[PATCH tip/core/rcu 10/20] rcu: Provide diagnostic option to slow down grace-period initialization

Paul E. McKenney
2015-03-03T17:52:18Z

Grace-period initialization normally proceeds quite quickly, so
that it is very difficult to reproduce races against grace-period
initialization. This commit therefore allows grace-period
initialization to be artificially slowed down, increasing
race-reproduction probability. A pair of new Kconfig parameters are
provided, CONFIG_RCU_TORTURE_TEST_SLOW_INIT to enable the slowdowns, and
CONFIG_RCU_TORTURE_TEST_SLOW_INIT_DELAY to specify the number of jiffies
of slowdown to apply. A boot-time parameter named rcutree.gp_init_delay
allows boot-time delay to be specified. By default, no delay will be
applied even if CONFIG_RCU_TORTURE_TEST_SLOW_INIT is set.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
---
Documentation/kernel-parameters.txt | 6 ++++++
kernel/rcu/tree.c | 10 ++++++++++
lib/Kconfig.debug | 24 ++++++++++++++++++++++++
3 files changed, 40 insertions(+)

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index bfcb1a62a7b4..94de410ec341 100644
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2968,6 +2968,12 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
Set maximum number of finished RCU callbacks to
process in one batch.

+ rcutree.gp_init_delay= [KNL]
+ Set the number of jiffies to delay each step of
+ RCU grace-period initialization. This only has
+ effect when CONFIG_RCU_TORTURE_TEST_SLOW_INIT is
+ set.
+
rcutree.rcu_fanout_leaf= [KNL]
Increase the number of CPUs assigned to each
leaf rcu_node structure. Useful for very large
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 3b7e4133ca99..b42001fd55fb 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -160,6 +160,12 @@ static void invoke_rcu_callbacks(struct rcu_state *rsp, struct rcu_data *rdp);
static int kthread_prio = CONFIG_RCU_KTHREAD_PRIO;
module_param(kthread_prio, int, 0644);

+/* Delay in jiffies for grace-period initialization delays. */
+static int gp_init_delay = IS_ENABLED(CONFIG_RCU_TORTURE_TEST_SLOW_INIT)
+ ? CONFIG_RCU_TORTURE_TEST_SLOW_INIT_DELAY
+ : 0;
+module_param(gp_init_delay, int, 0644);
+
/*
* Track the rcutorture test sequence number and the update version
* number within a given test. The rcutorture_testseq is incremented
@@ -1769,6 +1775,10 @@ static int rcu_gp_init(struct rcu_state *rsp)
raw_spin_unlock_irq(&rnp->lock);
cond_resched_rcu_qs();
ACCESS_ONCE(rsp->gp_activity) = jiffies;
+ if (IS_ENABLED(CONFIG_RCU_TORTURE_TEST_SLOW_INIT) &&
+ gp_init_delay > 0 &&
+ !(rsp->gpnum % (rcu_num_nodes * 10)))
+ schedule_timeout_uninterruptible(gp_init_delay);
}

mutex_unlock(&rsp->onoff_mutex);
diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
index c5cefb3c009c..4ddfc5d09161 100644
--- a/lib/Kconfig.debug
+++ b/lib/Kconfig.debug
@@ -1257,6 +1257,30 @@ config RCU_TORTURE_TEST_RUNNABLE
Say N here if you want the RCU torture tests to start only
after being manually enabled via /proc.

+config RCU_TORTURE_TEST_SLOW_INIT
+ bool "Slow down RCU grace-period initialization to expose races"
+ depends on RCU_TORTURE_TEST
+ help
+ This option makes grace-period initialization block for a
+ few jiffies between initializing each pair of consecutive
+ rcu_node structures. This helps to expose races involving
+ grace-period initialization, in other words, it makes your
+ kernel less stable. It can also greatly increase grace-period
+ latency, especially on systems with large numbers of CPUs.
+ This is useful when torture-testing RCU, but in almost no
+ other circumstance.
+
+ Say Y here if you want your system to crash and hang more often.
+ Say N if you want a sane system.
+
+config RCU_TORTURE_TEST_SLOW_INIT_DELAY
+ int "How must to slow down RCU grace-period initialization"
+ range 0 5
+ default 0
+ help
+ This option specifies the number of jiffies to wait between
+ each rcu_node structure initialization.
+
config RCU_CPU_STALL_TIMEOUT
int "RCU CPU stall timeout in seconds"
depends on RCU_STALL_COMMON
--
1.8.1.5