linux-kernel - Re: [PATCH 1/2] mem_cgroup: optimize the atomic count of wb

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Date:   Fri, 24 Sep 2021 23:21:19 +0800
From:   "brookxu.cn" <brookxu.cn@...il.com>
To:     Michal Hocko <mhocko@...e.com>
Cc:     hannes@...xchg.org, vdavydov.dev@...il.com,
        akpm@...ux-foundation.org, linux-kernel@...r.kernel.org,
        cgroups@...r.kernel.org
Subject: Re: [PATCH 1/2] mem_cgroup: optimize the atomic count of
 wb_completion

Thanks for your time.

On 2021/9/24 10:01 PM, Michal Hocko wrote:
> On Fri 24-09-21 21:02:52, brookxu wrote:
>> