linux-kernel - Re: [LKP] [mm] 9bc8039e71: will-it-scale.per_thread_ops -64.1% regression

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Date:   Fri, 28 Dec 2018 10:55:05 +0800
From:   kemi <kemi.wang@...el.com>
To:     Waiman Long <longman@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        "vbabka@...e.cz" <vbabka@...e.cz>,
        Davidlohr Bueso <dave@...olabs.net>
Cc:     "yang.shi@...ux.alibaba.com" <yang.shi@...ux.alibaba.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Matthew Wilcox <willy@...radead.org>,
        "mhocko@...nel.org" <mhocko@...nel.org>,
        Colin King <colin.king@...onical.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        "ldufour@...ux.vnet.ibm.com" <ldufour@...ux.vnet.ibm.com>,
        "lkp@...org" <lkp@...org>,
        "kirill.shutemov@...ux.intel.com" <kirill.shutemov@...ux.intel.com>
Subject: Re: [LKP] [mm] 9bc8039e71: will-it-scale.per_thread_ops -64.1%
 regression

On 2018/12/28 上午10:55, Waiman Long wrote:
> On 12/27/2018 08:31 PM, Wang, Kemi wrote:
>> Hi, Waiman
>>    Did you post that patch? Let's see if it helps.
> 
> I did post the patch a while ago. I will need to rebase it to a new
> baseline. Will do that in a week or 2.
> 

OK．I　will take a look at it and try to rebase it on shi's patch to see if 
the regression can be fixed.
May I know where I can get that patch, I didn't find it in my inbox. Thanks

> -Longman
> 
>>
>> -----Original Message-----
>> From: LKP [mailto:lkp-bounces@...ts.01.org] On Behalf Of Waiman Long
>> Sent: Tuesday, November 6, 2018 6:40 AM
>> To: Linus Torvalds <torvalds@...ux-foundation.org>; vbabka@...e.cz; Davidlohr Bueso <dave@...olabs.net>
>> Cc: yang.shi@...ux.alibaba.com; Linux Kernel Mailing List <linux-kernel@...r.kernel.org>; Matthew Wilcox <willy@...radead.org>; mhocko@...nel.org; Colin King <colin.king@...onical.com>; Andrew Morton <akpm@...ux-foundation.org>; ldufour@...ux.vnet.ibm.com; lkp@...org; kirill.shutemov@...ux.intel.com
>> Subject: Re: [LKP] [mm] 9bc8039e71: will-it-scale.per_thread_ops -64.1% regression
>>
>> On 11/05/2018 05:14 PM, Linus Torvalds wrote:
>>> On Mon, Nov 5, 2018 at 12:12 PM Vlastimil Babka <vbabka@...e.cz> wrote:
>>>> I didn't spot an obvious mistake in the patch itself, so it looks
>>>> like some bad interaction between scheduler and the mmap downgrade?
>>> I'm thinking it's RWSEM_SPIN_ON_OWNER that ends up being confused by
>>> the downgrade.
>>>
>>> It looks like the benchmark used to be basically CPU-bound, at about
>>> 800% CPU, and now it's somewhere in the 200% CPU region:
>>>
>>>                   will-it-scale.time.percent_of_cpu_this_job_got
>>>
>>>   800 +-+-------------------------------------------------------------------+
>>>       |.+.+.+.+.+.+.+.  .+.+.+.+.+.+.+.+.+.+.+.+.+.+.+.+.+..+.+.+.+. .+.+.+.|
>>>   700 +-+             +.                                            +       |
>>>       |                                                                     |
>>>   600 +-+                                                                   |
>>>       |                                                                     |
>>>   500 +-+                                                                   |
>>>       |                                                                     |
>>>   400 +-+                                                                   |
>>>       |                                                                     |
>>>   300 +-+                                                                   |
>>>       |                                                                     |
>>>   200 O-O O O O O                O                                          |
>>>       |           O O O  O O O O   O O O O O O O O O O O                    |
>>>   100 +-+-------------------------------------------------------------------+
>>>
>>> which sounds like the downgrade really messes with the "spin waiting
>>> for lock" logic.
>>>
>>> I'm thinking it's the "wake up waiter" logic that has some bad
>>> interaction with spinning, and breaks that whole optimization.
>>>
>>> Adding Waiman and Davidlohr to the participants, because they seem to
>>> be the obvious experts in this area.
>>>
>>>                             Linus
>> Optimistic spinning on rwsem is done only on writers spinning on a
>> writer-owned rwsem. If a write-lock is downgraded to a read-lock, all
>> the spinning waiters will quit. That may explain the drop in cpu
>> utilization. I do have a old patch that enable a certain amount of
>> reader spinning which may help the situation. I can rebase that and send
>> it out for review if people have interest.
>>
>> Cheers,
>> Longman
>>
>>
>> _______________________________________________
>> LKP mailing list
>> LKP@...ts.01.org
>> https://lists.01.org/mailman/listinfo/lkp
> 
> 

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