rcu, debug: detect stalled grace periods, cleanups

[deliverable/linux.git] / kernel / rcuclassic.c

/*
 * Read-Copy Update mechanism for mutual exclusion
 *
 * This program is free software; you can redistribute it and/or modify
 * it under the terms of the GNU General Public License as published by
 * the Free Software Foundation; either version 2 of the License, or
 * (at your option) any later version.
 *
 * This program is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
 * GNU General Public License for more details.
 *
 * You should have received a copy of the GNU General Public License
 * along with this program; if not, write to the Free Software
 * Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA.
 *
 * Copyright IBM Corporation, 2001
 *
 * Authors: Dipankar Sarma <dipankar@in.ibm.com>
 *          Manfred Spraul <manfred@colorfullife.com>
 *
 * Based on the original work by Paul McKenney <paulmck@us.ibm.com>
 * and inputs from Rusty Russell, Andrea Arcangeli and Andi Kleen.
 * Papers:
 * http://www.rdrop.com/users/paulmck/paper/rclockpdcsproof.pdf
 * http://lse.sourceforge.net/locking/rclock_OLS.2001.05.01c.sc.pdf (OLS2001)
 *
 * For detailed explanation of Read-Copy Update mechanism see -
 *              Documentation/RCU
 *
 */
#include <linux/types.h>
#include <linux/kernel.h>
#include <linux/init.h>
#include <linux/spinlock.h>
#include <linux/smp.h>
#include <linux/rcupdate.h>
#include <linux/interrupt.h>
#include <linux/sched.h>
#include <asm/atomic.h>
#include <linux/bitops.h>
#include <linux/module.h>
#include <linux/completion.h>
#include <linux/moduleparam.h>
#include <linux/percpu.h>
#include <linux/notifier.h>
#include <linux/cpu.h>
#include <linux/mutex.h>
#include <linux/time.h>

#ifdef CONFIG_DEBUG_LOCK_ALLOC
static struct lock_class_key rcu_lock_key;
struct lockdep_map rcu_lock_map =
        STATIC_LOCKDEP_MAP_INIT("rcu_read_lock", &rcu_lock_key);
EXPORT_SYMBOL_GPL(rcu_lock_map);
#endif


/* Definition for rcupdate control block. */
static struct rcu_ctrlblk rcu_ctrlblk = {
        .cur = -300,
        .completed = -300,
        .pending = -300,
        .lock = __SPIN_LOCK_UNLOCKED(&rcu_ctrlblk.lock),
        .cpumask = CPU_MASK_NONE,
};
static struct rcu_ctrlblk rcu_bh_ctrlblk = {
        .cur = -300,
        .completed = -300,
        .pending = -300,
        .lock = __SPIN_LOCK_UNLOCKED(&rcu_bh_ctrlblk.lock),
        .cpumask = CPU_MASK_NONE,
};

DEFINE_PER_CPU(struct rcu_data, rcu_data) = { 0L };
DEFINE_PER_CPU(struct rcu_data, rcu_bh_data) = { 0L };

static int blimit = 10;
static int qhimark = 10000;
static int qlowmark = 100;

#ifdef CONFIG_SMP
static void force_quiescent_state(struct rcu_data *rdp,
                        struct rcu_ctrlblk *rcp)
{
        int cpu;
        cpumask_t cpumask;
        set_need_resched();
        if (unlikely(!rcp->signaled)) {
                rcp->signaled = 1;
                /*
                 * Don't send IPI to itself. With irqs disabled,
                 * rdp->cpu is the current cpu.
                 *
                 * cpu_online_map is updated by the _cpu_down()
                 * using __stop_machine(). Since we're in irqs disabled
                 * section, __stop_machine() is not exectuting, hence
                 * the cpu_online_map is stable.
                 *
                 * However,  a cpu might have been offlined _just_ before
                 * we disabled irqs while entering here.
                 * And rcu subsystem might not yet have handled the CPU_DEAD
                 * notification, leading to the offlined cpu's bit
                 * being set in the rcp->cpumask.
                 *
                 * Hence cpumask = (rcp->cpumask & cpu_online_map) to prevent
                 * sending smp_reschedule() to an offlined CPU.
                 */
                cpus_and(cpumask, rcp->cpumask, cpu_online_map);
                cpu_clear(rdp->cpu, cpumask);
                for_each_cpu_mask_nr(cpu, cpumask)
                        smp_send_reschedule(cpu);
        }
}
#else
static inline void force_quiescent_state(struct rcu_data *rdp,
                        struct rcu_ctrlblk *rcp)
{
        set_need_resched();
}
#endif

static void __call_rcu(struct rcu_head *head, struct rcu_ctrlblk *rcp,
                struct rcu_data *rdp)
{
        long batch;
        smp_mb(); /* reads the most recently updated value of rcu->cur. */

