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Date:   Wed, 8 May 2019 19:14:17 -0700
From:   Subhra Mazumdar <>
To:     Aubrey Li <>
Cc:     Tim Chen <>,
        Aaron Lu <>,
        Vineeth Remanan Pillai <>,
        Nishanth Aravamudan <>,
        Julien Desfossez <>,
        Peter Zijlstra <>,
        Ingo Molnar <>,
        Thomas Gleixner <>,
        Paul Turner <>,
        Linus Torvalds <>,
        Linux List Kernel Mailing <>,
        Frédéric Weisbecker <>,
        Kees Cook <>,
        Greg Kerr <>, Phil Auld <>,
        Aaron Lu <>,
        Valentin Schneider <>,
        Mel Gorman <>,
        Pawan Gupta <>,
        Paolo Bonzini <>
Subject: Re: [RFC PATCH v2 11/17] sched: Basic tracking of matching tasks

On 5/8/19 6:38 PM, Aubrey Li wrote:
> On Thu, May 9, 2019 at 8:29 AM Subhra Mazumdar
> <> wrote:
>> On 5/8/19 5:01 PM, Aubrey Li wrote:
>>> On Thu, May 9, 2019 at 2:41 AM Subhra Mazumdar
>>> <> wrote:
>>>> 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?
>>> Yeah, I meant 2 different cookies on the system, but unlikely 2
>>> different cookies
>>> on one same rq.
>>> If I read the source correctly, for the sched_core_balance path, when try to
>>> steal cookie from another CPU, sched_core_find() uses dst's cookie to search
>>> if there is a cookie match in src's rq, and sched_core_find() returns idle or
>>> matched task, and later put this matched task onto dst's rq (activate_task() in
>>> sched_core_find()). At this moment, the nodes on the rq's rb tree should have
>>> same cookies.
>>> Thanks,
>>> -Aubrey
>> Yes, but sched_core_find is also called from pick_task to find a local
>> matching task.
> Can a local searching introduce a different cookies? Where is it from?
No. I meant the local search uses the same binary search of sched_core_find
so it has to be correct.
>> The enqueue side logic of the scheduler is unchanged with
>> core scheduling,
> But only the task with cookies is placed onto this rb tree?
>> so it is possible tasks with different cookies are
>> enqueued on the same rq. So while searching for a matching task locally
>> doing it correctly should matter.
> May I know how exactly?
select_task_rq_* seems to be unchanged. So the search logic to find a cpu
to enqueue when a task becomes runnable is same as before and doesn't do
any kind of cookie matching.
> Thanks,
> -Aubrey

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