Re: [ANNOUNCE] sched: schedtop utility

[Peter Zijlstra <peterz@infradead.org>]

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.