[RFC][PATCH] tracing: Introduce restart_critical_timings()

[Steven Rostedt]

From: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

I'm debugging some latency issues on a Chromebook and the preemptirqsoff
tracer hit this:

# tracer: preemptirqsoff
#
# preemptirqsoff latency trace v1.1.5 on 5.15.148-21853-g165fd2387469-dirty
# --------------------------------------------------------------------
# latency: 7813 us, #60/60, CPU#1 | (M:preempt VP:0, KP:0, SP:0 HP:0 #P:2)
#    -----------------
#    | task: init-1 (uid:0 nice:0 policy:0 rt_prio:0)
#    -----------------
#  => started at: rwsem_optimistic_spin
#  => ended at:   rwsem_optimistic_spin
#
#
#                    _------=> CPU#            
#                   / _-----=> irqs-off        
#                  | / _----=> need-resched    
#                  || / _---=> hardirq/softirq 
#                  ||| / _--=> preempt-depth   
#                  |||| / _-=> migrate-disable 
#                  ||||| /     delay           
#  cmd     pid     |||||| time  |   caller     
#     \   /        ||||||  \    |    /       
   <...>-1         1...1.    0us!: rwsem_optimistic_spin+0x20/0x194 <-rwsem_optimistic_spin+0x20/0x194
   <...>-1         1.N.1. 7813us : rwsem_optimistic_spin+0x140/0x194 <-rwsem_optimistic_spin+0x140/0x194
   <...>-1         1.N.1. 7814us+: tracer_preempt_on+0x4c/0x6a <-rwsem_optimistic_spin+0x140/0x194
   <...>-1         1.N.1. 7824us : <stack trace>
 => rwsem_optimistic_spin+0x140/0x194
 => rwsem_down_write_slowpath+0xc9/0x3fe
 => copy_process+0xd08/0x1b8a
 => kernel_clone+0x94/0x256
 => __x64_sys_clone+0x7a/0x9a
 => do_syscall_64+0x51/0xa1
 => entry_SYSCALL_64_after_hwframe+0x5c/0xc6

Which isn't a real issue, as it's in the rwsem_optimistic_spin() code that
spins with interrupts enabled, preempt disabled, and checks for
need_resched(). If it is true, it breaks out and schedules.

Hence, it's hiding real issues, because it can spin for a very long time
and this is not the source of the latency I'm tracking down.

I would like to introduce restart_critical_timings() and place it in
locations that have this behavior.

Signed-off-by: Steven Rostedt (Google) <rostedt@xxxxxxxxxxx>

diff --git a/include/linux/irqflags.h b/include/linux/irqflags.h
index 147feebd508c..e9f97f60bfc0 100644
--- a/include/linux/irqflags.h
+++ b/include/linux/irqflags.h
@@ -145,6 +145,13 @@ do {						\
 # define start_critical_timings() do { } while (0)
 #endif
 
+/* Used in spins that check need_resched() with preemption disabled */
+static inline void restart_critical_timings(void)
+{
+	stop_critical_timings();
+	start_critical_timings();
+}
+
 #ifdef CONFIG_DEBUG_IRQFLAGS
 extern void warn_bogus_irq_restore(void);
 #define raw_check_bogus_irq_restore()			\
diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
index 2340b6d90ec6..fa7b43e53d20 100644
--- a/kernel/locking/rwsem.c
+++ b/kernel/locking/rwsem.c
@@ -27,6 +27,7 @@
 #include <linux/export.h>
 #include <linux/rwsem.h>
 #include <linux/atomic.h>
+#include <linux/irqflags.h>
 #include <trace/events/lock.h>
 
 #ifndef CONFIG_PREEMPT_RT
@@ -780,6 +781,7 @@ rwsem_spin_on_owner(struct rw_semaphore *sem)
 		 */
 		barrier();
 
+		restart_critical_timings();
 		if (need_resched() || !owner_on_cpu(owner)) {
 			state = OWNER_NONSPINNABLE;
 			break;
@@ -912,6 +914,7 @@ static bool rwsem_optimistic_spin(struct rw_semaphore *sem)
 		 * a writer, need_resched() check needs to be done here.
 		 */
 		if (owner_state != OWNER_WRITER) {
+			restart_critical_timings();
 			if (need_resched())
 				break;
 			if (rt_task(current) &&