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

[Waiman Long <longman@redhat.com>]

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.

-Longman

>
> -----Original Message-----
> From: LKP [mailto:lkp-bounces@lists.01.org] On Behalf Of Waiman Long
> Sent: Tuesday, November 6, 2018 6:40 AM
> To: Linus Torvalds <torvalds@linux-foundation.org>; vbabka@suse.cz; Davidlohr Bueso <dave@stgolabs.net>
> Cc: yang.shi@linux.alibaba.com; Linux Kernel Mailing List <linux-kernel@vger.kernel.org>; Matthew Wilcox <willy@infradead.org>; mhocko@kernel.org; Colin King <colin.king@canonical.com>; Andrew Morton <akpm@linux-foundation.org>; ldufour@linux.vnet.ibm.com; lkp@01.org; kirill.shutemov@linux.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@suse.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
>
>
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