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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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