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authorPaul E. McKenney <paulmck@linux.vnet.ibm.com>2012-04-19 12:20:14 -0700
committerPaul E. McKenney <paulmck@linux.vnet.ibm.com>2012-04-24 20:54:52 -0700
commit8932a63d5edb02f714d50c26583152fe0a97a69c (patch)
treeec71159908f1a78eb21e736d284ffe7ed7584b6c
parentd8169d4c369e8aa2fda10df705a4957331b5a4db (diff)
rcu: Reduce cache-miss initialization latencies for large systems
Commit #0209f649 (rcu: limit rcu_node leaf-level fanout) set an upper limit of 16 on the leaf-level fanout for the rcu_node tree. This was needed to reduce lock contention that was induced by the synchronization of scheduling-clock interrupts, which was in turn needed to improve energy efficiency for moderate-sized lightly loaded servers. However, reducing the leaf-level fanout means that there are more leaf-level rcu_node structures in the tree, which in turn means that RCU's grace-period initialization incurs more cache misses. This is not a problem on moderate-sized servers with only a few tens of CPUs, but becomes a major source of real-time latency spikes on systems with many hundreds of CPUs. In addition, the workloads running on these large systems tend to be CPU-bound, which eliminates the energy-efficiency advantages of synchronizing scheduling-clock interrupts. Therefore, these systems need maximal values for the rcu_node leaf-level fanout. This commit addresses this problem by introducing a new kernel parameter named RCU_FANOUT_LEAF that directly controls the leaf-level fanout. This parameter defaults to 16 to handle the common case of a moderate sized lightly loaded servers, but may be set higher on larger systems. Reported-by: Mike Galbraith <efault@gmx.de> Reported-by: Dimitri Sivanich <sivanich@sgi.com> Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
-rw-r--r--init/Kconfig27
-rw-r--r--kernel/rcutree.c2
-rw-r--r--kernel/rcutree.h10
3 files changed, 31 insertions, 8 deletions
diff --git a/init/Kconfig b/init/Kconfig
index 85c6870ed47..6d18ef8071b 100644
--- a/init/Kconfig
+++ b/init/Kconfig
@@ -458,6 +458,33 @@ config RCU_FANOUT
Select a specific number if testing RCU itself.
Take the default if unsure.
+config RCU_FANOUT_LEAF
+ int "Tree-based hierarchical RCU leaf-level fanout value"
+ range 2 RCU_FANOUT if 64BIT
+ range 2 RCU_FANOUT if !64BIT
+ depends on TREE_RCU || TREE_PREEMPT_RCU
+ default 16
+ help
+ This option controls the leaf-level fanout of hierarchical
+ implementations of RCU, and allows trading off cache misses
+ against lock contention. Systems that synchronize their
+ scheduling-clock interrupts for energy-efficiency reasons will
+ want the default because the smaller leaf-level fanout keeps
+ lock contention levels acceptably low. Very large systems
+ (hundreds or thousands of CPUs) will instead want to set this
+ value to the maximum value possible in order to reduce the
+ number of cache misses incurred during RCU's grace-period
+ initialization. These systems tend to run CPU-bound, and thus
+ are not helped by synchronized interrupts, and thus tend to
+ skew them, which reduces lock contention enough that large
+ leaf-level fanouts work well.
+
+ Select a specific number if testing RCU itself.
+
+ Select the maximum permissible value for large systems.
+
+ Take the default if unsure.
+
config RCU_FANOUT_EXACT
bool "Disable tree-based hierarchical RCU auto-balancing"
depends on TREE_RCU || TREE_PREEMPT_RCU
diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index 1050d6d3922..780acf8e15e 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -2418,7 +2418,7 @@ static void __init rcu_init_levelspread(struct rcu_state *rsp)
for (i = NUM_RCU_LVLS - 1; i > 0; i--)
rsp->levelspread[i] = CONFIG_RCU_FANOUT;
- rsp->levelspread[0] = RCU_FANOUT_LEAF;
+ rsp->levelspread[0] = CONFIG_RCU_FANOUT_LEAF;
}
#else /* #ifdef CONFIG_RCU_FANOUT_EXACT */
static void __init rcu_init_levelspread(struct rcu_state *rsp)
diff --git a/kernel/rcutree.h b/kernel/rcutree.h
index cdd1be0a407..a905c200405 100644
--- a/kernel/rcutree.h
+++ b/kernel/rcutree.h
@@ -29,18 +29,14 @@
#include <linux/seqlock.h>
/*
- * Define shape of hierarchy based on NR_CPUS and CONFIG_RCU_FANOUT.
+ * Define shape of hierarchy based on NR_CPUS, CONFIG_RCU_FANOUT, and
+ * CONFIG_RCU_FANOUT_LEAF.
* In theory, it should be possible to add more levels straightforwardly.
* In practice, this did work well going from three levels to four.
* Of course, your mileage may vary.
*/
#define MAX_RCU_LVLS 4
-#if CONFIG_RCU_FANOUT > 16
-#define RCU_FANOUT_LEAF 16
-#else /* #if CONFIG_RCU_FANOUT > 16 */
-#define RCU_FANOUT_LEAF (CONFIG_RCU_FANOUT)
-#endif /* #else #if CONFIG_RCU_FANOUT > 16 */
-#define RCU_FANOUT_1 (RCU_FANOUT_LEAF)
+#define RCU_FANOUT_1 (CONFIG_RCU_FANOUT_LEAF)
#define RCU_FANOUT_2 (RCU_FANOUT_1 * CONFIG_RCU_FANOUT)
#define RCU_FANOUT_3 (RCU_FANOUT_2 * CONFIG_RCU_FANOUT)
#define RCU_FANOUT_4 (RCU_FANOUT_3 * CONFIG_RCU_FANOUT)