        /*
         * Determine the batch number of this callback.
         *
         * Using ACCESS_ONCE to avoid the following error when gcc eliminates
         * local variable "batch" and emits codes like this:
         *      1) rdp->batch = rcp->cur + 1 # gets old value
         *      ......
         *      2)rcu_batch_after(rcp->cur + 1, rdp->batch) # gets new value
         * then [*nxttail[0], *nxttail[1]) may contain callbacks
         * that batch# = rdp->batch, see the comment of struct rcu_data.
         */
        batch = ACCESS_ONCE(rcp->cur) + 1;

        if (rdp->nxtlist && rcu_batch_after(batch, rdp->batch)) {
                /* process callbacks */
                rdp->nxttail[0] = rdp->nxttail[1];
                rdp->nxttail[1] = rdp->nxttail[2];
                if (rcu_batch_after(batch - 1, rdp->batch))
                        rdp->nxttail[0] = rdp->nxttail[2];
        }

        rdp->batch = batch;
        *rdp->nxttail[2] = head;
        rdp->nxttail[2] = &head->next;

        if (unlikely(++rdp->qlen > qhimark)) {
                rdp->blimit = INT_MAX;
                force_quiescent_state(rdp, &rcu_ctrlblk);
        }
}

/**
 * call_rcu - Queue an RCU callback for invocation after a grace period.
 * @head: structure to be used for queueing the RCU updates.
 * @func: actual update function to be invoked after the grace period
 *
 * The update function will be invoked some time after a full grace
 * period elapses, in other words after all currently executing RCU
 * read-side critical sections have completed.  RCU read-side critical
 * sections are delimited by rcu_read_lock() and rcu_read_unlock(),
 * and may be nested.
 */
void call_rcu(struct rcu_head *head,
                                void (*func)(struct rcu_head *rcu))
{
        unsigned long flags;

        head->func = func;
        head->next = NULL;
        local_irq_save(flags);
        __call_rcu(head, &rcu_ctrlblk, &__get_cpu_var(rcu_data));
        local_irq_restore(flags);
}
EXPORT_SYMBOL_GPL(call_rcu);

/**
 * call_rcu_bh - Queue an RCU for invocation after a quicker grace period.
 * @head: structure to be used for queueing the RCU updates.
 * @func: actual update function to be invoked after the grace period
 *
 * The update function will be invoked some time after a full grace
 * period elapses, in other words after all currently executing RCU
 * read-side critical sections have completed. call_rcu_bh() assumes
 * that the read-side critical sections end on completion of a softirq
 * handler. This means that read-side critical sections in process
 * context must not be interrupted by softirqs. This interface is to be
 * used when most of the read-side critical sections are in softirq context.
 * RCU read-side critical sections are delimited by rcu_read_lock() and
 * rcu_read_unlock(), * if in interrupt context or rcu_read_lock_bh()
 * and rcu_read_unlock_bh(), if in process context. These may be nested.
 */
void call_rcu_bh(struct rcu_head *head,
                                void (*func)(struct rcu_head *rcu))
{
        unsigned long flags;

        head->func = func;
        head->next = NULL;
        local_irq_save(flags);
        __call_rcu(head, &rcu_bh_ctrlblk, &__get_cpu_var(rcu_bh_data));
        local_irq_restore(flags);
}
EXPORT_SYMBOL_GPL(call_rcu_bh);

/*
 * Return the number of RCU batches processed thus far.  Useful
 * for debug and statistics.
 */
long rcu_batches_completed(void)
{
        return rcu_ctrlblk.completed;
}
EXPORT_SYMBOL_GPL(rcu_batches_completed);

/*
 * Return the number of RCU batches processed thus far.  Useful
 * for debug and statistics.
 */
long rcu_batches_completed_bh(void)
{
        return rcu_bh_ctrlblk.completed;
}
EXPORT_SYMBOL_GPL(rcu_batches_completed_bh);

