[PATCH v2] tracing/hwlat: Do the latency calculations in nanoseconds

[Wenwu Hou <hwenwur@gmail.com>]

tr->max_latency is expected to hold nanoseconds: tracing_max_lat_read()
converts it with nsecs_to_usecs() to print the usecs documented in
hwlat_detector.rst, and the other latency tracers store nanoseconds there
too.

Do all the latency calculations in nanoseconds instead, and only convert
the durations to microseconds when recording the hwlat event.

Fixes: e7c15cd8a1133 ("tracing: Added hardware latency tracer")
Suggested-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Wenwu Hou <hwenwur@gmail.com>
---
Tested in a 4-vCPU KVM guest; all three hwlat modes worked as expected.

Changes in v2:
- Perform the latency calculations in nanoseconds instead of converting
  only the maximum latency, as suggested by Steven Rostedt.
- Convert the durations to microseconds only when recording the hwlat event
  to preserve the existing event format.

 kernel/trace/trace_hwlat.c | 14 ++++++--------
 1 file changed, 6 insertions(+), 8 deletions(-)

diff --git a/kernel/trace/trace_hwlat.c b/kernel/trace/trace_hwlat.c
index 65e184d800ff1..a16905ac12b0d 100644
--- a/kernel/trace/trace_hwlat.c
+++ b/kernel/trace/trace_hwlat.c
@@ -203,14 +203,12 @@ static int get_sample(void)
 	time_type start, t1, t2, last_t2;
 	s64 diff, outer_diff, total, last_total = 0;
 	u64 sample = 0;
-	u64 sample_width = READ_ONCE(hwlat_data.sample_width);
+	u64 sample_width = READ_ONCE(hwlat_data.sample_width) * NSEC_PER_USEC;
 	u64 thresh = tracing_thresh;
 	u64 outer_sample = 0;
 	int ret = -1;
 	unsigned int count = 0;
 
-	do_div(thresh, NSEC_PER_USEC); /* modifies interval value */
-
 	kdata->nmi_total_ts = 0;
 	kdata->nmi_count = 0;
 	/* Make sure NMIs see this first */
@@ -229,7 +227,7 @@ static int get_sample(void)
 
 		if (time_u64(last_t2)) {
 			/* Check the delta from outer loop (t2 to next t1) */
-			outer_diff = time_to_us(time_sub(t1, last_t2));
+			outer_diff = time_sub(t1, last_t2);
 			/* This shouldn't happen */
 			if (outer_diff < 0) {
 				hwlat_err(BANNER "time running backwards\n");
@@ -240,7 +238,7 @@ static int get_sample(void)
 		}
 		last_t2 = t2;
 
-		total = time_to_us(time_sub(t2, start)); /* sample width */
+		total = time_sub(t2, start); /* sample width */
 
 		/* Check for possible overflows */
 		if (total < last_total) {
@@ -250,7 +248,7 @@ static int get_sample(void)
 		last_total = total;
 
 		/* This checks the inner loop (t1 to t2) */
-		diff = time_to_us(time_sub(t2, t1));     /* current diff */
+		diff = time_sub(t2, t1);     /* current diff */
 
 		if (diff > thresh || outer_diff > thresh) {
 			if (!count)
@@ -286,8 +284,8 @@ static int get_sample(void)
 			do_div(kdata->nmi_total_ts, NSEC_PER_USEC);
 
 		s.seqnum = atomic64_inc_return(&hwlat_data.count);
-		s.duration = sample;
-		s.outer_duration = outer_sample;
+		s.duration = time_to_us(sample);
+		s.outer_duration = time_to_us(outer_sample);
 		s.nmi_total_ts = kdata->nmi_total_ts;
 		s.nmi_count = kdata->nmi_count;
 		s.count = count;
-- 
2.43.0