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Message-ID: <AANLkTikd3X4pLs5De-LO6=XD+JqUz3-Ku7YM8+0-oK8t@mail.gmail.com>
Date: Thu, 9 Dec 2010 09:43:36 -0800
From: Venkatesh Pallipadi <venki@...gle.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Russell King - ARM Linux <linux@....linux.org.uk>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Mikael Pettersson <mikpe@...uu.se>,
Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
linux-arm-kernel@...ts.infradead.org,
John Stultz <johnstul@...ibm.com>
Subject: Re: [BUG] 2.6.37-rc3 massive interactivity regression on ARM
On Thu, Dec 9, 2010 at 4:52 AM, Peter Zijlstra <peterz@...radead.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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