/* Raises the softirq for processing rcu_callbacks. */
static inline void raise_rcu_softirq(void)
{
        raise_softirq(RCU_SOFTIRQ);
}

/*
 * Invoke the completed RCU callbacks. They are expected to be in
 * a per-cpu list.
 */
static void rcu_do_batch(struct rcu_data *rdp)
{
        struct rcu_head *next, *list;
        int count = 0;

        list = rdp->donelist;
        while (list) {
                next = list->next;
                prefetch(next);
                list->func(list);
                list = next;
                if (++count >= rdp->blimit)
                        break;
        }
        rdp->donelist = list;

        local_irq_disable();
        rdp->qlen -= count;
        local_irq_enable();
        if (rdp->blimit == INT_MAX && rdp->qlen <= qlowmark)
                rdp->blimit = blimit;

        if (!rdp->donelist)
                rdp->donetail = &rdp->donelist;
        else
                raise_rcu_softirq();
}

/*
 * Grace period handling:
 * The grace period handling consists out of two steps:
 * - A new grace period is started.
 *   This is done by rcu_start_batch. The start is not broadcasted to
 *   all cpus, they must pick this up by comparing rcp->cur with
 *   rdp->quiescbatch. All cpus are recorded  in the
 *   rcu_ctrlblk.cpumask bitmap.
 * - All cpus must go through a quiescent state.
 *   Since the start of the grace period is not broadcasted, at least two
 *   calls to rcu_check_quiescent_state are required:
 *   The first call just notices that a new grace period is running. The
 *   following calls check if there was a quiescent state since the beginning
 *   of the grace period. If so, it updates rcu_ctrlblk.cpumask. If
 *   the bitmap is empty, then the grace period is completed.
 *   rcu_check_quiescent_state calls rcu_start_batch(0) to start the next grace
 *   period (if necessary).
 */

#ifdef CONFIG_DEBUG_RCU_STALL

static inline void record_gp_check_time(struct rcu_ctrlblk *rcp)
{
        rcp->gp_check = get_seconds() + 3;
}

static void print_other_cpu_stall(struct rcu_ctrlblk *rcp)
{
        int cpu;
        long delta;

        /* Only let one CPU complain about others per time interval. */

        spin_lock(&rcp->lock);
        delta = get_seconds() - rcp->gp_check;
        if (delta < 2L || cpus_empty(rcp->cpumask)) {
                spin_unlock(&rcp->lock);
                return;
        }
        spin_unlock(&rcp->lock);

        /* OK, time to rat on our buddy... */

        printk(KERN_ERR "RCU detected CPU stalls:");
        for_each_cpu_mask(cpu, rcp->cpumask)
                printk(" %d", cpu);
        printk(" (detected by %d, t=%lu/%lu)\n",
               smp_processor_id(), get_seconds(), rcp->gp_check);
}

static void print_cpu_stall(struct rcu_ctrlblk *rcp)
{
        printk(KERN_ERR "RCU detected CPU %d stall (t=%lu/%lu)\n",
                        smp_processor_id(), get_seconds(), rcp->gp_check);
        dump_stack();
        spin_lock(&rcp->lock);
        if ((long)(get_seconds() - rcp->gp_check) >= 0L)
                rcp->gp_check = get_seconds() + 30;
        spin_unlock(&rcp->lock);
}

static void check_cpu_stall(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
{
        long delta;

        delta = get_seconds() - rcp->gp_check;
        if (cpu_isset(smp_processor_id(), rcp->cpumask) && delta >= 0L) {

                /* We haven't checked in, so go dump stack. */

                print_cpu_stall(rcp);

        } else {
                if (!cpus_empty(rcp->cpumask) && delta >= 2L) {
                        /* They had two seconds to dump stack, so complain. */
                        print_other_cpu_stall(rcp);
                }
        }
}

#else /* #ifdef CONFIG_DEBUG_RCU_STALL */

static inline void record_gp_check_time(struct rcu_ctrlblk *rcp)
{
}

static inline void
check_cpu_stall(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
{
}

