linux-kernel - Re: [PATCH 1/3] sched, balancing: Update rq->max_idle_balance_cost whenever newidle balance is attempted

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Message-ID: <1398445392.2102.4.camel@j-VirtualBox>
Date:	Fri, 25 Apr 2014 10:03:12 -0700
From:	Jason Low <jason.low2@...com>
To:	Mike Galbraith <umgwanakikbuti@...il.com>
Cc:	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
	linux-kernel@...r.kernel.org, daniel.lezcano@...aro.org,
	alex.shi@...aro.org, vincent.guittot@...aro.org,
	morten.rasmussen@....com, aswin@...com, chegu_vinod@...com
Subject: Re: [PATCH 1/3] sched, balancing: Update rq->max_idle_balance_cost
 whenever newidle balance is attempted

On Fri, 2014-04-25 at 09:58 +0200, Mike Galbraith wrote:
> On Fri, 2014-04-25 at 00:13 -0700, Jason Low wrote: 
> > On Fri, 2014-04-25 at 10:42 +0530, Preeti U Murthy wrote:
> > > I agree with this. However I am concerned with an additional point that
> > > I have mentioned in my reply to Peter's mail on this thread.
> > > 
> > > Should we verify if rq->next_balance update is independent of
> > > pulled_tasks? sd->balance_interval is changed during load_balance() and
> > > rq->next_balance should perhaps consider that?
> > 
> > Hi Preeti,
> > 
> > I agree that we may want to consider having rq->next balance update be
> > independent of pulled_task. As you mentioned, load_balance() can modify
> > the balance_interval.
> > 
> > There are a few things I'm wondering if we would need to also add then:
> > 
> > 1. In the case that this_rq->avg_idle < sysctl_sched_migration_cost, we
> >    would need to also traverse the domains to properly compute
> >    next_balance (without the sd->busy_factor) as we would be going idle.
> >    Otherwise, next_balance could get set to jiffies + HZ while the
> >    CPU goes idle.
> 
> Avoiding high frequency cache misses and cycle wastage on micro-idle was
> what avg-idle was about.  If you're going to traverse anyway, or have a
> better way to not do that too frequently, you can just nuke it.

Yeah, we already compare avg-idle with the per-domain costs in that
function. I'll run some performance tests with the first check removed,
as such a change can potentially have a (+/-) impact on performance.

Thanks,
Jason


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