rcu: Introduce proper blocking to no-CBs kthreads GP waits
Currently, the no-CBs kthreads do repeated timed waits for grace periods to elapse. This is crude and energy inefficient, so this commit allows no-CBs kthreads to specify exactly which grace period they are waiting for and also allows them to block for the entire duration until the desired grace period completes. Signed-off-by: Paul E. McKenney <paul.mckenney@linaro.org> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
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@ -310,7 +310,7 @@ cpu_needs_another_gp(struct rcu_state *rsp, struct rcu_data *rdp)
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if (rcu_gp_in_progress(rsp))
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return 0; /* No, a grace period is already in progress. */
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if (rcu_nocb_needs_gp(rdp))
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if (rcu_nocb_needs_gp(rsp))
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return 1; /* Yes, a no-CBs CPU needs one. */
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if (!rdp->nxttail[RCU_NEXT_TAIL])
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return 0; /* No, this is a no-CBs (or offline) CPU. */
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@ -1364,6 +1364,7 @@ int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
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static void rcu_gp_cleanup(struct rcu_state *rsp)
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{
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unsigned long gp_duration;
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int nocb = 0;
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struct rcu_data *rdp;
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struct rcu_node *rnp = rcu_get_root(rsp);
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@ -1394,11 +1395,13 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
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rcu_for_each_node_breadth_first(rsp, rnp) {
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raw_spin_lock_irq(&rnp->lock);
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rnp->completed = rsp->gpnum;
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nocb += rcu_nocb_gp_cleanup(rsp, rnp);
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raw_spin_unlock_irq(&rnp->lock);
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cond_resched();
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}
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rnp = rcu_get_root(rsp);
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raw_spin_lock_irq(&rnp->lock);
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rcu_nocb_gp_set(rnp, nocb);
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rsp->completed = rsp->gpnum; /* Declare grace period done. */
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trace_rcu_grace_period(rsp->name, rsp->completed, "end");
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@ -3084,6 +3087,7 @@ static void __init rcu_init_one(struct rcu_state *rsp,
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}
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rnp->level = i;
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INIT_LIST_HEAD(&rnp->blkd_tasks);
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rcu_init_one_nocb(rnp);
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}
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}
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@ -196,6 +196,12 @@ struct rcu_node {
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/* Refused to boost: not sure why, though. */
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/* This can happen due to race conditions. */
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#endif /* #ifdef CONFIG_RCU_BOOST */
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#ifdef CONFIG_RCU_NOCB_CPU
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wait_queue_head_t nocb_gp_wq[2];
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/* Place for rcu_nocb_kthread() to wait GP. */
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int n_nocb_gp_requests[2];
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/* Counts of upcoming no-CB GP requests. */
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#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
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raw_spinlock_t fqslock ____cacheline_internodealigned_in_smp;
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} ____cacheline_internodealigned_in_smp;
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@ -326,7 +332,6 @@ struct rcu_data {
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int nocb_p_count_lazy; /* (approximate). */
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wait_queue_head_t nocb_wq; /* For nocb kthreads to sleep on. */
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struct task_struct *nocb_kthread;
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bool nocb_needs_gp;
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#endif /* #ifdef CONFIG_RCU_NOCB_CPU */
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int cpu;
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@ -524,7 +529,10 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu);
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static void print_cpu_stall_info_end(void);
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static void zero_cpu_stall_ticks(struct rcu_data *rdp);
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static void increment_cpu_stall_ticks(void);
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static int rcu_nocb_needs_gp(struct rcu_data *rdp);
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static int rcu_nocb_needs_gp(struct rcu_state *rsp);
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static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq);
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static int rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp);
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static void rcu_init_one_nocb(struct rcu_node *rnp);
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static bool is_nocb_cpu(int cpu);
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static bool __call_rcu_nocb(struct rcu_data *rdp, struct rcu_head *rhp,
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bool lazy);
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@ -2176,11 +2176,51 @@ static int __init parse_rcu_nocb_poll(char *arg)
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early_param("rcu_nocb_poll", parse_rcu_nocb_poll);
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/*
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* Does this CPU needs a grace period due to offloaded callbacks?
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* Do any no-CBs CPUs need another grace period?
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*
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* Interrupts must be disabled. If the caller does not hold the root
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* rnp_node structure's ->lock, the results are advisory only.
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*/
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static int rcu_nocb_needs_gp(struct rcu_data *rdp)
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static int rcu_nocb_needs_gp(struct rcu_state *rsp)
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{
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return rdp->nocb_needs_gp;
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struct rcu_node *rnp = rcu_get_root(rsp);
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return rnp->n_nocb_gp_requests[(ACCESS_ONCE(rnp->completed) + 1) & 0x1];
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}
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/*
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* Clean up this rcu_node structure's no-CBs state at the end of
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* a grace period, and also return whether any no-CBs CPU associated
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* with this rcu_node structure needs another grace period.