#endif /* #else #ifdef CONFIG_DEBUG_RCU_STALL */

/*
 * Register a new batch of callbacks, and start it up if there is currently no
 * active batch and the batch to be registered has not already occurred.
 * Caller must hold rcu_ctrlblk.lock.
 */
static void rcu_start_batch(struct rcu_ctrlblk *rcp)
{
        if (rcp->cur != rcp->pending &&
                        rcp->completed == rcp->cur) {
                rcp->cur++;
                record_gp_check_time(rcp);

                /*
                 * Accessing nohz_cpu_mask before incrementing rcp->cur needs a
                 * Barrier  Otherwise it can cause tickless idle CPUs to be
                 * included in rcp->cpumask, which will extend graceperiods
                 * unnecessarily.
                 */
                smp_mb();
                cpus_andnot(rcp->cpumask, cpu_online_map, nohz_cpu_mask);

                rcp->signaled = 0;
        }
}

/*
 * cpu went through a quiescent state since the beginning of the grace period.
 * Clear it from the cpu mask and complete the grace period if it was the last
 * cpu. Start another grace period if someone has further entries pending
 */
static void cpu_quiet(int cpu, struct rcu_ctrlblk *rcp)
{
        cpu_clear(cpu, rcp->cpumask);
        if (cpus_empty(rcp->cpumask)) {
                /* batch completed ! */
                rcp->completed = rcp->cur;
                rcu_start_batch(rcp);
        }
}

/*
 * Check if the cpu has gone through a quiescent state (say context
 * switch). If so and if it already hasn't done so in this RCU
 * quiescent cycle, then indicate that it has done so.
 */
static void rcu_check_quiescent_state(struct rcu_ctrlblk *rcp,
                                        struct rcu_data *rdp)
{
        if (rdp->quiescbatch != rcp->cur) {
                /* start new grace period: */
                rdp->qs_pending = 1;
                rdp->passed_quiesc = 0;
                rdp->quiescbatch = rcp->cur;
                return;
        }

        /* Grace period already completed for this cpu?
         * qs_pending is checked instead of the actual bitmap to avoid
         * cacheline trashing.
         */
        if (!rdp->qs_pending)
                return;

        /*
         * Was there a quiescent state since the beginning of the grace
         * period? If no, then exit and wait for the next call.
         */
        if (!rdp->passed_quiesc)
                return;
        rdp->qs_pending = 0;

        spin_lock(&rcp->lock);
        /*
         * rdp->quiescbatch/rcp->cur and the cpu bitmap can come out of sync
         * during cpu startup. Ignore the quiescent state.
         */
        if (likely(rdp->quiescbatch == rcp->cur))
                cpu_quiet(rdp->cpu, rcp);

        spin_unlock(&rcp->lock);
}


#ifdef CONFIG_HOTPLUG_CPU

/* warning! helper for rcu_offline_cpu. do not use elsewhere without reviewing
 * locking requirements, the list it's pulling from has to belong to a cpu
 * which is dead and hence not processing interrupts.
 */
static void rcu_move_batch(struct rcu_data *this_rdp, struct rcu_head *list,
                                struct rcu_head **tail, long batch)
{
        if (list) {
                local_irq_disable();
                this_rdp->batch = batch;
                *this_rdp->nxttail[2] = list;
                this_rdp->nxttail[2] = tail;
                local_irq_enable();
        }
}

static void __rcu_offline_cpu(struct rcu_data *this_rdp,
                                struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
{
        /* if the cpu going offline owns the grace period
         * we can block indefinitely waiting for it, so flush
         * it here
         */
        spin_lock_bh(&rcp->lock);
        if (rcp->cur != rcp->completed)
                cpu_quiet(rdp->cpu, rcp);
        spin_unlock_bh(&rcp->lock);
        /* spin_lock implies smp_mb() */
        rcu_move_batch(this_rdp, rdp->donelist, rdp->donetail, rcp->cur + 1);
        rcu_move_batch(this_rdp, rdp->nxtlist, rdp->nxttail[2], rcp->cur + 1);

        local_irq_disable();
        this_rdp->qlen += rdp->qlen;
        local_irq_enable();
}

static void rcu_offline_cpu(int cpu)
{
        struct rcu_data *this_rdp = &get_cpu_var(rcu_data);
        struct rcu_data *this_bh_rdp = &get_cpu_var(rcu_bh_data);

