Messages in this thread Patch in this message | | | Date | Thu, 5 Jun 2008 10:50:08 +0530 | From | Ankita Garg <> | Subject | Re: [ANNOUNCE] sched: schedtop utility |
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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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