X-Git-Url: https://git.lttng.org/?a=blobdiff_plain;f=ltt%2Fbranches%2Fpoly%2Fdoc%2Fdeveloper%2Ftime-monotonic-accurate.txt;h=52de9ae84b3ae044298bdd16f39c85afd8274e50;hb=0c8bbfb74fe44e05e943223c0f697cb2ae0a3d38;hp=c1bc89d4d267a9a13550c056f42d943728c9e947;hpb=d8ce0927bf851fadc23714ad3a242a347c6431bc;p=lttv.git

diff --git a/ltt/branches/poly/doc/developer/time-monotonic-accurate.txt b/ltt/branches/poly/doc/developer/time-monotonic-accurate.txt
index c1bc89d4..52de9ae8 100644
--- a/ltt/branches/poly/doc/developer/time-monotonic-accurate.txt
+++ b/ltt/branches/poly/doc/developer/time-monotonic-accurate.txt
@@ -27,6 +27,9 @@ static struct time_struct {
 
 DECLARE_PERCPU(struct time_struct, cpu_time);
 
+/* Number of times the scheduler is called on each CPU */
+DECLARE_PERCPU(unsigned long, sched_nr);
+
 /* On frequency change event */
 /* In irq context */
 void freq_change_cb(unsigned int new_freq)
@@ -83,20 +86,32 @@ wake_from_hlt()
 /* If the update_count changes while we read the context, it may be invalid.
  * This would happen if we are scheduled out for a period of time long enough to
  * permit 2 frequency changes. We simply start the loop again if it happens.
- * We detect it by comparing the update_count running counter. */
+ * We detect it by comparing the update_count running counter.
+ * We detect preemption by incrementing a counter sched_nr within schedule(). 
+ * This counter is readable by user space through the vsyscall page. */
+ */
 u64 read_time(void)
 {
 	u64 walltime;
 	long update_count;
-	struct time_struct this_cpu_time = 
-		per_cpu(cpu_time, smp_processor_id());
+	struct time_struct this_cpu_time;
 	struct time_info *current_time;
+	unsigned int cpu;
+	long prev_sched_nr;
 	do {
+		cpu = _smp_processor_id();
+		prev_sched_nr = per_cpu(sched_nr, cpu);
+		if(cpu != _smp_processor_id())
+			continue;	/* changed CPU between CPUID and getting
+					   sched_nr */
+		this_cpu_time = per_cpu(cpu_time, cpu);
 		update_count = this_cpu_time->update_count;
 		current_time = this_cpu_time->time_sel[update_count&1];
 		walltime = current_time->walltime + 
 				(get_cycles() - current_time->tsc) /
 				current_time->freq;
+		if(per_cpu(sched_nr, cpu) != prev_sched_nr)
+			continue;	/* been preempted */
 	} while(this_cpu_time->update_count != update_count);
 	return walltime;
 }