        __rcu_offline_cpu(this_rdp, &rcu_ctrlblk,
                                        &per_cpu(rcu_data, cpu));
        __rcu_offline_cpu(this_bh_rdp, &rcu_bh_ctrlblk,
                                        &per_cpu(rcu_bh_data, cpu));
        put_cpu_var(rcu_data);
        put_cpu_var(rcu_bh_data);
}

#else

static void rcu_offline_cpu(int cpu)
{
}

#endif

/*
 * This does the RCU processing work from softirq context.
 */
static void __rcu_process_callbacks(struct rcu_ctrlblk *rcp,
                                        struct rcu_data *rdp)
{
        if (rdp->nxtlist) {
                local_irq_disable();

                /*
                 * move the other grace-period-completed entries to
                 * [rdp->nxtlist, *rdp->nxttail[0]) temporarily
                 */
                if (!rcu_batch_before(rcp->completed, rdp->batch))
                        rdp->nxttail[0] = rdp->nxttail[1] = rdp->nxttail[2];
                else if (!rcu_batch_before(rcp->completed, rdp->batch - 1))
                        rdp->nxttail[0] = rdp->nxttail[1];

                /*
                 * the grace period for entries in
                 * [rdp->nxtlist, *rdp->nxttail[0]) has completed and
                 * move these entries to donelist
                 */
                if (rdp->nxttail[0] != &rdp->nxtlist) {
                        *rdp->donetail = rdp->nxtlist;
                        rdp->donetail = rdp->nxttail[0];
                        rdp->nxtlist = *rdp->nxttail[0];
                        *rdp->donetail = NULL;

                        if (rdp->nxttail[1] == rdp->nxttail[0])
                                rdp->nxttail[1] = &rdp->nxtlist;
                        if (rdp->nxttail[2] == rdp->nxttail[0])
                                rdp->nxttail[2] = &rdp->nxtlist;
                        rdp->nxttail[0] = &rdp->nxtlist;
                }

                local_irq_enable();

                if (rcu_batch_after(rdp->batch, rcp->pending)) {
                        /* and start it/schedule start if it's a new batch */
                        spin_lock(&rcp->lock);
                        if (rcu_batch_after(rdp->batch, rcp->pending)) {
                                rcp->pending = rdp->batch;
                                rcu_start_batch(rcp);
                        }
                        spin_unlock(&rcp->lock);
                }
        }

        rcu_check_quiescent_state(rcp, rdp);
        if (rdp->donelist)
                rcu_do_batch(rdp);
}

static void rcu_process_callbacks(struct softirq_action *unused)
{
        __rcu_process_callbacks(&rcu_ctrlblk, &__get_cpu_var(rcu_data));
        __rcu_process_callbacks(&rcu_bh_ctrlblk, &__get_cpu_var(rcu_bh_data));
}

static int __rcu_pending(struct rcu_ctrlblk *rcp, struct rcu_data *rdp)
{
        /* Check for CPU stalls, if enabled. */
        check_cpu_stall(rcp, rdp);

        if (rdp->nxtlist) {
                /*
                 * This cpu has pending rcu entries and the grace period
                 * for them has completed.
                 */
                if (!rcu_batch_before(rcp->completed, rdp->batch))
                        return 1;
                if (!rcu_batch_before(rcp->completed, rdp->batch - 1) &&
                                rdp->nxttail[0] != rdp->nxttail[1])
                        return 1;
                if (rdp->nxttail[0] != &rdp->nxtlist)
                        return 1;

                /*
                 * This cpu has pending rcu entries and the new batch
                 * for then hasn't been started nor scheduled start
                 */
                if (rcu_batch_after(rdp->batch, rcp->pending))
                        return 1;
        }

        /* This cpu has finished callbacks to invoke */
        if (rdp->donelist)
                return 1;

        /* The rcu core waits for a quiescent state from the cpu */
        if (rdp->quiescbatch != rcp->cur || rdp->qs_pending)
                return 1;

        /* nothing to do */
        return 0;
}

/*
 * Check to see if there is any immediate RCU-related work to be done
 * by the current CPU, returning 1 if so.  This function is part of the
 * RCU implementation; it is -not- an exported member of the RCU API.
 */
int rcu_pending(int cpu)
{
        return __rcu_pending(&rcu_ctrlblk, &per_cpu(rcu_data, cpu)) ||
                __rcu_pending(&rcu_bh_ctrlblk, &per_cpu(rcu_bh_data, cpu));
}

