Messages in this thread | | | Date | Mon, 02 Jun 2008 07:20:09 -0600 | From | "Gregory Haskins" <> | Subject | Re: [ANNOUNCE] sched: schedtop utility |
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Hi Ankita, For some reason, I didn't get your original email. I had to go find it on the lkml.org archives.
But anyway, see inline
>>> On Mon, Jun 2, 2008 at 9:07 AM, in message <1212412051.6269.5.camel@lappy.programming.kicks-ass.net>, Peter Zijlstra <peterz@infradead.org> 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.
I suspect we must be talking about those stats that are always moving pretty fast. I see that too, and I use the (potentially unknown) filtering feature of schedtop: "-i REGEX" will set the include filter, and "-x REGEX" will set the exclude filter. By default, include allows everything, and exclude filters nothing. Changing it to "-x sched_info" will exclude all those pesky stats that move fast and do not convey useful (to me, anyway) data. I hope that helps!
Also, about your idea for the /proc/<pid>/schedstats, I was thinking the same thing while on my trip on Friday. I will add this feature. Thanks!
-Greg
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