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rcu: Switch __rcu_process_callbacks() to rcu_accelerate_cbs()
The __rcu_process_callbacks() function currently checks to see if the current CPU needs a grace period and also if there is any other reason to kick off a new grace period. This is one of the fail-safe checks that has been rendered unnecessary by the changes that increase the accuracy of rcu_gp_cleanup()'s estimate as to whether another grace period is required. Because this particular fail-safe involved acquiring the root rcu_node structure's ->lock, which has seen excessive contention in real life, this fail-safe needs to go. However, one check must remain, namely the check for newly arrived RCU callbacks that have not yet been associated with a grace period. One might hope that the checks in __note_gp_changes(), which is invoked indirectly from rcu_check_quiescent_state(), would suffice, but this function won't be invoked at all if RCU is idle. It is therefore necessary to replace the fail-safe checks with a simpler check for newly arrived callbacks during an RCU idle period, which is exactly what this commit does. This change removes the final call to rcu_start_gp(), so this function is removed as well. Note that lockless use of cpu_needs_another_gp() is racy, but that these races are harmless in this case. If RCU really is idle, the values will not change, so the return value from cpu_needs_another_gp() will be correct. If RCU is not idle, the resulting redundant call to rcu_accelerate_cbs() will be harmless, and might even have the benefit of reducing grace-period latency a bit. This commit also moves interrupt disabling into the "if" statement to improve real-time response a bit. Reported-by: Nicholas Piggin <npiggin@gmail.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com> Tested-by: Nicholas Piggin <npiggin@gmail.com>
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parent
a6058d85a2
commit
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1 changed files with 15 additions and 38 deletions
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@ -2334,34 +2334,6 @@ rcu_start_gp_advanced(struct rcu_state *rsp, struct rcu_node *rnp,
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return true;
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}
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/*
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* Similar to rcu_start_gp_advanced(), but also advance the calling CPU's
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* callbacks. Note that rcu_start_gp_advanced() cannot do this because it
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* is invoked indirectly from rcu_advance_cbs(), which would result in
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* endless recursion -- or would do so if it wasn't for the self-deadlock
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* that is encountered beforehand.
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*
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* Returns true if the grace-period kthread needs to be awakened.
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*/
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static bool rcu_start_gp(struct rcu_state *rsp)
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{
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struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
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struct rcu_node *rnp = rcu_get_root(rsp);
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bool ret = false;
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/*
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* If there is no grace period in progress right now, any
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* callbacks we have up to this point will be satisfied by the
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* next grace period. Also, advancing the callbacks reduces the
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* probability of false positives from cpu_needs_another_gp()
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* resulting in pointless grace periods. So, advance callbacks
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* then start the grace period!
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*/
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ret = rcu_advance_cbs(rsp, rnp, rdp) || ret;
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ret = rcu_start_gp_advanced(rsp, rnp, rdp) || ret;
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return ret;
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}
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/*
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* Report a full set of quiescent states to the specified rcu_state data
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* structure. Invoke rcu_gp_kthread_wake() to awaken the grace-period
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@ -2889,22 +2861,27 @@ __rcu_process_callbacks(struct rcu_state *rsp)
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unsigned long flags;
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bool needwake;
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struct rcu_data *rdp = raw_cpu_ptr(rsp->rda);
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struct rcu_node *rnp;
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WARN_ON_ONCE(!rdp->beenonline);
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/* Update RCU state based on any recent quiescent states. */
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rcu_check_quiescent_state(rsp, rdp);
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/* Does this CPU require a not-yet-started grace period? */
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local_irq_save(flags);
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if (cpu_needs_another_gp(rsp, rdp)) {
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raw_spin_lock_rcu_node(rcu_get_root(rsp)); /* irqs disabled. */
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needwake = rcu_start_gp(rsp);
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raw_spin_unlock_irqrestore_rcu_node(rcu_get_root(rsp), flags);
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if (needwake)
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rcu_gp_kthread_wake(rsp);
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} else {
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local_irq_restore(flags);
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/* No grace period and unregistered callbacks? */
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if (!rcu_gp_in_progress(rsp) &&
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rcu_segcblist_is_enabled(&rdp->cblist)) {
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local_irq_save(flags);
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if (rcu_segcblist_restempty(&rdp->cblist, RCU_NEXT_READY_TAIL)) {
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local_irq_restore(flags);
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} else {
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rnp = rdp->mynode;
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raw_spin_lock_rcu_node(rnp); /* irqs disabled. */
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needwake = rcu_accelerate_cbs(rsp, rnp, rdp);
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raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
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if (needwake)
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rcu_gp_kthread_wake(rsp);
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}
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}
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/* If there are callbacks ready, invoke them. */
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