/*
 * Check to see if any future RCU-related work will need to be done
 * by the current CPU, even if none need be done immediately, returning
 * 1 if so.  This function is part of the RCU implementation; it is -not-
 * an exported member of the RCU API.
 */
int rcu_needs_cpu(int cpu)
{
        struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
        struct rcu_data *rdp_bh = &per_cpu(rcu_bh_data, cpu);

        return !!rdp->nxtlist || !!rdp_bh->nxtlist || rcu_pending(cpu);
}

void rcu_check_callbacks(int cpu, int user)
{
        if (user ||
            (idle_cpu(cpu) && !in_softirq() &&
                                hardirq_count() <= (1 << HARDIRQ_SHIFT))) {

                /*
                 * Get here if this CPU took its interrupt from user
                 * mode or from the idle loop, and if this is not a
                 * nested interrupt.  In this case, the CPU is in
                 * a quiescent state, so count it.
                 *
                 * Also do a memory barrier.  This is needed to handle
                 * the case where writes from a preempt-disable section
                 * of code get reordered into schedule() by this CPU's
                 * write buffer.  The memory barrier makes sure that
                 * the rcu_qsctr_inc() and rcu_bh_qsctr_inc() are see
                 * by other CPUs to happen after any such write.
                 */

                smp_mb();  /* See above block comment. */
                rcu_qsctr_inc(cpu);
                rcu_bh_qsctr_inc(cpu);

        } else if (!in_softirq()) {

                /*
                 * Get here if this CPU did not take its interrupt from
                 * softirq, in other words, if it is not interrupting
                 * a rcu_bh read-side critical section.  This is an _bh
                 * critical section, so count it.  The memory barrier
                 * is needed for the same reason as is the above one.
                 */

                smp_mb();  /* See above block comment. */
                rcu_bh_qsctr_inc(cpu);
        }
        raise_rcu_softirq();
}

static void rcu_init_percpu_data(int cpu, struct rcu_ctrlblk *rcp,
                                                struct rcu_data *rdp)
{
        memset(rdp, 0, sizeof(*rdp));
        rdp->nxttail[0] = rdp->nxttail[1] = rdp->nxttail[2] = &rdp->nxtlist;
        rdp->donetail = &rdp->donelist;
        rdp->quiescbatch = rcp->completed;
        rdp->qs_pending = 0;
        rdp->cpu = cpu;
        rdp->blimit = blimit;
}

static void __cpuinit rcu_online_cpu(int cpu)
{
        struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
        struct rcu_data *bh_rdp = &per_cpu(rcu_bh_data, cpu);

        rcu_init_percpu_data(cpu, &rcu_ctrlblk, rdp);
        rcu_init_percpu_data(cpu, &rcu_bh_ctrlblk, bh_rdp);
        open_softirq(RCU_SOFTIRQ, rcu_process_callbacks);
}

static int __cpuinit rcu_cpu_notify(struct notifier_block *self,
                                unsigned long action, void *hcpu)
{
        long cpu = (long)hcpu;

        switch (action) {
        case CPU_UP_PREPARE:
        case CPU_UP_PREPARE_FROZEN:
                rcu_online_cpu(cpu);
                break;
        case CPU_DEAD:
        case CPU_DEAD_FROZEN:
                rcu_offline_cpu(cpu);
                break;
        default:
                break;
        }
        return NOTIFY_OK;
}

static struct notifier_block __cpuinitdata rcu_nb = {
        .notifier_call  = rcu_cpu_notify,
};

/*
 * Initializes rcu mechanism.  Assumed to be called early.
 * That is before local timer(SMP) or jiffie timer (uniproc) is setup.
 * Note that rcu_qsctr and friends are implicitly
 * initialized due to the choice of ``0'' for RCU_CTR_INVALID.
 */
void __init __rcu_init(void)
{
        rcu_cpu_notify(&rcu_nb, CPU_UP_PREPARE,
                        (void *)(long)smp_processor_id());
        /* Register notifier for non-boot CPUs */
        register_cpu_notifier(&rcu_nb);
}

module_param(blimit, int, 0);
module_param(qhimark, int, 0);
module_param(qlowmark, int, 0);