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SubjectRe: Negative values in /proc/latency_stats
On Mon, 2 Feb 2009 20:55:45 -0800
Andrew Morton <akpm@linux-foundation.org> wrote:

> On Sat, 31 Jan 2009 14:42:20 +0100 Corrado Zoccolo
> <czoccolo@gmail.com> wrote:
>
> > Hello,
> > I found negative numbers sometimes appear in /proc/latency_stats
> > (vanilla kernel 2.6.29-rc3, on x86 cpu, configuration attached)
> >
> > [corrado@localhost ~]$ while sleep 1; do grep --
> > - /proc/latency_stats
> > >> neg_stats; done
> > ^Z
> > [1]+ Stopped sleep 1
> > [corrado@localhost ~]$ cat neg_stats
> > 1 -486373534 -486373534 sys_rt_sigsuspend sysenter_do_call
> > 1 -486373534 -486373534 sys_rt_sigsuspend sysenter_do_call
> > 1 -486373534 -486373534 sys_rt_sigsuspend sysenter_do_call
> >
> >
> > I suspect this can be the cause for
> > https://bugs.launchpad.net/ubuntu/+source/latencytop/+bug/297776 ,
> > as I saw it happening on my machine during a kernel recompilation.
> >
>
> <discovers kernel/latencytop.c>

how about this?

From 4384fb13a27994949a75a31a8bbd2c8be10a2a2e Mon Sep 17 00:00:00 2001
From: Arjan van de Ven <arjan@linux.intel.com>
Date: Tue, 3 Feb 2009 16:12:48 -0800
Subject: [PATCH] latencytop: incorporate review feedback from Andrew Morton

Andrew had some suggestions for the latencytop file; this
patch takes care of most of these:
* Add documentation
* Turn account_scheduler_latency into an inline function
* Don't report negative values to userspace
* Make the file operations struct const
* Fix a few checkpatch.pl warnings

Signed-off-by: Arjan van de Ven <arjan@linux.intel.com>
---
include/linux/latencytop.h | 10 +++++-
kernel/latencytop.c | 83 +++++++++++++++++++++++++++++++++++++------
2 files changed, 80 insertions(+), 13 deletions(-)
diff --git a/include/linux/latencytop.h b/include/linux/latencytop.h
index 901c2d6..b0e9989 100644
--- a/include/linux/latencytop.h
+++ b/include/linux/latencytop.h
@@ -9,6 +9,7 @@
#ifndef _INCLUDE_GUARD_LATENCYTOP_H_
#define _INCLUDE_GUARD_LATENCYTOP_H_

+#include <linux/compiler.h>
#ifdef CONFIG_LATENCYTOP

#define LT_SAVECOUNT 32
@@ -24,7 +25,14 @@ struct latency_record {

struct task_struct;

-void account_scheduler_latency(struct task_struct *task, int usecs, int inter);
+extern int latencytop_enabled;
+void __account_scheduler_latency(struct task_struct *task, int usecs, int inter);
+static inline void
+account_scheduler_latency(struct task_struct *task, int usecs, int inter)
+{
+ if (unlikely(latencytop_enabled))
+ __account_scheduler_latency(task, usecs, inter);
+}

void clear_all_latency_tracing(struct task_struct *p);

diff --git a/kernel/latencytop.c b/kernel/latencytop.c
index 449db46..1f80c7e 100644
--- a/kernel/latencytop.c
+++ b/kernel/latencytop.c
@@ -9,6 +9,44 @@
* as published by the Free Software Foundation; version 2
* of the License.
*/
+
+/*
+ * CONFIG_LATENCYTOP enables a kernel latency tracking infrastructure that is
+ * used by the "latencytop" userspace tool. The latency that is tracked is not
+ * the 'traditional' interrupt latency (which is primarily caused by something
+ * else consuming CPU), but instead, it is the latency an application encounters
+ * because the kernel sleeps on its behalf for various reasons.
+ *
+ * This code tracks 2 levels of statistics:
+ * 1) System level latency
+ * 2) Per process latency
+ *
+ * The latency is stored in fixed sized datastructures in an accumulated form;
+ * if the "same" latency cause is hit twice, this will be tracked as one entry
+ * in the datastructure. Both the count, total accumulated latency and maximum
+ * latency are tracked in this data structure. When the fixed size structure is
+ * full, no new causes are tracked until the buffer is flushed by writing to
+ * the /proc file; the userspace tool does this on a regular basis.
+ *
+ * A latency cause is identified by a stringified backtrace at the point that
+ * the scheduler gets invoked. The userland tool will use this string to
+ * identify the cause of the latency in human readable form.
+ *
+ * The information is exported via /proc/latency_stats and /proc/<pid>/latency.
+ * These files look like this:
+ *
+ * Latency Top version : v0.1
+ * 70 59433 4897 i915_irq_wait drm_ioctl vfs_ioctl do_vfs_ioctl sys_ioctl
+ * | | | |
+ * | | | +----> the stringified backtrace
+ * | | +---------> The maximum latency for this entry in microseconds
+ * | +--------------> The accumulated latency for this entry (microseconds)
+ * +-------------------> The number of times this entry is hit
+ *
+ * (note: the average latency is the acummulated latency deviced by the number
+ * of times)
+ */
+
#include <linux/latencytop.h>
#include <linux/kallsyms.h>
#include <linux/seq_file.h>
@@ -72,7 +110,7 @@ account_global_scheduler_latency(struct task_struct *tsk, struct latency_record
firstnonnull = i;
continue;
}
- for (q = 0 ; q < LT_BACKTRACEDEPTH ; q++) {
+ for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
unsigned long record = lat->backtrace[q];

