Török Edwin wrote:
>
>> The performance aspect... collecting the data isn't cheap (which is
>> why it's made a sysctl),
>> I still plan to look at optimizing it but it won't ever be free.
>>
>
> Yes, I understand that. Is there a way latencytop could track its own
> overhead? I suppose it would lead to more accurate results
> (not that there would be anything wrong with the current ones).
>
Latencytop userspace tool shows latencies > 0.1 msec, thus capturing
backtraces for latencies <0.1msec could be avoided.
If I apply the patch below, then enabling latencytop doesn't freeze X
when running the "10-threads doing infloop usleep(1)" test.
Still, I don't want to loose track of the latencies we didn't collect
backtraces for, so I added a special "untraced" category, reported as
first line in /proc/latency_stats. If needed, instead of hardcoding the
threshold, it could be made a sysctl, or set via writing to
/proc/latency_stats,...
While I am running the test-program:
$ grep untraced /proc/latency_stats
4875605 5120414 49 untraced
On an idle system:
$ grep untraced /proc/latency_stats
532 3287 47 untraced
$ grep untraced /proc/latency_stats
853 5778 47 untraced
$ grep untraced /proc/latency_stats
950 6788 47 untraced
$ grep untraced /proc/latency_stats
1343 9977 49 untraced
$ grep untraced /proc/latency_stats
1448 11075 49 untraced
Best regards,
--Edwin
---
latencytop.c | 19 ++++++++++++++++++-
1 file changed, 18 insertions(+), 1 deletion(-)
--- /tmp/linux-2.6.24-rc8/kernel/latencytop.c 2008-01-24
11:27:32.727487585 +0200
+++ kernel/latencytop.c 2008-01-24 10:42:25.000000000 +0200
@@ -24,8 +24,11 @@
static DEFINE_SPINLOCK(latency_lock);
#define MAXLR 128
+/* we are not interested in latencies less than 0.1msec, so
+ * don't get backtraces for latencies <0.05msec.*/
+#define LATENCY_TRACE_THRESHOLD 50
static struct latency_record latency_record[MAXLR];
-
+static struct latency_record untraced;
int latencytop_enabled;
void clear_all_latency_tracing(struct task_struct *p)
@@ -47,6 +50,7 @@
spin_lock_irqsave(&latency_lock, flags);
memset(&latency_record, 0, sizeof(latency_record));
+ memset(&untraced, 0, sizeof(untraced));
spin_unlock_irqrestore(&latency_lock, flags);
}
@@ -124,6 +128,15 @@
if (inter && usecs > 5000)
return;
+ if(usecs < LATENCY_TRACE_THRESHOLD) {
+ /* don't get stacktrace for very low latencies */
+ untraced.time += usecs;
+ if(usecs > untraced.max)
+ untraced.max = usecs;
+ untraced.count++;
+ return;
+ }
+
memset(&lat, 0, sizeof(lat));
lat.count = 1;
lat.time = usecs;
@@ -177,6 +190,10 @@
seq_puts(m, "Latency Top version : v0.1\n");
+ seq_printf(m, "%i %li %li untraced \n",
+ untraced.count,
+ untraced.time,
+ untraced.max);
for (i = 0; i < MAXLR; i++) {
if (latency_record[i].backtrace[0]) {
int q;
--
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