From: Uladzislau Rezki <uladzislau.rezki@xxxxxxxx>
Currently for both expedited and regular grace periods stall
warnings are emitted based on one timeout value that is defined
in seconds. The problem is that, a stall timeout in seconds for
expedited grace period is a way long.
The idea of expedited grace period is to force CPUs to report
their quiescent states as fast as possible. If in RCU read-side
critical section, it will request the next rcu_read_unlock() to
record the quiescent state. If not either it reports immediately
or set TIF_NEED_RESCHED to initiate the task switch.
Therefore introduce the CONFIG_RCU_EXP_CPU_STALL_TIMEOUT kernel
configuration that is set to 20 msec. It also can be changed in
run-time via: /sys/.../parameters/rcu_exp_cpu_stall_timeout.
Signed-off-by: Uladzislau Rezki <uladzislau.rezki@xxxxxxxx>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@xxxxxxxxx>
---
Documentation/RCU/stallwarn.rst | 18 +++++++++++++
.../admin-guide/kernel-parameters.txt | 6 +++++
kernel/rcu/Kconfig.debug | 12 +++++++++
kernel/rcu/rcu.h | 2 ++
kernel/rcu/tree_exp.h | 4 +--
kernel/rcu/tree_stall.h | 26 +++++++++++++++++++
kernel/rcu/update.c | 2 ++
7 files changed, 68 insertions(+), 2 deletions(-)
diff --git a/Documentation/RCU/stallwarn.rst b/Documentation/RCU/stallwarn.rst
index 78404625bad2..b9ce89980779 100644
--- a/Documentation/RCU/stallwarn.rst
+++ b/Documentation/RCU/stallwarn.rst
@@ -162,6 +162,24 @@ CONFIG_RCU_CPU_STALL_TIMEOUT
Stall-warning messages may be enabled and disabled completely via
/sys/module/rcupdate/parameters/rcu_cpu_stall_suppress.
+CONFIG_RCU_EXP_CPU_STALL_TIMEOUT
+--------------------------------
+
+ Same as the CONFIG_RCU_CPU_STALL_TIMEOUT parameter but only for
+ the expedited grace period. This parameter defines the period of
+ time that RCU will wait from the beginning of an expedited grace
+ period until it issues an RCU CPU stall warning. This time period
+ is normally 20 milliseconds.
+
+ This configuration parameter may be changed at runtime via the
+ /sys/module/rcupdate/parameters/rcu_exp_cpu_stall_timeout, however
+ this parameter is checked only at the beginning of a cycle. If you
+ are in a current stall cycle, setting it to a new value will change
+ the timeout for the -next- stall.
+
+ Stall-warning messages may be enabled and disabled completely via
+ /sys/module/rcupdate/parameters/rcu_cpu_stall_suppress.
+
RCU_STALL_DELAY_DELTA
---------------------
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 9ab6a4a5be06..9a32030f5d1e 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4817,6 +4817,12 @@
rcupdate.rcu_cpu_stall_timeout= [KNL]
Set timeout for RCU CPU stall warning messages.
+ rcupdate.rcu_exp_cpu_stall_timeout= [KNL]
+ Set timeout for expedited RCU CPU stall warning messages.
+ The value is in milliseconds and a maximum allowed parameter
+ is 21000 milliseconds. Please note, a set value is adjusted
+ to an arch timer tick resolution.
+
rcupdate.rcu_expedited= [KNL]
Use expedited grace-period primitives, for
example, synchronize_rcu_expedited() instead
diff --git a/kernel/rcu/Kconfig.debug b/kernel/rcu/Kconfig.debug
index 4fd64999300f..e0f2c5edef44 100644
--- a/kernel/rcu/Kconfig.debug
+++ b/kernel/rcu/Kconfig.debug
@@ -91,6 +91,18 @@ config RCU_CPU_STALL_TIMEOUT
RCU grace period persists, additional CPU stall warnings are
printed at more widely spaced intervals.
+config RCU_EXP_CPU_STALL_TIMEOUT
+ int "Expedited RCU CPU stall timeout in milliseconds"
+ depends on RCU_STALL_COMMON
+ range 1 21000
+ default 20
+
+ help
+ If a given expedited RCU grace period extends more than the
+ specified number of milliseconds, a CPU stall warning is printed.
