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

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Message-ID: <BANLkTikhAF=nzJA4qg_8By=CL6t9iFPQwg@mail.gmail.com>
Date:	Thu, 28 Apr 2011 11:51:59 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Nikhil Rao <ncrao@...gle.com>
Cc:	vatsa@...ux.vnet.ibm.com,
	"Nikunj A. Dadhania" <nikunj@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	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 11:33 AM, Nikhil Rao <ncrao@...gle.com> wrote:
> 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.
>

I don't think you have much room to maneuver here, the calculations in
c_d_m() are already u64 based, even on 32bit.  Changing the external
load factors to 64 bit doesn't change this.

We lose fairness in cdm beyond 32 bits, at the old LOAD_SCALE=10
you've got 22 bits with which you can maintain fairness. This gives
total accuracy in total curr on any delta <= ~4ms (for a NICE_0 task).
 If you bump this up (and don't downshift before computing the inverse
as you are) then you start introducing rounding errors beyond ~4us.

This would also be further exacerbated in sched_period() since that's
using the total cfs_rq weight.

> -Thanks,
> Nikhil
>
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