linux-kernel - Re: [RFC][PATCH v4 3/3] sched: Periodically decay max cost of idle balance

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Date:	Mon, 9 Sep 2013 13:49:19 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@...com>
Cc:	mingo@...hat.com, linux-kernel@...r.kernel.org, efault@....de,
	pjt@...gle.com, preeti@...ux.vnet.ibm.com,
	akpm@...ux-foundation.org, mgorman@...e.de, riel@...hat.com,
	aswin@...com, scott.norton@...com, srikar@...ux.vnet.ibm.com
Subject: Re: [RFC][PATCH v4 3/3] sched: Periodically decay max cost of idle
 balance

On Wed, Sep 04, 2013 at 12:10:01AM -0700, Jason Low wrote:
> On Fri, 2013-08-30 at 12:18 +0200, Peter Zijlstra wrote:
> > On Thu, Aug 29, 2013 at 01:05:36PM -0700, Jason Low wrote:
> > > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> > > index 58b0514..bba5a07 100644
> > > --- a/kernel/sched/core.c
> > > +++ b/kernel/sched/core.c
> > > @@ -1345,7 +1345,7 @@ ttwu_do_wakeup(struct rq *rq, struct task_struct *p, int wake_flags)
> > >  
> > >  	if (rq->idle_stamp) {
> > >  		u64 delta = rq_clock(rq) - rq->idle_stamp;
> > > -		u64 max = 2*rq->max_idle_balance_cost;
> > > +		u64 max = 2*(sysctl_sched_migration_cost + rq->max_idle_balance_cost);
> > 
> > You re-introduce sched_migration_cost here because max_idle_balance_cost
> > can now drop down to 0 again?
> 
> Yes it was so that max_idle_balance_cost would be at least sched_migration_cost
> and that we would still skip idle_balance if avg_idle < sched_migration_cost.
> 
> I also initially thought that adding sched_migration_cost would also account for
> the extra "costs" of idle balancing that are not accounted for in the time spent
> on each newidle load balance. Come to think of it though, sched_migration_cost
> might be too large when used in that context considering we're already using the
> max cost.

Right, so shall we do as Srikar suggests and drop that initial check?
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