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*/
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static int rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
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{
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int c = rnp->completed;
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int needmore;
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wake_up_all(&rnp->nocb_gp_wq[c & 0x1]);
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rnp->n_nocb_gp_requests[c & 0x1] = 0;
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needmore = rnp->n_nocb_gp_requests[(c + 1) & 0x1];
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return needmore;
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}
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/*
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* Set the root rcu_node structure's ->n_nocb_gp_requests field
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* based on the sum of those of all rcu_node structures. This does
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* double-count the root rcu_node structure's requests, but this
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* is necessary to handle the possibility of a rcu_nocb_kthread()
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* having awakened during the time that the rcu_node structures
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* were being updated for the end of the previous grace period.
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*/
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static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq)
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{
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rnp->n_nocb_gp_requests[(rnp->completed + 1) & 0x1] += nrq;
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}
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static void rcu_init_one_nocb(struct rcu_node *rnp)
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{
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init_waitqueue_head(&rnp->nocb_gp_wq[0]);
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init_waitqueue_head(&rnp->nocb_gp_wq[1]);
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}
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/* Is the specified CPU a no-CPUs CPU? */
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@ -2289,31 +2329,73 @@ static bool __maybe_unused rcu_nocb_adopt_orphan_cbs(struct rcu_state *rsp,
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static void rcu_nocb_wait_gp(struct rcu_data *rdp)
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{
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unsigned long c;
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bool d;
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unsigned long flags;
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unsigned long j;
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unsigned long flags1;
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struct rcu_node *rnp = rdp->mynode;
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struct rcu_node *rnp_root = rcu_get_root(rdp->rsp);
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raw_spin_lock_irqsave(&rnp->lock, flags);
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c = rnp->completed + 2;
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rdp->nocb_needs_gp = true;
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raw_spin_unlock_irqrestore(&rnp->lock, flags);
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/* Count our request for a grace period. */
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rnp->n_nocb_gp_requests[c & 0x1]++;
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if (rnp->gpnum != rnp->completed) {
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/*
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* This rcu_node structure believes that a grace period
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* is in progress, so we are done. When this grace
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* period ends, our request will be acted upon.
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*/
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raw_spin_unlock_irqrestore(&rnp->lock, flags);
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} else {
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/*
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* Might not be a grace period, check root rcu_node
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* structure to see if we must start one.
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*/
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if (rnp != rnp_root)
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raw_spin_lock(&rnp_root->lock); /* irqs disabled. */
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if (rnp_root->gpnum != rnp_root->completed) {
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raw_spin_unlock(&rnp_root->lock); /* irqs disabled. */
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} else {
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/*
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* No grace period, so we need to start one.
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* The good news is that we can wait for exactly
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* one grace period instead of part of the current
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* grace period and all of the next grace period.
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* Adjust counters accordingly and start the
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* needed grace period.
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*/
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rnp->n_nocb_gp_requests[c & 0x1]--;
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c = rnp_root->completed + 1;
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rnp->n_nocb_gp_requests[c & 0x1]++;
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rnp_root->n_nocb_gp_requests[c & 0x1]++;
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local_save_flags(flags1);
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rcu_start_gp(rdp->rsp, flags1); /* Rlses ->lock. */
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}
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/* Clean up locking and irq state. */
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if (rnp != rnp_root)
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raw_spin_unlock_irqrestore(&rnp->lock, flags);
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else
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local_irq_restore(flags);
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}
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/*
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* Wait for the grace period. Do so interruptibly to avoid messing
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* up the load average.
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*/
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for (;;) {
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j = jiffies;
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schedule_timeout_interruptible(2);
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raw_spin_lock_irqsave(&rnp->lock, flags);
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if (ULONG_CMP_GE(rnp->completed, c)) {
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rdp->nocb_needs_gp = false;
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raw_spin_unlock_irqrestore(&rnp->lock, flags);
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wait_event_interruptible(
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rnp->nocb_gp_wq[c & 0x1],
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(d = ULONG_CMP_GE(ACCESS_ONCE(rnp->completed), c)));
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if (likely(d))
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break;
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}
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if (j == jiffies)
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flush_signals(current);
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raw_spin_unlock_irqrestore(&rnp->lock, flags);
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flush_signals(current);
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}
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smp_mb(); /* Ensure that CB invocation happens after GP end. */
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}
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@ -2416,11 +2498,24 @@ static bool init_nocb_callback_list(struct rcu_data *rdp)
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#else /* #ifdef CONFIG_RCU_NOCB_CPU */
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static int rcu_nocb_needs_gp(struct rcu_data *rdp)
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static int rcu_nocb_needs_gp(struct rcu_state *rsp)
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{
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return 0;
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}
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static int rcu_nocb_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
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{
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return 0;
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}
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static void rcu_nocb_gp_set(struct rcu_node *rnp, int nrq)
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{
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}
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static void rcu_init_one_nocb(struct rcu_node *rnp)
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{
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}
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static bool is_nocb_cpu(int cpu)
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{
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return false;
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