if (latency_record[i].backtrace[q] != record) {
@@ -101,31 +139,52 @@ account_global_scheduler_latency(struct task_struct *tsk, struct latency_record
memcpy(&latency_record[i], lat, sizeof(struct latency_record));
}

-static inline void store_stacktrace(struct task_struct *tsk, struct latency_record *lat)
+/*
+ * Iterator to store a backtrace into a latency record entry
+ */
+static inline void store_stacktrace(struct task_struct *tsk,
+ struct latency_record *lat)
{
struct stack_trace trace;

memset(&trace, 0, sizeof(trace));
trace.max_entries = LT_BACKTRACEDEPTH;
trace.entries = &lat->backtrace[0];
- trace.skip = 0;
save_stack_trace_tsk(tsk, &trace);
}

+/**
+ * __account_scheduler_latency - record an occured latency
+ * @tsk - the task struct of the task hitting the latency
+ * @usecs - the duration of the latency in microseconds
+ * @inter - 1 if the sleep was interruptible, 0 if uninterruptible
+ *
+ * This function is the main entry point for recording latency entries
+ * as called by the scheduler.
+ *
+ * This function has a few special cases to deal with normal 'non-latency'
+ * sleeps: specifically, interruptible sleep longer than 5 msec is skipped
+ * since this usually is caused by waiting for events via select() and co.
+ *
+ * Negative latencies (caused by time going backwards) are also explicitly
+ * skipped.
+ */
void __sched
-account_scheduler_latency(struct task_struct *tsk, int usecs, int inter)
+__account_scheduler_latency(struct task_struct *tsk, int usecs, int inter)
{
unsigned long flags;
int i, q;
struct latency_record lat;

- if (!latencytop_enabled)
- return;
-
/* Long interruptible waits are generally user requested... */
if (inter && usecs > 5000)
return;

+ /* Negative sleeps are time going backwards */
+ /* Zero-time sleeps are non-interesting */
+ if (usecs <= 0)
+ return;
+
memset(&lat, 0, sizeof(lat));
lat.count = 1;
lat.time = usecs;
@@ -143,12 +202,12 @@ account_scheduler_latency(struct task_struct *tsk, int usecs, int inter)
if (tsk->latency_record_count >= LT_SAVECOUNT)
goto out_unlock;

- for (i = 0; i < LT_SAVECOUNT ; i++) {
+ for (i = 0; i < LT_SAVECOUNT; i++) {
struct latency_record *mylat;
int same = 1;

mylat = &tsk->latency_record[i];
- for (q = 0 ; q < LT_BACKTRACEDEPTH ; q++) {
+ for (q = 0; q < LT_BACKTRACEDEPTH; q++) {
unsigned long record = lat.backtrace[q];

if (mylat->backtrace[q] != record) {
@@ -186,7 +245,7 @@ static int lstats_show(struct seq_file *m, void *v)
for (i = 0; i < MAXLR; i++) {
if (latency_record[i].backtrace[0]) {
int q;
- seq_printf(m, "%i %li %li ",
+ seq_printf(m, "%i %lu %lu ",
latency_record[i].count,
latency_record[i].time,
latency_record[i].max);
@@ -223,7 +282,7 @@ static int lstats_open(struct inode *inode, struct file *filp)
return single_open(filp, lstats_show, NULL);
}

-static struct file_operations lstats_fops = {
+static const struct file_operations lstats_fops = {
.open = lstats_open,
.read = seq_read,
.write = lstats_write,
@@ -236,4 +295,4 @@ static int __init init_lstats_procfs(void)
proc_create("latency_stats", 0644, NULL, &lstats_fops);
return 0;
}
-__initcall(init_lstats_procfs);
+device_initcall(init_lstats_procfs);
--
1.6.0.6

--
Arjan van de Ven Intel Open Source Technology Centre
For development, discussion and tips for power savings,
visit http://www.lesswatts.org


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