linux-kernel - Re: CFS scheduler unfairly prefers pinned tasks

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Date:	Sat, 10 Oct 2015 15:58:03 +0800
From:	Wanpeng Li <wanpeng.li@...mail.com>
To:	paul.szabo@...ney.edu.au, Peter Zijlstra <peterz@...radead.org>
CC:	Mike Galbraith <umgwanakikbuti@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: CFS scheduler unfairly prefers pinned tasks

On 10/10/15 11:59 AM, Wanpeng Li wrote:
> Hi Paul,
> On 10/8/15 4:19 PM, Mike Galbraith wrote:
>> On Tue, 2015-10-06 at 04:45 +0200, Mike Galbraith wrote:
>>> On Tue, 2015-10-06 at 08:48 +1100, paul.szabo@...ney.edu.au wrote:
>>>> The Linux CFS scheduler prefers pinned tasks and unfairly
>>>> gives more CPU time to tasks that have set CPU affinity.
>>>> This effect is observed with or without CGROUP controls.
>>>>
>>>> To demonstrate: on an otherwise idle machine, as some user
>>>> run several processes pinned to each CPU, one for each CPU
>>>> (as many as CPUs present in the system) e.g. for a quad-core
>>>> non-HyperThreaded machine:
>>>>
>>>>    taskset -c 0 perl -e 'while(1){1}' &
>>>>    taskset -c 1 perl -e 'while(1){1}' &
>>>>    taskset -c 2 perl -e 'while(1){1}' &
>>>>    taskset -c 3 perl -e 'while(1){1}' &
>>>>
>>>> and (as that same or some other user) run some without
>>>> pinning:
>>>>
>>>>    perl -e 'while(1){1}' &
>>>>    perl -e 'while(1){1}' &
>>>>
>>>> and use e.g.   top   to observe that the pinned processes get
>>>> more CPU time than "fair".
>
> Interesting, I can reproduce it w/ your simple script. However, they 
> are fair when the number of pinned perl tasks is equal to unpinned 
> perl tasks. I will dig into it more deeply.

For the pinned tasks, when set the task affinity to all the available 
cpus instead of the separate cpu as in your test, there is fair between 
pinned tasks and unpinned tasks. So I suspect that if it is the overhead 
associated with migration stuff.

Regards,
Wanpeng Li

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