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