[PATCH 1/3] rcu: genericize RCU stall suppression functions

[Robert Elliott]

Convert the functions that temporarily suppress RCU CPU
stall reporting:
	rcu_sysrq_start()
	rcu_sysrq_end()

to more generic functions that may be called by functions
other than the SysRq handler:
	rcu_suppress_start()
	rcu_suppress_end()

Covert the underlying variable:
	rcu_cpu_stall_suppress

to an atomic variable so multiple threads can manipulate it at the
same time, incrementing it in start() and decrementing in end().

Expose that in sysfs in a read-only
	/sys/module/rcupdate/parameters/rcu_cpu_stall_suppress_dyn

Keep
	/sys/module/rcupdate/parameters/rcu_cpu_stall_suppress
as writeable by userspace, but OR that into the equations rather than
directly manipulate the atomic value.

Signed-off-by: Robert Elliott <elliott@xxxxxxx>
---
 .../RCU/Design/Requirements/Requirements.rst  |  6 +++---
 Documentation/RCU/stallwarn.rst               | 19 +++++++++++++++----
 arch/parisc/kernel/process.c                  |  2 +-
 drivers/tty/sysrq.c                           |  4 ++--
 include/linux/rcupdate.h                      |  8 ++++----
 kernel/rcu/rcu.h                              | 11 ++++++-----
 kernel/rcu/tree_stall.h                       | 18 ++++++++++--------
 kernel/rcu/update.c                           | 11 ++++++++++-
 8 files changed, 51 insertions(+), 28 deletions(-)

diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
index 49387d823619..5083490bb6fc 100644
--- a/Documentation/RCU/Design/Requirements/Requirements.rst
+++ b/Documentation/RCU/Design/Requirements/Requirements.rst
@@ -1639,9 +1639,9 @@ against mishaps and misuse:
    ``rcupdate.rcu_cpu_stall_suppress`` to suppress the splats. This
    kernel parameter may also be set via ``sysfs``. Furthermore, RCU CPU
    stall warnings are counter-productive during sysrq dumps and during
-   panics. RCU therefore supplies the rcu_sysrq_start() and
-   rcu_sysrq_end() API members to be called before and after long
-   sysrq dumps. RCU also supplies the rcu_panic() notifier that is
+   panics. RCU therefore supplies the rcu_suppress_start() and
+   rcu_suppress_end() API members to be called before and after long
+   CPU usage. RCU also supplies the rcu_panic() notifier that is
    automatically invoked at the beginning of a panic to suppress further
    RCU CPU stall warnings.
 
diff --git a/Documentation/RCU/stallwarn.rst b/Documentation/RCU/stallwarn.rst
index e38c587067fc..9e6fba72f56d 100644
--- a/Documentation/RCU/stallwarn.rst
+++ b/Documentation/RCU/stallwarn.rst
@@ -135,13 +135,24 @@ see include/trace/events/rcu.h.
 Fine-Tuning the RCU CPU Stall Detector
 ======================================
 
-The rcuupdate.rcu_cpu_stall_suppress module parameter disables RCU's
-CPU stall detector, which detects conditions that unduly delay RCU grace
-periods.  This module parameter enables CPU stall detection by default,
-but may be overridden via boot-time parameter or at runtime via sysfs.
+RCU's CPU stall detector detects conditions that unduly delay RCU grace
+periods.  CPU stall detection is enabled by default, but may be overridden
+via boot-time parameter or at runtime via sysfs (accessible via
+/sys/module/rcupdate/parameters).
+
+The rcupdate.rcu_cpu_stall_suppress module parameter disables RCU's
+CPU stall detector.
+
+/sys/module/rcu_cpu_stall_suppress_dyn reports an internal semaphore
+used by the RCU's CPU stall detector. Functions holding a CPU for a
+long time (e.g., sysrq printouts) increment this value before use
+and decrement it when done, so the value reports the number of
+functions currently disabling stalls.
+
 The stall detector's idea of what constitutes "unduly delayed" is
 controlled by a set of kernel configuration variables and cpp macros:
 
