linux-kernel - Re: [PATCH v3 05/22] sched: remove domain iterations in fork/exec/wake

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Message-ID: <20130111100757.GH2046@e103034-lin>
Date:	Fri, 11 Jan 2013 10:07:57 +0000
From:	Morten Rasmussen <Morten.Rasmussen@....com>
To:	Alex Shi <alex.shi@...el.com>
Cc:	"mingo@...hat.com" <mingo@...hat.com>,
	"peterz@...radead.org" <peterz@...radead.org>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"arjan@...ux.intel.com" <arjan@...ux.intel.com>,
	"bp@...en8.de" <bp@...en8.de>, "pjt@...gle.com" <pjt@...gle.com>,
	"namhyung@...nel.org" <namhyung@...nel.org>,
	"efault@....de" <efault@....de>,
	"vincent.guittot@...aro.org" <vincent.guittot@...aro.org>,
	"gregkh@...uxfoundation.org" <gregkh@...uxfoundation.org>,
	"preeti@...ux.vnet.ibm.com" <preeti@...ux.vnet.ibm.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 05/22] sched: remove domain iterations in
 fork/exec/wake

On Fri, Jan 11, 2013 at 02:46:31AM +0000, Alex Shi wrote:
> On 01/10/2013 02:21 AM, Morten Rasmussen wrote:
> >>  		new_cpu = find_idlest_cpu(group, p, cpu);
> >> > -
> >> > -		/* Now try balancing at a lower domain level of new_cpu */
> >> > -		cpu = new_cpu;
> >> > -		weight = sd->span_weight;
> >> > -		sd = NULL;
> >> > -		for_each_domain(cpu, tmp) {
> >> > -			if (weight <= tmp->span_weight)
> >> > -				break;
> >> > -			if (tmp->flags & sd_flag)
> >> > -				sd = tmp;
> >> > -		}
> >> > -		/* while loop will break here if sd == NULL */
> > I agree that this should be a major optimization. I just can't figure
> > out why the existing recursive search for an idle cpu switches to the
> > new cpu near the end and then starts a search for an idle cpu in the new
> > cpu's domain. Is this to handle some exotic sched domain configurations?
> > If so, they probably wouldn't work with your optimizations.
> 
> I did not find odd configuration that asking for old logical.
> 
> According to Documentation/scheduler/sched-domains.txt, Maybe never.
> "A domain's span MUST be a superset of it child's span (this restriction
> could be relaxed if the need arises), and a base domain for CPU i MUST
> span at least i."  etc. etc.

The reason for my suspicion is the SD_OVERLAP flag, which has something
to do overlapping sched domains. I haven't looked into what it does or
how it works. I'm just wondering if this optimization will affect the
use of that flag.

Morten

> 
> 
> -- 
> Thanks Alex
> 

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