linux-kernel - Re: [PATCH 3/9] sched: Move idle

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Message-ID: <52E129A8.5050503@linaro.org>
Date:	Thu, 23 Jan 2014 15:39:36 +0100
From:	Daniel Lezcano <daniel.lezcano@...aro.org>
To:	Peter Zijlstra <peterz@...radead.org>, linux-kernel@...r.kernel.org
CC:	mingo@...nel.org, pjt@...gle.com, bsegall@...gle.com,
	Jason Low <jason.low2@...com>
Subject: Re: [PATCH 3/9] sched: Move idle_stamp up to the core

On 01/23/2014 01:58 PM, Peter Zijlstra wrote:
> On Tue, Jan 21, 2014 at 12:17:57PM +0100, Peter Zijlstra wrote:
>> From: Daniel Lezcano <daniel.lezcano@...aro.org>
>>
>> The idle_balance modifies the idle_stamp field of the rq, making this
>> information to be shared across core.c and fair.c. As we can know if the
>> cpu is going to idle or not with the previous patch, let's encapsulate the
>> idle_stamp information in core.c by moving it up to the caller. The
>> idle_balance function returns true in case a balancing occured and the cpu
>> won't be idle, false if no balance happened and the cpu is going idle.
>>
>> Cc: alex.shi@...aro.org
>> Cc: peterz@...radead.org
>> Cc: mingo@...nel.org
>> Signed-off-by: Daniel Lezcano <daniel.lezcano@...aro.org>
>> Signed-off-by: Peter Zijlstra <peterz@...radead.org>
>> Link: http://lkml.kernel.org/r/1389949444-14821-3-git-send-email-daniel.lezcano@linaro.org
>> ---
>>   kernel/sched/core.c  |    2 +-
>>   kernel/sched/fair.c  |   14 ++++++--------
>>   kernel/sched/sched.h |    2 +-
>>   3 files changed, 8 insertions(+), 10 deletions(-)
>>
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -2680,7 +2680,7 @@ static void __sched __schedule(void)
>>   	pre_schedule(rq, prev);
>>
>>   	if (unlikely(!rq->nr_running))
>> -		idle_balance(rq);
>> +		rq->idle_stamp = idle_balance(rq) ? 0 : rq_clock(rq);
>
> OK, spotted a problem here..
>
> So previously idle_stamp was set _before_ actually doing idle_balance(),
> and that was RIGHT, because that way we include the cost of actually
> doing idle_balance() into the idle time.
>
> By not including the cost of idle_balance() you underestimate the 'idle'
> time in that if idle_balance() filled the entire idle time we account 0
> idle, even though we had 'plenty' of time to run the entire thing.
>
> This leads to not running idle_balance() even though we have the time
> for it.

Good catch. How did you notice that ?

> So we very much want something like:
>
>
>    if (!rq->nr_running)
>    	rq->idle_stamp = rq_clock(rq);
>
>    p = pick_next_task(rq, prev);
>
>    if (!is_idle_task(p))
>    	rq->idle_stamp = 0;

Is this code assuming idle_balance() is in pick_next_task ?

For this specific patch 3/9, will be ok the following ?

+       if (unlikely(!rq->nr_running)) {
+               rq->idle_stamp = rq_clock(rq);
+               if (idle_balance(rq))
+                       rq->idle_stamp = 0;
+       }

So the patch 9/9 is wrong also because it does not fill idle_stamp 
before idle_balance, the fix would be.

  kernel/sched/core.c |    8 +++++---
  1 file changed, 5 insertions(+), 3 deletions(-)

Index: cpuidle-next/kernel/sched/core.c
===================================================================
--- cpuidle-next.orig/kernel/sched/core.c
+++ cpuidle-next/kernel/sched/core.c
@@ -2579,15 +2579,17 @@ again:
  		}
  	}

+	rq->idle_stamp = rq_clock(rq);
+
  	/*
  	 * If there is a task balanced on this cpu, pick the next task,
  	 * otherwise fall in the optimization by picking the idle task
  	 * directly.
  	 */
-	if (idle_balance(rq))
+	if (idle_balance(rq)) {
+		rq->idle_stamp = 0;
  		goto again;
-
-	rq->idle_stamp = rq_clock(rq);
+	}

  	return idle_sched_class.pick_next_task(rq, prev);
  }


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