+
 CONFIG_RCU_CPU_STALL_TIMEOUT
 ----------------------------
 
diff --git a/arch/parisc/kernel/process.c b/arch/parisc/kernel/process.c
index c4f8374c7018..038378fe7bfa 100644
--- a/arch/parisc/kernel/process.c
+++ b/arch/parisc/kernel/process.c
@@ -126,7 +126,7 @@ void machine_power_off(void)
 	       "Please power this system off now.");
 
 	/* prevent soft lockup/stalled CPU messages for endless loop. */
-	rcu_sysrq_start();
+	rcu_suppress_start();
 	lockup_detector_soft_poweroff();
 	for (;;);
 }
diff --git a/drivers/tty/sysrq.c b/drivers/tty/sysrq.c
index d2b2720db6ca..81ab63a473a7 100644
--- a/drivers/tty/sysrq.c
+++ b/drivers/tty/sysrq.c
@@ -579,7 +579,7 @@ void __handle_sysrq(int key, bool check_mask)
 	orig_suppress_printk = suppress_printk;
 	suppress_printk = 0;
 
-	rcu_sysrq_start();
+	rcu_suppress_start();
 	rcu_read_lock();
 	/*
 	 * Raise the apparent loglevel to maximum so that the sysrq header
@@ -623,7 +623,7 @@ void __handle_sysrq(int key, bool check_mask)
 		console_loglevel = orig_log_level;
 	}
 	rcu_read_unlock();
-	rcu_sysrq_end();
+	rcu_suppress_end();
 
 	suppress_printk = orig_suppress_printk;
 }
diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 03abf883a281..c0d8a4aae7ff 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -131,11 +131,11 @@ static inline void rcu_init_tasks_generic(void) { }
 #endif
 
 #ifdef CONFIG_RCU_STALL_COMMON
-void rcu_sysrq_start(void);
-void rcu_sysrq_end(void);
+void rcu_suppress_start(void);
+void rcu_suppress_end(void);
 #else /* #ifdef CONFIG_RCU_STALL_COMMON */
-static inline void rcu_sysrq_start(void) { }
-static inline void rcu_sysrq_end(void) { }
+static inline void rcu_suppress_start(void) { }
+static inline void rcu_suppress_end(void) { }
 #endif /* #else #ifdef CONFIG_RCU_STALL_COMMON */
 
 #if defined(CONFIG_NO_HZ_FULL) && (!defined(CONFIG_GENERIC_ENTRY) || !defined(CONFIG_KVM_XFER_TO_GUEST_WORK))
diff --git a/kernel/rcu/rcu.h b/kernel/rcu/rcu.h
index c5aa934de59b..a658955a1174 100644
--- a/kernel/rcu/rcu.h
+++ b/kernel/rcu/rcu.h
@@ -224,24 +224,25 @@ 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;
+extern atomic_t rcu_cpu_stall_suppress_dyn;
 int rcu_jiffies_till_stall_check(void);
 int rcu_exp_jiffies_till_stall_check(void);
 
 static inline bool rcu_stall_is_suppressed(void)
 {
-	return rcu_stall_is_suppressed_at_boot() || rcu_cpu_stall_suppress;
+	return rcu_stall_is_suppressed_at_boot() ||
+	       rcu_cpu_stall_suppress ||
+	       atomic_read(&rcu_cpu_stall_suppress_dyn);
 }
 
 #define rcu_ftrace_dump_stall_suppress() \
 do { \
-	if (!rcu_cpu_stall_suppress) \
-		rcu_cpu_stall_suppress = 3; \
+	atomic_inc(&rcu_cpu_stall_suppress_dyn); \
 } while (0)
 
 #define rcu_ftrace_dump_stall_unsuppress() \
 do { \
-	if (rcu_cpu_stall_suppress == 3) \
-		rcu_cpu_stall_suppress = 0; \
+	atomic_dec(&rcu_cpu_stall_suppress_dyn); \
 } while (0)
 
