From c9336643e1440f4dfc89ad4ac6185813619abb8c Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Wed, 18 Apr 2012 16:20:18 -0700 Subject: rcu: Clarify help text for RCU_BOOST_PRIO MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit The old text confused real-time applications with real-time threads, so that you pretty much needed to understand how this kernel configuration parameter worked to understand the help text. This commit therefore attempts to make the help text human-readable. Reported-by: Jörn Engel Signed-off-by: Paul E. McKenney Signed-off-by: Paul E. McKenney --- init/Kconfig | 23 +++++++++++++++++++---- 1 file changed, 19 insertions(+), 4 deletions(-) (limited to 'init') diff --git a/init/Kconfig b/init/Kconfig index 6cfd71d0646..85c6870ed47 100644 --- a/init/Kconfig +++ b/init/Kconfig @@ -515,10 +515,25 @@ config RCU_BOOST_PRIO depends on RCU_BOOST default 1 help - This option specifies the real-time priority to which preempted - RCU readers are to be boosted. If you are working with CPU-bound - real-time applications, you should specify a priority higher then - the highest-priority CPU-bound application. + This option specifies the real-time priority to which long-term + preempted RCU readers are to be boosted. If you are working + with a real-time application that has one or more CPU-bound + threads running at a real-time priority level, you should set + RCU_BOOST_PRIO to a priority higher then the highest-priority + real-time CPU-bound thread. The default RCU_BOOST_PRIO value + of 1 is appropriate in the common case, which is real-time + applications that do not have any CPU-bound threads. + + Some real-time applications might not have a single real-time + thread that saturates a given CPU, but instead might have + multiple real-time threads that, taken together, fully utilize + that CPU. In this case, you should set RCU_BOOST_PRIO to + a priority higher than the lowest-priority thread that is + conspiring to prevent the CPU from running any non-real-time + tasks. For example, if one thread at priority 10 and another + thread at priority 5 are between themselves fully consuming + the CPU time on a given CPU, then RCU_BOOST_PRIO should be + set to priority 6 or higher. Specify the real-time priority, or take the default if unsure. -- cgit v1.2.3 From 8932a63d5edb02f714d50c26583152fe0a97a69c Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Thu, 19 Apr 2012 12:20:14 -0700 Subject: 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 Reported-by: Dimitri Sivanich Signed-off-by: Paul E. McKenney --- init/Kconfig | 27 +++++++++++++++++++++++++++ kernel/rcutree.c | 2 +- kernel/rcutree.h | 10 +++------- 3 files changed, 31 insertions(+), 8 deletions(-) (limited to 'init') 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 /* - * 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) -- cgit v1.2.3