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

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Date:	Sat, 10 Oct 2015 11:59:41 +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

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.

Regards,
Wanpeng Li
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