linux-kernel - Re: [PATCH] sched: Improve readability in update_cpu

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Message-ID: <b647ffbd0805150831p1b61437cjbd47697d5dd72f01@mail.gmail.com>
Date:	Thu, 15 May 2008 17:31:52 +0200
From:	"Dmitry Adamushko" <dmitry.adamushko@...il.com>
To:	vatsa@...ux.vnet.ibm.com
Cc:	"Gautham R Shenoy" <ego@...ibm.com>, "Ingo Molnar" <mingo@...e.hu>,
	npiggin@...e.de, linux-kernel@...r.kernel.org,
	"Srivatsa Vaddagiri" <vatsa@...ibm.com>
Subject: Re: [PATCH] sched: Improve readability in update_cpu_load() code

2008/5/15 Dmitry Adamushko <dmitry.adamushko@...il.com>:
> 2008/5/15 Srivatsa Vaddagiri <vatsa@...ux.vnet.ibm.com>:
>> On Thu, May 15, 2008 at 06:34:59PM +0530, Gautham R Shenoy wrote:
>>> Author: Gautham R Shenoy <ego@...ibm.com>
>>> Date:   Thu May 15 17:55:49 2008 +0530
>>>
>>>     sched: Improve readability in update_cpu_load() code
>>>
>>>     Currently the cpu_load[i] is calculated as:
>>>       this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) >> i;
>>>
>>>     However, since scale = 2^i, this can be simplified as:
>>>       this_rq->cpu_load[i] = old_load + ((new_load - old_load) >> i);
>>>
>>>     Makes it easier to read.
>>>     Signed-off-by: Gautham R Shenoy <ego@...ibm.com>
>>>
>>> diff --git a/kernel/sched.c b/kernel/sched.c
>>> index 2d7d8f1..e1a6985 100644
>>> --- a/kernel/sched.c
>>> +++ b/kernel/sched.c
>>> @@ -2921,7 +2921,7 @@ static void update_cpu_load(struct rq *this_rq)
>>>                */
>>>               if (new_load > old_load)
>>>                       new_load += scale-1;
>>> -             this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) >> i;
>>> +             this_rq->cpu_load[i] = old_load + ((new_load - old_load) >> i);
>>
>> This wont work when new_load < old_load ..
>>
>> For ex: I tried this prog:
>>
>> #include <stdio.h>
>>
>> main()
>> {
>>        unsigned long old_load = 100, new_load = 90, this_load, this_load1;
>>        int i = 1, scale = 2 << i;
>>
>>        this_load = (old_load*(scale-1) + new_load) >> i;
>>        this_load1 = old_load + ((new_load - old_load) >> i);
>
> it should be
>
> this_load2 = 2 * old_load + (new_load >> i) - (old_load >> i);
>
> scale == 2 << i == 1 << (i + 1), so scale >> i = 2.

argh.. sorry, it's (scale = 2 << i) in your test-program which is
wrong. 2^i == (1 << i), so the (2 *) part is redundant, of course.

this_load2 = old_load + (new_load >> i) - (old_load >> i);


-- 
Best regards,
Dmitry Adamushko
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