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rcu/nocb: Check for deferred nocb wakeups before nohz_full early exit

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In theory, a timer is used to defer wakeups of no-CBs grace-period
kthreads when the wakeup cannot be done safely directly from the
call_rcu().  In practice, the one-jiffy delay is not always consistent
with timely callback invocation under heavy call_rcu() loads.  Therefore,
there are a number of checks for a pending deferred wakeup, including
from the scheduling-clock interrupt.  Unfortunately, this check follows
the rcu_nohz_full_cpu() early exit, which renders it useless on such CPUs.

This commit therefore moves the check for the pending deferred no-CB
wakeup to precede the rcu_nohz_full_cpu() early exit.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
This commit is contained in:
Paul E. McKenney 2019-04-16 14:48:28 -07:00
parent c00045be32
commit 85f69b3212
1 changed files with 4 additions and 4 deletions
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8
kernel/rcu/tree.c
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@ -2739,6 +2739,10 @@ static int rcu_pending(void)
/* Check for CPU stalls, if enabled. */
check_cpu_stall(rdp);
/* Does this CPU need a deferred NOCB wakeup? */
if (rcu_nocb_need_deferred_wakeup(rdp))
return 1;
/* Is this CPU a NO_HZ_FULL CPU that should ignore RCU? */
if (rcu_nohz_full_cpu())
return 0;
@ -2763,10 +2767,6 @@ static int rcu_pending(void)
unlikely(READ_ONCE(rdp->gpwrap))) /* outside lock */
return 1;
/* Does this CPU need a deferred NOCB wakeup? */
if (rcu_nocb_need_deferred_wakeup(rdp))
return 1;
/* nothing to do */
return 0;
}