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SubjectRe: [ANNOUNCE] sched: schedtop utility
Hi,

On Mon, Jun 02, 2008 at 03:07:31PM +0200, Peter Zijlstra wrote:
> On Mon, 2008-06-02 at 18:18 +0530, Ankita Garg wrote:
> > Hi Gregory,
> >
> > On Thu, May 22, 2008 at 08:06:44AM -0600, Gregory Haskins wrote:
> > > Hi all scheduler developers,
> > > I had an itch to scratch w.r.t. watching the stats in /proc/schedstats, and it appears that the perl scripts referenced in Documentation/scheduler/sched-stats.txt do not support v14 from HEAD so I whipped up a little utility I call "schedtop".
> > >
> >
> > Nice tool! Helps in better visualization of the data in schedstats.
> >
> > Using the tool, realized that most of the timing related stats therein
> > might not be completely usable in many scenarios, as might already be
> > known.
> >
> > Without any additional load on the system, all the stats are nice and
> > sane. But, as soon as I ran my particular testcase, the data
> > pertaining to the delta of run_delay/cpu_time went haywire! I understand
> > that all the values are based on top of rq->clock, which relies on tsc that
> > is not synced across cpus and would result in skews/incorrect values.
> > But, turns out to be not so reliable data for debugging. This is
> > ofcourse nothing related to the tool, but for schedstat in
> > general...rather just adding on to the already existing woes with non-syned
> > tscs :-)
>
> Thing is, things runtime should be calculated by using per cpu deltas.
> You take a stamp when you get scheduled on the cpu and another one when
> you stop running, then the delta is added to runtime.
>
> This is always on the same cpu - when you get migrated you're stopped
> and re-scheduled so that should work out nicely.
>
> So in that sense it shouldn't matter that the rq->clock values can get
> skewed between cpus.
>
> So I'm still a little puzzled by your observations; though it could be
> that the schedstat stuff got broken - I've never really looked too
> closely at it.
>

Thanks Peter for the explanation...
I agree with the above and that is the reason why I did not see weird
values with cpu_time. But, run_delay still would suffer skews as the end
points for delta could be taken on different cpus due to migration (more
so on RT kernel due to the push-pull operations). With the below patch,
I could not reproduce the issue I had seen earlier. After every dequeue,
we take the delta and start wait measurements from zero when moved to a
different rq.

Signed-off-by: Ankita Garg <ankita@in.ibm.com>
Index: linux-2.6.24.4/kernel/sched.c
===================================================================
--- linux-2.6.24.4.orig/kernel/sched.c 2008-06-03 14:14:07.000000000 +0530
+++ linux-2.6.24.4/kernel/sched.c 2008-06-04 12:48:34.000000000 +0530
@@ -948,6 +948,7 @@

static void dequeue_task(struct rq *rq, struct task_struct *p, int sleep)
{
+ sched_info_dequeued(p);
p->sched_class->dequeue_task(rq, p, sleep);
p->se.on_rq = 0;
}
Index: linux-2.6.24.4/kernel/sched_stats.h
===================================================================
--- linux-2.6.24.4.orig/kernel/sched_stats.h 2008-06-03 14:14:28.000000000 +0530
+++ linux-2.6.24.4/kernel/sched_stats.h 2008-06-05 10:39:39.000000000 +0530
@@ -113,6 +113,13 @@
if (rq)
rq->rq_sched_info.cpu_time += delta;
}
+
+static inline void
+rq_sched_info_dequeued(struct rq *rq, unsigned long long delta)
+{
+ if (rq)
+ rq->rq_sched_info.run_delay += delta;
+}
# define schedstat_inc(rq, field) do { (rq)->field++; } while (0)
# define schedstat_add(rq, field, amt) do { (rq)->field += (amt); } while (0)
# define schedstat_set(var, val) do { var = (val); } while (0)
@@ -129,6 +136,11 @@
#endif

#if defined(CONFIG_SCHEDSTATS) || defined(CONFIG_TASK_DELAY_ACCT)
+static inline void sched_info_reset_dequeued(struct task_struct *t)
+{
+ t->sched_info.last_queued = 0;
+}
+
/*
* Called when a process is dequeued from the active array and given
* the cpu. We should note that with the exception of interactive
@@ -138,15 +150,22 @@
* active queue, thus delaying tasks in the expired queue from running;
* see scheduler_tick()).
*
- * This function is only called from sched_info_arrive(), rather than
- * dequeue_task(). Even though a task may be queued and dequeued multiple
- * times as it is shuffled about, we're really interested in knowing how
- * long it was from the *first* time it was queued to the time that it
- * finally hit a cpu.
+ * Though we are interested in knowing how long it was from the *first* time a
+ * task was queued to the time that it finally hit a cpu, we call this routine
+ * from dequeue_task() to account for possible rq->clock skew across cpus. The
+ * delta taken on each cpu would annul the skew.
*/
static inline void sched_info_dequeued(struct task_struct *t)
{
- t->sched_info.last_queued = 0;
+ unsigned long long now = task_rq(t)->clock, delta = 0;
+
+ if(unlikely(sched_info_on()))
+ if(t->sched_info.last_queued)
+ delta = now - t->sched_info.last_queued;
+ sched_info_reset_dequeued(t);
+ t->sched_info.run_delay += delta;
+
+ rq_sched_info_dequeued(task_rq(t), delta);
}

/*
@@ -160,7 +179,7 @@

if (t->sched_info.last_queued)
delta = now - t->sched_info.last_queued;
- sched_info_dequeued(t);
+ sched_info_reset_dequeued(t);
t->sched_info.run_delay += delta;
t->sched_info.last_arrival = now;
t->sched_info.pcount++;
--
Regards,
Ankita Garg (ankita@in.ibm.com)
Linux Technology Center
IBM India Systems & Technology Labs,
Bangalore, India


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