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Subject[PATCH tip/core/rcu 04/12] rcu: Covert CONFIG_RCU_FANOUT_EXACT to boot parameter
Date
From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>

The CONFIG_RCU_FANOUT_EXACT Kconfig parameter is used primarily (and
perhaps only) by rcutorture to verify that RCU works correctly in specific
rcu_node combining-tree configurations. It therefore does not make
much sense have this as a question to people attempting to configure
their kernels. So this commit creates an rcutree.rcu_fanout_exact=
boot parameter that rcutorture can use, and eliminates the original
CONFIG_RCU_FANOUT_EXACT Kconfig parameter.

Reported-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/kernel-parameters.txt | 6 ++++++
init/Kconfig | 14 --------------
kernel/rcu/tree.c | 7 +++++--
kernel/rcu/tree_plugin.h | 2 +-
4 files changed, 12 insertions(+), 17 deletions(-)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index 64528a2fc67c..24b0d95acd45 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2986,6 +2986,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
the rcu_node combining tree. This only has effect
when CONFIG_RCU_TORTURE_TEST_SLOW_PREINIT is set.

+ rcutree.rcu_fanout_exact= [KNL]
+ Disable autobalancing of the rcu_node combining
+ tree. This is used by rcutorture, and might
+ possibly be useful for architectures having high
+ cache-to-cache transfer latencies.
+
rcutree.rcu_fanout_leaf= [KNL]
Increase the number of CPUs assigned to each
leaf rcu_node structure. Useful for very large
diff --git a/init/Kconfig b/init/Kconfig
index 6d2cf6453be7..67598f3143f5 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -609,20 +609,6 @@ config RCU_FANOUT_LEAF

Take the default if unsure.

-config RCU_FANOUT_EXACT
- bool "Disable tree-based hierarchical RCU auto-balancing"
- depends on TREE_RCU || PREEMPT_RCU
- default n
- help
- This option forces use of the exact RCU_FANOUT value specified,
- regardless of imbalances in the hierarchy. This is useful for
- testing RCU itself, and might one day be useful on systems with
- strong NUMA behavior.
-
- Without RCU_FANOUT_EXACT, the code will balance the hierarchy.
-
- Say N if unsure.
-
config RCU_FAST_NO_HZ
bool "Accelerate last non-dyntick-idle CPU's grace periods"
depends on NO_HZ_COMMON && SMP
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 3467e321280c..fdca17e7a1b5 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -114,6 +114,9 @@ static struct rcu_state *const rcu_state_p;
static struct rcu_data __percpu *const rcu_data_p;
LIST_HEAD(rcu_struct_flavors);

+/* Control rcu_node-tree auto-balancing at boot time. */
+static bool rcu_fanout_exact;
+module_param(rcu_fanout_exact, bool, 0444);
/* Increase (but not decrease) the CONFIG_RCU_FANOUT_LEAF at boot time. */
static int rcu_fanout_leaf = CONFIG_RCU_FANOUT_LEAF;
module_param(rcu_fanout_leaf, int, 0444);
@@ -3937,13 +3940,13 @@ void rcu_scheduler_starting(void)

/*
* Compute the per-level fanout, either using the exact fanout specified
- * or balancing the tree, depending on CONFIG_RCU_FANOUT_EXACT.
+ * or balancing the tree, depending on the rcu_fanout_exact boot parameter.
*/
static void __init rcu_init_levelspread(int *levelspread, const int *levelcnt)
{
int i;

- if (IS_ENABLED(CONFIG_RCU_FANOUT_EXACT)) {
+ if (rcu_fanout_exact) {
levelspread[rcu_num_lvls - 1] = rcu_fanout_leaf;
for (i = rcu_num_lvls - 2; i >= 0; i--)
levelspread[i] = CONFIG_RCU_FANOUT;
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index e6ef194da4b7..074689cedda9 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -74,7 +74,7 @@ static void __init rcu_bootup_announce_oddness(void)
(!IS_ENABLED(CONFIG_64BIT) && CONFIG_RCU_FANOUT != 32))
pr_info("\tCONFIG_RCU_FANOUT set to non-default value of %d\n",
CONFIG_RCU_FANOUT);
- if (IS_ENABLED(CONFIG_RCU_FANOUT_EXACT))
+ if (rcu_fanout_exact)
pr_info("\tHierarchical RCU autobalancing is disabled.\n");
if (IS_ENABLED(CONFIG_RCU_FAST_NO_HZ))
pr_info("\tRCU dyntick-idle grace-period acceleration is enabled.\n");
--
1.8.1.5


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