Subject: [PATCH] latency-hist: indicate correct cpu of timer offset, if known
From: Carsten Emde <[email-masked]>
Date: Sun, 27 Nov 2016 20:16:15 +0100

The CPU of the missed timer offset histogram indicated the CPU
that was executing the timer interrupt rather than the CPU of
the task to switch to, since the latter may not be known. In
cases where it can be predicted correctly, since only a single
CPU is allowed, we now indicate this one.

Signed-off-by: Carsten Emde <[email-masked]>

---
 kernel/trace/latency_hist.c |   14 ++++++++++----
 1 file changed, 10 insertions(+), 4 deletions(-)

Index: linux-4.14.15-rt12/kernel/trace/latency_hist.c
===================================================================
--- linux-4.14.15-rt12.orig/kernel/trace/latency_hist.c
+++ linux-4.14.15-rt12/kernel/trace/latency_hist.c
@@ -1168,6 +1168,7 @@ static notrace void probe_hrtimer_interr
 	    (task->prio < curr->prio ||
 	    (task->prio == curr->prio &&
 	    !cpumask_test_cpu(cpu, task->cpus_ptr)))) {
+		int nextcpu = -1;
 		long latency;
 		u64 now;
 
@@ -1177,27 +1178,32 @@ static notrace void probe_hrtimer_interr
 				return;
 		}
 
+		if (cpumask_weight(task->cpus_ptr) == 1)
+			nextcpu = cpumask_first(task->cpus_ptr);
+
 #ifdef CONFIG_SWITCHTIME_HIST
-		if (!switchtime_pid) {
+		if (!switchtime_pid && nextcpu != -1) {
 			unsigned long flags;
 			struct task_struct *cpu_switchtime_task;
 
 			raw_spin_lock_irqsave(&switchtime_lock, flags);
-			cpu_switchtime_task = per_cpu(switchtime_task, cpu);
+			cpu_switchtime_task = per_cpu(switchtime_task, nextcpu);
 			if (cpu_switchtime_task &&
 			    task->prio < cpu_switchtime_task->prio) {
 				cpu_switchtime_task->switchtime_timestamp_hist =
 				    0;
 				put_task_struct(cpu_switchtime_task);
-				per_cpu(switchtime_task, cpu) = NULL;
+				per_cpu(switchtime_task, nextcpu) = NULL;
 			}
 			raw_spin_unlock_irqrestore(&switchtime_lock, flags);
 		}
 #endif
 
 		now = ftrace_now(cpu);
+		if (nextcpu == -1)
+			nextcpu = cpu;
 		latency = (long) div_s64(-latency_ns, NSECS_PER_USECS);
-		latency_hist(MISSED_TIMER_OFFSETS, cpu, latency, 0, 0, now,
+		latency_hist(MISSED_TIMER_OFFSETS, nextcpu, latency, 0, 0, now,
 		    task, -1);
 #ifdef CONFIG_WAKEUP_LATENCY_HIST
 		task->timer_offset = latency;