linux-kernel - Re: Add rq->nr_uninterruptible count to dest cpu's rq while CPU goes down.

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Date:	Mon, 20 Aug 2012 18:16:40 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rakib Mullick <rakib.mullick@...il.com>
Cc:	mingo@...nel.org, linux-kernel@...r.kernel.org,
	paulmck <paulmck@...ux.vnet.ibm.com>
Subject: Re: Add rq->nr_uninterruptible count to dest cpu's rq while CPU
 goes down.

On Mon, 2012-08-20 at 22:10 +0600, Rakib Mullick wrote:
> >
> First of all, you've misspelled my name, it's Rakib not Rabik.

Damn, sorry!, lysdexic that.. 

> > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > index 4376c9f..06d23c6 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -5338,27 +5338,17 @@ void idle_task_exit(void)
> >  }
> >
> >  /*
> > - * While a dead CPU has no uninterruptible tasks queued at this point,
> > - * it might still have a nonzero ->nr_uninterruptible counter, because
> > - * for performance reasons the counter is not stricly tracking tasks to
> > - * their home CPUs. So we just add the counter to another CPU's counter,
> > - * to keep the global sum constant after CPU-down:
> > - */
> > -static void migrate_nr_uninterruptible(struct rq *rq_src)
> > -{
> > -     struct rq *rq_dest = cpu_rq(cpumask_any(cpu_active_mask));
> > -
> > -     rq_dest->nr_uninterruptible += rq_src->nr_uninterruptible;
> > -     rq_src->nr_uninterruptible = 0;
> > -}
> > -
> > -/*
> > - * remove the tasks which were accounted by rq from calc_load_tasks.
> > + * Since this CPU is going 'away' for a while, fold any nr_active delta
> > + * we might have. Assumes we're called after migrate_tasks() so that the
> > + * nr_active count is stable.
> > + *
> But after migrate_tasks(), it's likely that rq->nr_running will be 1.
> Then, nr_active will be screwed. No? 

Gah indeed. let me try that again.
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