linux-kernel - Re: [RFC PATCH v2 11/17] sched: Basic tracking of matching tasks

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Date:   Wed, 8 May 2019 11:37:21 -0700
From:   Subhra Mazumdar <subhra.mazumdar@...cle.com>
To:     Aubrey Li <aubrey.intel@...il.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Cc:     Aaron Lu <aaron.lu@...ux.alibaba.com>,
        Vineeth Remanan Pillai <vpillai@...italocean.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...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>,
        Aaron Lu <aaron.lwe@...il.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 v2 11/17] sched: Basic tracking of matching tasks


On 5/8/19 11:19 AM, Subhra Mazumdar wrote:
>
> On 5/8/19 8:49 AM, Aubrey Li wrote:
>>> Pawan ran an experiment setting up 2 VMs, with one VM doing a 
>>> parallel kernel build and one VM doing sysbench,
>>> limiting both VMs to run on 16 cpu threads (8 physical cores), with 
>>> 8 vcpu for each VM.
>>> Making the fix did improve kernel build time by 7%.
>> I'm gonna agree with the patch below, but just wonder if the testing
>> result is consistent,
>> as I didn't see any improvement in my testing environment.
>>
>> IIUC, from the code behavior, especially for 2 VMs case(only 2
>> different cookies), the
>> per-rq rb tree unlikely has nodes with different cookies, that is, all
>> the nodes on this
>> tree should have the same cookie, so:
>> - if the parameter cookie is equal to the rb tree cookie, we meet a
>> match and go the
>> third branch
>> - else, no matter we go left or right, we can't find a match, and
>> we'll return idle thread
>> finally.
>>
>> Please correct me if I was wrong.
>>
>> Thanks,
>> -Aubrey
> This is searching in the per core rb tree (rq->core_tree) which can have
> 2 different cookies. But having said that, even I didn't see any
> improvement with the patch for my DB test case. But logically it is
> correct.
>
Ah, my bad. It is per rq. But still can have 2 different cookies. Not sure
why you think it is unlikely?