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

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Date:	Mon, 09 Sep 2013 14:07:53 -0700
From:	Jason Low <jason.low2@...com>
To:	Peter Zijlstra <peterz@...radead.org>
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 Mon, 2013-09-09 at 13:49 +0200, Peter Zijlstra wrote:
> 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?

I agree that we can delete the check between avg_idle and max_idle_balance_cost
so that large costs in higher domains don't cause balancing to be skipped in
lower domains as Srikar suggested. Should we keep the old
"if (this_rq->avg_idle < sysctl_sched_migration_cost)" check?

Also, are the other costs of idle balancing, specifically the cost of cache
refreshes, not considered in the max_newidle_lb_cost? I was wondering if there
is a non expensive way we can also take those into account. For example, can we
multiply max_newidle_lb_cost by a factor of 1.2x to 2x, but also increase the
decay factor (4% to 10% per second)?

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