linux-kernel - Re: [RFC][PATCH 00/18] Increase resolution of load weights

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Message-ID: <BANLkTimKUSGU_8YzBMQcf2tH4tJ7-o6eBg@mail.gmail.com>
Date:	Thu, 28 Apr 2011 11:33:31 -0700
From:	Nikhil Rao <ncrao@...gle.com>
To:	vatsa@...ux.vnet.ibm.com
Cc:	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Paul Turner <pjt@...gle.com>, Mike Galbraith <efault@....de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>
Subject: Re: [RFC][PATCH 00/18] Increase resolution of load weights

On Thu, Apr 28, 2011 at 5:12 AM, Srivatsa Vaddagiri
<vatsa@...ux.vnet.ibm.com> wrote:
> On Thu, Apr 28, 2011 at 05:18:27PM +0530, Nikunj A. Dadhania wrote:
>> --- kernel/sched.c.orig       2011-04-28 16:34:24.000000000 +0530
>> +++ kernel/sched.c    2011-04-28 16:36:29.000000000 +0530
>> @@ -1336,7 +1336,7 @@ calc_delta_mine(unsigned long delta_exec
>>                       lw->inv_weight = 1 + (WMULT_CONST - w/2) / (w + 1);
>>       }
>>
>> -     tmp = (u64)delta_exec * weight;
>> +     tmp = (u64)delta_exec * (weight >> SCHED_LOAD_RESOLUTION);
>
> Should we be fixing inv_weight rather to account for SCHED_LOAD_RESOLUTION here?
>

Yes, I have been looking into fixing inv_weight and calc_delta_mine()
calculations based on the assumption that we have u64 weights. IMO the
function is complicated because the return value needs to be
calculated to fit into unsigned long. I would like to update users of
calc_delta_mine() to use u64 instead of unsigned longs and I think
this can be easily done (quick inspection of the code shows two call
sites that need to be updated - update_curr() and wakeup_gran()).
Without the restriction to fit into unsigned long, I think we can make
calc_delta_mine() and the inv_weight calculations simpler.

-Thanks,
Nikhil
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