[tip: core/rcu] rcu: Restrict RCU_STRICT_GRACE_PERIOD to at most four CPUs

[tip-bot2 for Paul E. McKenney]

The following commit has been merged into the core/rcu branch of tip:

Commit-ID:     4d80b8e196fad9852050f3c8624eea09a6bbeada
Gitweb:        https://git.kernel.org/tip/4d80b8e196fad9852050f3c8624eea09a6bbeada
Author:        Paul E. McKenney <paulmck@xxxxxxxxxx>
AuthorDate:    Wed, 07 Apr 2021 15:21:32 -07:00
Committer:     Paul E. McKenney <paulmck@xxxxxxxxxx>
CommitterDate: Mon, 10 May 2021 16:22:54 -07:00

rcu: Restrict RCU_STRICT_GRACE_PERIOD to at most four CPUs

Kernels built with CONFIG_RCU_STRICT_GRACE_PERIOD=y can experience
significant lock contention due to RCU's resulting focus on ending grace
periods as soon as possible.  This is OK, but only if there are not very
many CPUs.  This commit therefore puts this Kconfig option off-limits
to systems with more than four CPUs.

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
---
 kernel/rcu/Kconfig.debug | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/rcu/Kconfig.debug b/kernel/rcu/Kconfig.debug
index 1942c1f..4fd6499 100644
--- a/kernel/rcu/Kconfig.debug
+++ b/kernel/rcu/Kconfig.debug
@@ -116,7 +116,7 @@ config RCU_EQS_DEBUG
 
 config RCU_STRICT_GRACE_PERIOD
 	bool "Provide debug RCU implementation with short grace periods"
-	depends on DEBUG_KERNEL && RCU_EXPERT
+	depends on DEBUG_KERNEL && RCU_EXPERT && NR_CPUS <= 4
 	default n
 	select PREEMPT_COUNT if PREEMPT=n
 	help