linux-kernel - Re: [RFC PATCH v3 00/16] Core scheduling v3

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Date:   Wed, 11 Sep 2019 09:19:02 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Aaron Lu <aaron.lu@...ux.alibaba.com>,
        Julien Desfossez <jdesfossez@...italocean.com>
Cc:     Dario Faggioli <dfaggioli@...e.com>,
        "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Subhra Mazumdar <subhra.mazumdar@...cle.com>,
        Vineeth Remanan Pillai <vpillai@...italocean.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Paul Turner <pjt@...gle.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Frédéric Weisbecker <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kerr <kerrnel@...gle.com>, Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>
Subject: Re: [RFC PATCH v3 00/16] Core scheduling v3

On 9/11/19 7:02 AM, Aaron Lu wrote:
> Hi Tim & Julien,
> 
> On Fri, Sep 06, 2019 at 11:30:20AM -0700, Tim Chen wrote:
>> On 8/7/19 10:10 AM, Tim Chen wrote:
>>
>>> 3) Load balancing between CPU cores
>>> -----------------------------------
>>> Say if one CPU core's sibling threads get forced idled
>>> a lot as it has mostly incompatible tasks between the siblings,
>>> moving the incompatible load to other cores and pulling
>>> compatible load to the core could help CPU utilization.
>>>
>>> So just considering the load of a task is not enough during
>>> load balancing, task compatibility also needs to be considered.
>>> Peter has put in mechanisms to balance compatible tasks between
>>> CPU thread siblings, but not across cores.
>>>
>>> Status:
>>> I have not seen patches on this issue.  This issue could lead to
>>> large variance in workload performance based on your luck
>>> in placing the workload among the cores.
>>>
>>
>> I've made an attempt in the following two patches to address
>> the load balancing of mismatched load between the siblings.
>>
>> It is applied on top of Aaron's patches:
>> - sched: Fix incorrect rq tagged as forced idle
>> - wrapper for cfs_rq->min_vruntime
>>   https://lore.kernel.org/lkml/20190725143127.GB992@aaronlu/
>> - core vruntime comparison
>>   https://lore.kernel.org/lkml/20190725143248.GC992@aaronlu/
> 
> So both of you are working on top of my 2 patches that deal with the
> fairness issue, but I had the feeling Tim's alternative patches[1] are
> simpler than mine and achieves the same result(after the force idle tag

I think Julien's result show that my patches did not do as well as
your patches for fairness. Aubrey did some other testing with the same
conclusion.  So I think keeping the forced idle time balanced is not
enough for maintaining fairness.

Will love to see if my load balancing patches help for your workload.

Tim

> fix), so unless there is something I missed, I think we should go with
> the simpler one?
> 
> [1]: https://lore.kernel.org/lkml/b7a83fcb-5c34-9794-5688-55c52697fd84@linux.intel.com/
>