+ If the RCU grace period persists, additional CPU stall warnings
+ are printed at more widely spaced intervals.
+
config RCU_TRACE
bool "Enable tracing for RCU"
depends on DEBUG_KERNEL
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index 819f9d979e61..f8ec1df4b91e 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -217,7 +217,9 @@ static inline bool rcu_stall_is_suppressed_at_boot(void)
extern int rcu_cpu_stall_ftrace_dump;
extern int rcu_cpu_stall_suppress;
extern int rcu_cpu_stall_timeout;
+extern int rcu_exp_cpu_stall_timeout;
int rcu_jiffies_till_stall_check(void);
+int rcu_exp_jiffies_till_stall_check(void);
static inline bool rcu_stall_is_suppressed(void)
{
diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
index 404198808683..30f05fd080ca 100644
--- a/kernel/rcu/tree_exp.h
+++ b/kernel/rcu/tree_exp.h
@@ -496,7 +496,7 @@ static void synchronize_rcu_expedited_wait(void)
struct rcu_node *rnp_root = rcu_get_root();
trace_rcu_exp_grace_period(rcu_state.name, rcu_exp_gp_seq_endval(), TPS("startwait"));
- jiffies_stall = rcu_jiffies_till_stall_check();
+ jiffies_stall = rcu_exp_jiffies_till_stall_check();
jiffies_start = jiffies;
if (tick_nohz_full_enabled() && rcu_inkernel_boot_has_ended()) {
if (synchronize_rcu_expedited_wait_once(1))
@@ -571,7 +571,7 @@ static void synchronize_rcu_expedited_wait(void)
dump_cpu_task(cpu);
}
}
- jiffies_stall = 3 * rcu_jiffies_till_stall_check() + 3;
+ jiffies_stall = 3 * rcu_exp_jiffies_till_stall_check() + 3;
}
}
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 0c5d8516516a..84b812a3ab44 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -25,6 +25,32 @@ int sysctl_max_rcu_stall_to_panic __read_mostly;
#define RCU_STALL_MIGHT_DIV 8
#define RCU_STALL_MIGHT_MIN (2 * HZ)
+int rcu_exp_jiffies_till_stall_check(void)
+{
+ int cpu_stall_timeout = READ_ONCE(rcu_exp_cpu_stall_timeout);
+ int exp_stall_delay_delta = 0;
+ int till_stall_check;
+
+ /*
+ * Limit check must be consistent with the Kconfig limits for
+ * CONFIG_RCU_EXP_CPU_STALL_TIMEOUT, so check the allowed range.
+ * The minimum clamped value is "2UL", because at least one full
+ * tick has to be guaranteed.
+ */
+ till_stall_check = clamp(msecs_to_jiffies(cpu_stall_timeout), 2UL, 21UL * HZ);
+
+ if (jiffies_to_msecs(till_stall_check) != cpu_stall_timeout)
+ WRITE_ONCE(rcu_exp_cpu_stall_timeout, jiffies_to_msecs(till_stall_check));
+
+#ifdef CONFIG_PROVE_RCU
+ /* Add extra ~25% out of till_stall_check. */
+ exp_stall_delay_delta = ((till_stall_check * 25) / 100) + 1;
+#endif
+
+ return till_stall_check + exp_stall_delay_delta;
+}
+EXPORT_SYMBOL_GPL(rcu_exp_jiffies_till_stall_check);
+
/* Limit-check stall timeouts specified at boottime and runtime. */
int rcu_jiffies_till_stall_check(void)
{
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index 180ff9c41fa8..fc7fef575606 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -506,6 +506,8 @@ EXPORT_SYMBOL_GPL(rcu_cpu_stall_suppress);
module_param(rcu_cpu_stall_suppress, int, 0644);
int rcu_cpu_stall_timeout __read_mostly = CONFIG_RCU_CPU_STALL_TIMEOUT;
module_param(rcu_cpu_stall_timeout, int, 0644);
+int rcu_exp_cpu_stall_timeout __read_mostly = CONFIG_RCU_EXP_CPU_STALL_TIMEOUT;
+module_param(rcu_exp_cpu_stall_timeout, int, 0644);
#endif /* #ifdef CONFIG_RCU_STALL_COMMON */
// Suppress boot-time RCU CPU stall warnings and rcutorture writer stall
--
2.30.2
