[PATCH] sched: reorder root_domain to remove 64 bit alignment padding

[PATCH] sched: reorder root_domain to remove 64 bit alignment padding

[Richard Kennedy]

Reorder root_domain to remove 8 bytes of alignment padding on 64 bit
builds, this shrinks the size from 1736 to 1728 bytes, therefore using
one fewer cachelines.

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
---

patch against v3.0-rc7

compiled & booted on x86_64.

I've been running this patch for 2 days without any obvious problems,
but I don't have any specific RT test cases.

regards
Richard



diff --git a/kernel/sched.c b/kernel/sched.c
index 9769c75..4ecf9db 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -422,6 +422,7 @@ struct rt_rq {
  */
 struct root_domain {
 	atomic_t refcount;
+	atomic_t rto_count;
 	struct rcu_head rcu;
 	cpumask_var_t span;
 	cpumask_var_t online;
@@ -431,7 +432,6 @@ struct root_domain {
 	 * one runnable RT task.
 	 */
 	cpumask_var_t rto_mask;
-	atomic_t rto_count;
 	struct cpupri cpupri;
 };
 

Re: [PATCH] sched: reorder root_domain to remove 64 bit alignment padding

[Peter Zijlstra]

On Fri, 2011-07-15 at 11:41 +0100, Richard Kennedy wrote:
> Reorder root_domain to remove 8 bytes of alignment padding on 64 bit
> builds, this shrinks the size from 1736 to 1728 bytes, therefore using
> one fewer cachelines.

depends a bit on how insane struct cpupri is, but yeah, good compaction.
Thanks!

Re: [PATCH] sched: reorder root_domain to remove 64 bit alignment padding

[Richard Kennedy]

On Fri, 2011-07-15 at 12:44 +0200, Peter Zijlstra wrote:
> On Fri, 2011-07-15 at 11:41 +0100, Richard Kennedy wrote:
> > Reorder root_domain to remove 8 bytes of alignment padding on 64 bit
> > builds, this shrinks the size from 1736 to 1728 bytes, therefore using
> > one fewer cachelines.
> 
> depends a bit on how insane struct cpupri is, but yeah, good compaction.
> Thanks!

Thanks Peter,

argh -- I hadn't spotted that cpupri used NR_CPUS and was so variable in
size. I guess there won't be much benefit then, as the distros seem to
always set NR_CPUS to some insanely huge value ;)

regards
Richard 

Re: [PATCH] sched: reorder root_domain to remove 64 bit alignment padding

[Henrique de Moraes Holschuh]

On Fri, 15 Jul 2011, Richard Kennedy wrote:
> On Fri, 2011-07-15 at 12:44 +0200, Peter Zijlstra wrote:
> > On Fri, 2011-07-15 at 11:41 +0100, Richard Kennedy wrote:
> > > Reorder root_domain to remove 8 bytes of alignment padding on 64 bit
> > > builds, this shrinks the size from 1736 to 1728 bytes, therefore using
> > > one fewer cachelines.
> > 
> > depends a bit on how insane struct cpupri is, but yeah, good compaction.
> > Thanks!
> 
> Thanks Peter,
> 
> argh -- I hadn't spotted that cpupri used NR_CPUS and was so variable in
> size. I guess there won't be much benefit then, as the distros seem to
> always set NR_CPUS to some insanely huge value ;)

There will be, for those who _do_ care about performance (and thus have sane
NR_CPUS for their boxes, among other things).  Please don't be discouraged
over this.

-- 
  "One disk to rule them all, One disk to find them. One disk to bring
  them all and in the darkness grind them. In the Land of Redmond
  where the shadows lie." -- The Silicon Valley Tarot
  Henrique Holschuh

[tip:sched/core] sched: Reorder root_domain to remove 64 bit alignment padding

[tip-bot for Richard Kennedy]

Commit-ID:  26a148eb9c790149750f7e77da0d96029443d400
Gitweb:     http://git.kernel.org/tip/26a148eb9c790149750f7e77da0d96029443d400
Author:     Richard Kennedy <richard@rsk.demon.co.uk>
AuthorDate: Fri, 15 Jul 2011 11:41:31 +0100
Committer:  Ingo Molnar <mingo@elte.hu>
CommitDate: Thu, 21 Jul 2011 18:01:52 +0200

sched: Reorder root_domain to remove 64 bit alignment padding

Reorder root_domain to remove 8 bytes of alignment padding on 64 bit
builds, this shrinks the size from 1736 to 1728 bytes, therefore using
one fewer cachelines.

Signed-off-by: Richard Kennedy <richard@rsk.demon.co.uk>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1310726492.1977.5.camel@castor.rsk
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
 kernel/sched.c |    2 +-
 1 files changed, 1 insertions(+), 1 deletions(-)

diff --git a/kernel/sched.c b/kernel/sched.c
index f107204..e3f0bac 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -422,6 +422,7 @@ struct rt_rq {
  */
 struct root_domain {
 	atomic_t refcount;
+	atomic_t rto_count;
 	struct rcu_head rcu;
 	cpumask_var_t span;
 	cpumask_var_t online;
@@ -431,7 +432,6 @@ struct root_domain {
 	 * one runnable RT task.
 	 */
 	cpumask_var_t rto_mask;
-	atomic_t rto_count;
 	struct cpupri cpupri;
 };