Re: [RFC patch 1/4] sched: change cfs_rq load avg to unsigned long

From: Paul Turner
Date: Mon Jun 17 2013 - 05:49:40 EST

On Fri, Jun 7, 2013 at 7:18 PM, Alex Shi <alex.shi@xxxxxxxxx> wrote:
> On 06/07/2013 05:07 PM, Vincent Guittot wrote:
>> On 7 June 2013 09:29, Alex Shi <alex.shi@xxxxxxxxx> wrote:
>>> > Since the 'u64 runnable_load_avg, blocked_load_avg' in cfs_rq struct are
>>> > smaller than 'unsigned long' cfs_rq->load.weight. We don't need u64
>>> > vaiables to describe them. unsigned long is more efficient and convenience.
>>> >
>> Hi Alex,
>> I just want to point out that we can't have more than 48388 tasks with
>> highest priority on a runqueue with an unsigned long on a 32 bits
>> system. I don't know if we can reach such kind of limit on a 32bits
>> machine ? For sure, not on an embedded system.

This should be ok.

Note that:
runnable_load_avg = \Sum se->load_avg_contrib <= \Sum
se->load.weight = cfs_rq->load.weight

And load_weight uses unsigned longs also.

blocked_load_avg must be also safe since anything appearing in blocked
load could have appeared in runnable load and we've said that was ok

Reviewed-By: Paul Turner <pjt@xxxxxxxxxx>

> Thanks question!
> It should be a talked problem. I just remember the conclusion is when
> you get the up bound task number, you already run out the memory space
> on 32 bit.
> Just for kernel resource for a process, it need 2 pages stack.
> mm_struct, task_struct, task_stats, vm_area_struct, page table etc.
> these are already beyond 4 pages. so 4 * 4k * 48388 = 774MB. plus user
> level resources.
> So, usually the limited task number in Linux is often far lower this
> number: $ulimit -u.
> Anyway, at least, the runnable_load_avg is smaller then load.weight. if
> load.weight can use long type, runablle_load_avg is no reason can't.
> --
> Thanks
> Alex
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