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rcu/nocb: Use build-time no-CBs check in rcu_do_batch()

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Currently, rcu_do_batch() invokes rcu_segcblist_is_offloaded() each time
it needs to know whether the current CPU is a no-CBs CPU.  Given that it
is not possible to change the no-CBs status of a CPU after boot, and given
that it is not possible to even have no-CBs CPUs in CONFIG_RCU_NOCB_CPU=n
kernels, this per-callback invocation wastes CPU.  This commit therefore
created a const on-stack variable to allow this check to be done only
once per rcu_do_batch() invocation.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
This commit is contained in:
Paul E. McKenney 2019-05-21 13:03:49 -07:00
parent 4f9c1bc727
commit ec5ef87bac
1 changed files with 5 additions and 5 deletions
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10
kernel/rcu/tree.c
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@ -2087,6 +2087,8 @@ int rcutree_dead_cpu(unsigned int cpu)
static void rcu_do_batch(struct rcu_data *rdp)
{
unsigned long flags;
const bool offloaded = IS_ENABLED(CONFIG_RCU_NOCB_CPU) &&
rcu_segcblist_is_offloaded(&rdp->cblist);
struct rcu_head *rhp;
struct rcu_cblist rcl = RCU_CBLIST_INITIALIZER(rcl);
long bl, count;
@ -2128,12 +2130,11 @@ static void rcu_do_batch(struct rcu_data *rdp)
* Stop only if limit reached and CPU has something to do.
* Note: The rcl structure counts down from zero.
*/
if (-rcl.len >= bl &&
!rcu_segcblist_is_offloaded(&rdp->cblist) &&
if (-rcl.len >= bl && !offloaded &&
(need_resched() ||
(!is_idle_task(current) && !rcu_is_callbacks_kthread())))
break;
if (rcu_segcblist_is_offloaded(&rdp->cblist)) {
if (offloaded) {
WARN_ON_ONCE(in_serving_softirq());
local_bh_enable();
lockdep_assert_irqs_enabled();
@ -2175,8 +2176,7 @@ static void rcu_do_batch(struct rcu_data *rdp)
rcu_nocb_unlock_irqrestore(rdp, flags);
/* Re-invoke RCU core processing if there are callbacks remaining. */
if (!rcu_segcblist_is_offloaded(&rdp->cblist) &&
rcu_segcblist_ready_cbs(&rdp->cblist))
if (!offloaded && rcu_segcblist_ready_cbs(&rdp->cblist))
invoke_rcu_core();
}