linux-kernel - Re: [patch] sched: fix broken smt/mc optimizations with CFS

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Date:	Tue, 04 Sep 2007 19:35:21 -0400
From:	Chuck Ebbert <cebbert@...hat.com>
To:	"Siddha, Suresh B" <suresh.b.siddha@...el.com>
CC:	mingo@...e.hu, nickpiggin@...oo.com.au,
	linux-kernel@...r.kernel.org, akpm@...ux-foundation.org
Subject: Re: [patch] sched: fix broken smt/mc optimizations with CFS

On 08/28/2007 06:27 PM, Siddha, Suresh B wrote:
> On Mon, Aug 27, 2007 at 12:31:03PM -0700, Siddha, Suresh B wrote:
>> Essentially I observed that nice 0 tasks still endup on two cores of same
>> package, with out getting spread out to two different packages. This behavior
>> is same with out this fix and this fix doesn't help in any way.
> 
> Ingo, Appended patch seems to fix the issue and as far as I can test, seems ok
> to me.
> 
> This is a quick fix for .23. Peter Williams and myself plan to look at
> code cleanups in this area (HT/MC optimizations) post .23
> 
> BTW, with this fix, do you want to retain the current FUZZ value?
> 
> thanks,
> suresh
> --
> 
> Try to fix MC/HT scheduler optimization breakage again, with out breaking
> the FUZZ logic.
> 
> First fix the check
> 	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task)
> with this
> 	if (*imbalance < busiest_load_per_task)
> 
> As the current check is always false for nice 0 tasks (as SCHED_LOAD_SCALE_FUZZ
> is same as busiest_load_per_task for nice 0 tasks).
> 
> With the above change, imbalance was getting reset to 0 in the corner case
> condition, making the FUZZ logic fail. Fix it by not corrupting the
> imbalance and change the imbalance, only when it finds that the
> HT/MC optimization is needed.
> 
> Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>
> ---
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 9fe473a..03e5e8d 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -2511,7 +2511,7 @@ group_next:
>  	 * a think about bumping its value to force at least one task to be
>  	 * moved
>  	 */
> -	if (*imbalance + SCHED_LOAD_SCALE_FUZZ < busiest_load_per_task) {
> +	if (*imbalance < busiest_load_per_task) {
>  		unsigned long tmp, pwr_now, pwr_move;
>  		unsigned int imbn;
>  
> @@ -2563,10 +2563,8 @@ small_imbalance:
>  		pwr_move /= SCHED_LOAD_SCALE;
>  
>  		/* Move if we gain throughput */
> -		if (pwr_move <= pwr_now)
> -			goto out_balanced;
> -
> -		*imbalance = busiest_load_per_task;
> +		if (pwr_move > pwr_now)
> +			*imbalance = busiest_load_per_task;
>  	}
>  
>  	return busiest;

Seems this didn't get merged? Latest git as of today still has the code
as it was before this patch.
-
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