 #else /* #endif #ifdef CONFIG_RCU_STALL_COMMON */
diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
index 5653560573e2..260748bc5bc8 100644
--- a/kernel/rcu/tree_stall.h
+++ b/kernel/rcu/tree_stall.h
@@ -103,23 +103,25 @@ bool rcu_gp_might_be_stalled(void)
 	return !time_before(j, READ_ONCE(rcu_state.gp_start) + d);
 }
 
-/* Don't do RCU CPU stall warnings during long sysrq printouts. */
-void rcu_sysrq_start(void)
+/* Don't do RCU CPU stall warnings during functions holding the CPU
+ * for a long period of time such as long sysrq printouts.
+ */
+void rcu_suppress_start(void)
 {
-	if (!rcu_cpu_stall_suppress)
-		rcu_cpu_stall_suppress = 2;
+	atomic_inc(&rcu_cpu_stall_suppress_dyn);
 }
+EXPORT_SYMBOL_GPL(rcu_suppress_start);
 
-void rcu_sysrq_end(void)
+void rcu_suppress_end(void)
 {
-	if (rcu_cpu_stall_suppress == 2)
-		rcu_cpu_stall_suppress = 0;
+	atomic_dec(&rcu_cpu_stall_suppress_dyn);
 }
+EXPORT_SYMBOL_GPL(rcu_suppress_end);
 
 /* Don't print RCU CPU stall warnings during a kernel panic. */
 static int rcu_panic(struct notifier_block *this, unsigned long ev, void *ptr)
 {
-	rcu_cpu_stall_suppress = 1;
+	atomic_inc(&rcu_cpu_stall_suppress_dyn);
 	return NOTIFY_DONE;
 }
 
diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
index f5e6a2f95a2a..ceee9d777384 100644
--- a/kernel/rcu/update.c
+++ b/kernel/rcu/update.c
@@ -501,11 +501,18 @@ EXPORT_SYMBOL_GPL(rcutorture_sched_setaffinity);
 #ifdef CONFIG_RCU_STALL_COMMON
 int rcu_cpu_stall_ftrace_dump __read_mostly;
 module_param(rcu_cpu_stall_ftrace_dump, int, 0644);
-int rcu_cpu_stall_suppress __read_mostly; // !0 = suppress stall warnings.
+
+int rcu_cpu_stall_suppress __read_mostly; // !0 = suppress stall warnings from sysfs
 EXPORT_SYMBOL_GPL(rcu_cpu_stall_suppress);
 module_param(rcu_cpu_stall_suppress, int, 0644);
+
+atomic_t rcu_cpu_stall_suppress_dyn __read_mostly; // !0 = suppress stall warnings from functions
+EXPORT_SYMBOL_GPL(rcu_cpu_stall_suppress_dyn);
+module_param_named(rcu_cpu_stall_suppress_dyn, rcu_cpu_stall_suppress_dyn.counter, int, 0444);
+
 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 */
@@ -616,6 +623,8 @@ void __init rcupdate_announce_bootup_oddness(void)
 		pr_info("\tAll grace periods are expedited (rcu_expedited).\n");
 	if (rcu_cpu_stall_suppress)
 		pr_info("\tRCU CPU stall warnings suppressed (rcu_cpu_stall_suppress).\n");
+	if (atomic_read(&rcu_cpu_stall_suppress_dyn))
+		pr_info("\tDynamic RCU CPU stall warnings suppressed (rcu_cpu_stall_suppress_dyn).\n");
 	if (rcu_cpu_stall_timeout != CONFIG_RCU_CPU_STALL_TIMEOUT)
 		pr_info("\tRCU CPU stall warnings timeout set to %d (rcu_cpu_stall_timeout).\n", rcu_cpu_stall_timeout);
 	rcu_tasks_bootup_oddness();
-- 
2.38.1