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SubjectRe: [patch 3/7] sched: set skip_clock_update in yield_task_fair()
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On Wed, 2011-11-23 at 15:48 +0100, Peter Zijlstra wrote:
> On Tue, 2011-11-22 at 15:21 +0100, Mike Galbraith wrote:
> > This is another case where we are on our way to schedule(),
> > so can save a useless clock update and resulting microscopic
> > vruntime update.
> >
>
> Everytime I see one of these skip_clock_update thingies I get the idea
> we should do something smarter, because its most fragile and getting it
> wrong results in a subtle trainwreck..
>
> Would something like the below help in validating this stuff?

No, because switch itself is irrelevant to fair-beans counting.

> ---
> Index: linux-2.6/kernel/sched/core.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched/core.c
> +++ linux-2.6/kernel/sched/core.c
> @@ -113,12 +113,27 @@ void update_rq_clock(struct rq *rq)
> {
> s64 delta;
>
> - if (rq->skip_clock_update > 0)
> + if (rq->skip_clock_update > 0) {
> + /*
> + * We skipped an update even though we had a full context
> + * switch in between, badness.
> + */
> + if (rq->clock_update_stamp != rq->sched_switch)
> + trace_printk("lost an update!!\n");
> return;
> + }
> +
> + /*
> + * We're updating the clock even though we didn't switch yet.
> + */
> + if (rq->clock_update_stamp == rq->sched_switch)
> + trace_printk("too many updates!!\n");
>
> delta = sched_clock_cpu(cpu_of(rq)) - rq->clock;
> rq->clock += delta;
> update_rq_clock_task(rq, delta);
> +
> + rq->clock_update_stamp = rq->sched_switch
> }
>
> /*
> @@ -1922,6 +1937,9 @@ static void finish_task_switch(struct rq
> #ifdef __ARCH_WANT_INTERRUPTS_ON_CTXSW
> local_irq_enable();
> #endif /* __ARCH_WANT_INTERRUPTS_ON_CTXSW */
> +#ifdef CONFIG_SCHED_DEBUG
> + schedstat_inc(rq, sched_switch);
> +#endif
> finish_lock_switch(rq, prev);
>
> fire_sched_in_preempt_notifiers(current);
> Index: linux-2.6/kernel/sched/sched.h
> ===================================================================
> --- linux-2.6.orig/kernel/sched/sched.h
> +++ linux-2.6/kernel/sched/sched.h
> @@ -374,6 +374,8 @@ struct rq {
> unsigned char nohz_balance_kick;
> #endif
> int skip_clock_update;
> + unsigned int clock_update_stamp;
> +
>
> /* capture load from *all* tasks on this cpu: */
> struct load_weight load;
>




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