Commit 7fa27001 authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

rcu: Convert CONFIG_RCU_FANOUT_EXACT to boot parameter 



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: default avatarIngo Molnar <mingo@kernel.org>
Signed-off-by: default avatarPaul E. McKenney <paulmck@linux.vnet.ibm.com>
Reviewed-by: default avatarPranith Kumar <bobby.prani@gmail.com>
parent 7db21edf

Documentation/kernel-parameters.txt

+6 −0
Original line number Diff line number Diff line
@@ -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

init/Kconfig

+0 −14
Original line number Diff line number Diff line
@@ -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

kernel/rcu/tree.c

+5 −2
Original line number Diff line number Diff line
@@ -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;

kernel/rcu/tree_plugin.h

+1 −1
Original line number Diff line number Diff line
@@ -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");

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