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    SubjectRe: [BUG] 2.6.37-rc3 massive interactivity regression on ARM
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    On Thu, Dec 9, 2010 at 4:52 AM, Peter Zijlstra <peterz@infradead.org> wrote:
    > On Wed, 2010-12-08 at 15:31 -0800, Venkatesh Pallipadi wrote:
    >
    >> [PATCH] update_rq_clock() with irq_time should handle 64 bit wraparound
    >>
    >> update_rq_clock with IRQ_TIME_ACCOUNTING assumed 64 bit sched_clock that
    >> practically will not wraparound. Change to code to handle wraparounds,
    >> still preventing rq->clock_task from going backwards.
    >
    > All we should do is deal with the 64wrap, not filter backward motion, I
    > came up with something like the below.

    The original intent of this check
    - if (rq->clock - irq_time > rq->clock_task)
    was to filter the potential backward motion. As otherwise, downstream
    users of clock_task can see
    huge positive numbers from curr - prev calculations.

    The same problem will be there with below code, with irq_delta >
    delta, clock_task can go backwards which is not good.
    + delta -= irq_delta;
    + rq->clock_task += delta;

    The reason for this is rq->clock and irqtime updates kind of happen
    independently and specifically, if a rq->clock update happens while we
    are in a softirq, we may have this case of going backwards on the next
    update.

    >
    > I think for now the best solution to the early wrap problem for ARM is
    > for them to select HAVE_UNSTABLE_SCHED_CLOCK, it will mostly deal with
    > the short wrap by filtering out the occasional negative value.
    >
    > Then later we can look at cleaning/breaking-up the kernel/sched_clock.c
    > code to provide smaller bits of functionality and possibly merging it
    > with some of the clocksource code.
    >

    Yes. That should resolve the early wrap.


    Thanks,
    Venki

    > ---
    >  kernel/sched.c |   54 +++++++++++++++++++++++++-----------------------------
    >  1 files changed, 25 insertions(+), 29 deletions(-)
    >
    > diff --git a/kernel/sched.c b/kernel/sched.c
    > index 8e885c1..0fb7de8 100644
    > --- a/kernel/sched.c
    > +++ b/kernel/sched.c
    > @@ -642,23 +642,19 @@ static inline struct task_group *task_group(struct task_struct *p)
    >
    >  #endif /* CONFIG_CGROUP_SCHED */
    >
    > -static u64 irq_time_cpu(int cpu);
    > -static void sched_irq_time_avg_update(struct rq *rq, u64 irq_time);
    > +static void update_rq_clock_task(struct rq *rq, s64 delta);
    >
    >  inline void update_rq_clock(struct rq *rq)
    >  {
    > -       int cpu = cpu_of(rq);
    > -       u64 irq_time;
    > +       s64 delta;
    >
    >        if (rq->skip_clock_update)
    >                return;
    >
    > -       rq->clock = sched_clock_cpu(cpu);
    > -       irq_time = irq_time_cpu(cpu);
    > -       if (rq->clock - irq_time > rq->clock_task)
    > -               rq->clock_task = rq->clock - irq_time;
    > +       delta = sched_clock_cpu(cpu_of(rq)) - rq->clock;
    > +       rq->clock += delta;
    > +       update_rq_clock_task(rq, delta);
    >
    > -       sched_irq_time_avg_update(rq, irq_time);
    >  }
    >
    >  /*
    > @@ -1817,14 +1813,6 @@ void disable_sched_clock_irqtime(void)
    >        sched_clock_irqtime = 0;
    >  }
    >
    > -static u64 irq_time_cpu(int cpu)
    > -{
    > -       if (!sched_clock_irqtime)
    > -               return 0;
    > -
    > -       return per_cpu(cpu_softirq_time, cpu) + per_cpu(cpu_hardirq_time, cpu);
    > -}
    > -
    >  void account_system_vtime(struct task_struct *curr)
    >  {
    >        unsigned long flags;
    > @@ -1855,25 +1843,33 @@ void account_system_vtime(struct task_struct *curr)
    >  }
    >  EXPORT_SYMBOL_GPL(account_system_vtime);
    >
    > -static void sched_irq_time_avg_update(struct rq *rq, u64 curr_irq_time)
    > +static inline u64 irq_time_cpu(int cpu)
    >  {
    > -       if (sched_clock_irqtime && sched_feat(NONIRQ_POWER)) {
    > -               u64 delta_irq = curr_irq_time - rq->prev_irq_time;
    > -               rq->prev_irq_time = curr_irq_time;
    > -               sched_rt_avg_update(rq, delta_irq);
    > -       }
    > +       return per_cpu(cpu_softirq_time, cpu) + per_cpu(cpu_hardirq_time, cpu);
    >  }
    >
    > -#else
    > -
    > -static u64 irq_time_cpu(int cpu)
    > +static void update_rq_clock_task(struct rq *rq, s64 delta)
    >  {
    > -       return 0;
    > +       s64 irq_delta;
    > +
    > +       irq_delta = irq_time_cpu(cpu_of(rq)) - rq->prev_irq_time;
    > +       rq->prev_irq_time += irq_delta;
    > +
    > +       delta -= irq_delta;
    > +       rq->clock_task += delta;
    > +
    > +       if (irq_delta && sched_feat(NONIRQ_POWER))
    > +               sched_rt_avg_update(rq, irq_delta);
    >  }
    >
    > -static void sched_irq_time_avg_update(struct rq *rq, u64 curr_irq_time) { }
    > +#else /* CONFIG_IRQ_TIME_ACCOUNTING */
    >
    > -#endif
    > +static inline void update_rq_clock_task(struct rq *rq, s64 delta)
    > +{
    > +       rq->clock_task += delta;
    > +}
    > +
    > +#endif /* CONFIG_IRQ_TIME_ACCOUNTING */
    >
    >  #include "sched_idletask.c"
    >  #include "sched_fair.c"
    >
    >
    --
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