linux-kernel - Re: [PATCH 1/2] sched: Optimize build_sched_domains() for saving first SD node for a cpu

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Message-ID: <CAKohpokGrgFOtOT6Y3e4MOJBUQWT4FG4Mk-F6_M0V-pEEf3KYw@mail.gmail.com>
Date:	Wed, 5 Jun 2013 10:37:29 +0530
From:	Viresh Kumar <viresh.kumar@...aro.org>
To:	Michael Wang <wangyun@...ux.vnet.ibm.com>
Cc:	mingo@...hat.com, peterz@...radead.org,
	linaro-kernel@...ts.linaro.org, patches@...aro.org,
	linux-kernel@...r.kernel.org, robin.randhawa@....com,
	Steve.Bannister@....com, Liviu.Dudau@....com,
	charles.garcia-tobin@....com, arvind.chauhan@....com
Subject: Re: [PATCH 1/2] sched: Optimize build_sched_domains() for saving
 first SD node for a cpu

On 5 June 2013 10:12, Michael Wang <wangyun@...ux.vnet.ibm.com> wrote:
> Hi, Viresh
>
> On 06/04/2013 07:20 PM, Viresh Kumar wrote:
> [snip]
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 58453b8..638f6cb 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -6533,16 +6533,13 @@ static int build_sched_domains(const struct cpumask *cpu_map,
>>               sd = NULL;
>>               for (tl = sched_domain_topology; tl->init; tl++) {
>>                       sd = build_sched_domain(tl, &d, cpu_map, attr, sd, i);
>> +                     if (!*per_cpu_ptr(d.sd, i))
>
> What about:
>                         if (tl == sched_domain_topology)
>
> It cost less than per_cpu_ptr(), isn't it?

How can I miss it.. Obviously its better :)

See if below one looks better (Attached too in case gmail screws up
my mail)..

--------x-------------x------------------

From: Viresh Kumar <viresh.kumar@...aro.org>
Date: Tue, 4 Jun 2013 15:41:15 +0530
Subject: [PATCH] sched: Optimize build_sched_domains() for saving first SD
 node for a cpu

We are saving first scheduling domain for a cpu in build_sched_domains() by
iterating over the nested sd->child list. We don't actually need to do it this
way.

tl will be equal to sched_domain_topology for the first iteration and so we can
set *per_cpu_ptr(d.sd, i) based on that.  So, save pointer to first SD while
running the iteration loop over tl's.

Signed-off-by: Viresh Kumar <viresh.kumar@...aro.org>
---
 kernel/sched/core.c | 7 ++-----
 1 file changed, 2 insertions(+), 5 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 58453b8..08a27be 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6533,16 +6533,13 @@ static int build_sched_domains(const struct
cpumask *cpu_map,
 		sd = NULL;
 		for (tl = sched_domain_topology; tl->init; tl++) {
 			sd = build_sched_domain(tl, &d, cpu_map, attr, sd, i);
+			if (tl == sched_domain_topology)
+				*per_cpu_ptr(d.sd, i) = sd;
 			if (tl->flags & SDTL_OVERLAP || sched_feat(FORCE_SD_OVERLAP))
 				sd->flags |= SD_OVERLAP;
 			if (cpumask_equal(cpu_map, sched_domain_span(sd)))
 				break;
 		}
-
-		while (sd->child)
-			sd = sd->child;
-
-		*per_cpu_ptr(d.sd, i) = sd;
 	}

 	/* Build the groups for the domains */

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