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rcu: Convert CONFIG_RCU_FANOUT_EXACT to boot parameter

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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>
Reviewed-by: Pranith Kumar <bobby.prani@gmail.com>
This commit is contained in:
Paul E. McKenney 2015-04-20 10:27:15 -07:00
parent 7db21edfec
commit 7fa270010e
4 changed files with 12 additions and 17 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

6
Documentation/kernel-parameters.txt
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@ -3010,6 +3010,12 @@ 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

14
init/Kconfig
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@ -611,20 +611,6 @@ 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

7
kernel/rcu/tree.c
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@ -113,6 +113,9 @@ RCU_STATE_INITIALIZER(rcu_bh, 'b', call_rcu_bh);
static struct rcu_state *rcu_state_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);
@ -3956,13 +3959,13 @@ 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(struct rcu_state *rsp)
{
int i;
if (IS_ENABLED(CONFIG_RCU_FANOUT_EXACT)) {
if (rcu_fanout_exact) {
rsp->levelspread[rcu_num_lvls - 1] = rcu_fanout_leaf;
for (i = rcu_num_lvls - 2; i >= 0; i--)
rsp->levelspread[i] = CONFIG_RCU_FANOUT;

2
kernel/rcu/tree_plugin.h
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@ -64,7 +64,7 @@ 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");