linux-kernel - Re: [PATCH] sched: Accelerate "pick_next

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Date:	Mon, 16 Jan 2012 18:34:45 +0800
From:	Michael Wang <wangyun@...ux.vnet.ibm.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
CC:	ingo Molnar <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] sched: Accelerate "pick_next_entity" under special condition

Hi, peter

Thanks so much for your reply :)

On 01/16/2012 05:51 PM, Peter Zijlstra wrote:

> On Mon, 2012-01-16 at 17:37 +0800, Michael Wang wrote:
>> From: Michael Wang <wangyun@...ux.vnet.ibm.com>
>>
>> We can avoid some useless operation in some special condition.
> 
> This is a pretty empty statement.
> 
>> For example:
>> If we have "cfs_rq->next" and it can be use, we just return it directly.
> 
> What it doesn't state is what it actually does, if it affects the common
> case and performance numbers (or a good reason for the lack thereof).
> 


Please help me to understand the logic, I think in the original code,
even if we have cfs_rq->next and wakeup_preempt_entity check passed, we
still need to do a lot of work (check cfs_rq->last for example) which
have no influence on result, will it be better if we skip them and just
do what really needed?

>> Signed-off-by: Michael Wang <wangyun@...ux.vnet.ibm.com>
>> ---
>>  kernel/sched/fair.c |   28 +++++++++++++++++-----------
>>  1 files changed, 17 insertions(+), 11 deletions(-)
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index 84adb2d..9fc2c3c 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -1295,6 +1295,8 @@ set_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *se)
>>  static int
>>  wakeup_preempt_entity(struct sched_entity *curr, struct sched_entity *se);
>>  
>> +#define ENTITY_PREEMPT_ALLOWED(prev,next)	(wakeup_preempt_entity(prev, next) < 1)
> 

> This is just uglification imo, its shouting and it doesn't actually win
> you much space.
> 


I see, sorry for the bad idea.

Best regards,
Michael Wang

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