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rcu: Check for NOCB CPUs and empty lists earlier in CB migration 

The current CPU-hotplug RCU-callback-migration code checks
for the source (newly offlined) CPU being a NOCBs CPU down in
rcu_send_cbs_to_orphanage().  This commit simplifies callback migration a
bit by moving this check up to rcu_migrate_callbacks().  This commit also
adds a check for the source CPU having no callbacks, which eases analysis
of the rcu_send_cbs_to_orphanage() and rcu_adopt_orphan_cbs() functions.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
This commit is contained in:
Paul E. McKenney 2017-06-26 10:49:50 -07:00
parent c47e067a3c
commit 95335c0355
1 changed files with 3 additions and 4 deletions
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7
kernel/rcu/tree.c
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@ -3862,10 +3862,6 @@ rcu_send_cbs_to_orphanage(int cpu, struct rcu_state *rsp,
{
lockdep_assert_held(&rsp->orphan_lock);
/* No-CBs CPUs do not have orphanable callbacks. */
if (!IS_ENABLED(CONFIG_HOTPLUG_CPU) || rcu_is_nocb_cpu(rdp->cpu))
return;
/*
* Orphan the callbacks. First adjust the counts. This is safe
* because _rcu_barrier() excludes CPU-hotplug operations, so it
@ -3935,6 +3931,9 @@ static void rcu_migrate_callbacks(int cpu, struct rcu_state *rsp)
struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
struct rcu_node *rnp = rdp->mynode; /* Outgoing CPU's rdp & rnp. */
if (rcu_is_nocb_cpu(cpu) || rcu_segcblist_empty(&rdp->cblist))
return; /* No callbacks to migrate. */
raw_spin_lock_irqsave(&rsp->orphan_lock, flags);
rcu_send_cbs_to_orphanage(cpu, rsp, rnp, rdp);
rcu_adopt_orphan_cbs(rsp, flags);