = RCU_NEEDS_CPU_FLUSHES;
    - per_cpu(rcu_awake_at_gp_end, cpu) = 0;
    } else if (--per_cpu(rcu_dyntick_drain, cpu) <= 0) {
    /* We have hit the limit, so time to give up. */
    per_cpu(rcu_dyntick_holdoff, cpu) = jiffies;
    @@ -2113,50 +2085,4 @@ static void rcu_prepare_for_idle(int cpu)
    }
    }

    -/*
    - * Wake up a CPU by invoking the RCU core. Intended for use by
    - * rcu_wake_cpus_for_gp_end(), which passes this function to
    - * smp_call_function_single().
    - */
    -static void rcu_wake_cpu(void *unused)
    -{
    - trace_rcu_prep_idle("CPU awakened at GP end");
    - invoke_rcu_core();
    -}
    -
    -/*
    - * If an RCU grace period ended recently, scan the rcu_awake_at_gp_end
    - * per-CPU variables, and wake up any CPUs that requested a wakeup.
    - */
    -static void rcu_wake_cpus_for_gp_end(void)
    -{
    - int cpu;
    - struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
    -
    - if (!rdtp->wake_gp_end)
    - return;
    - rdtp->wake_gp_end = 0;
    - for_each_online_cpu(cpu) {
    - if (per_cpu(rcu_awake_at_gp_end, cpu)) {
    - per_cpu(rcu_awake_at_gp_end, cpu) = 0;
    - smp_call_function_single(cpu, rcu_wake_cpu, NULL, 0);
    - }
    - }
    -}
    -
    -/*
    - * A grace period has just ended, and so we will need to awaken CPUs
    - * that now have work to do. But we cannot send IPIs with interrupts
    - * disabled, so just set a flag so that this will happen upon exit
    - * from RCU core processing.
    - */
    -static void rcu_schedule_wake_gp_end(void)
    -{
    - struct rcu_dynticks *rdtp = &__get_cpu_var(rcu_dynticks);
    -
    - rdtp->wake_gp_end = 1;
    -}
    -
    -/* @@@ need tracing as well. */
    -
    #endif /* #else #if !defined(CONFIG_RCU_FAST_NO_HZ) */
    --
    1.7.3.2


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