linux-kernel - Re: [RFC patch 1/4] sched: change cfs

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Date:	Sat, 08 Jun 2013 10:18:50 +0800
From:	Alex Shi <alex.shi@...el.com>
To:	Vincent Guittot <vincent.guittot@...aro.org>
CC:	"mingo@...hat.com" <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Borislav Petkov <bp@...en8.de>, Paul Turner <pjt@...gle.com>,
	Namhyung Kim <namhyung@...nel.org>,
	Mike Galbraith <efault@....de>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	linux-kernel <linux-kernel@...r.kernel.org>,
	Mel Gorman <mgorman@...e.de>, riel@...hat.com,
	Michael Wang <wangyun@...ux.vnet.ibm.com>,
	Jason Low <jason.low2@...com>
Subject: Re: [RFC patch 1/4] sched: change cfs_rq load avg to unsigned long

On 06/07/2013 05:07 PM, Vincent Guittot wrote:
> On 7 June 2013 09:29, Alex Shi <alex.shi@...el.com> 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.

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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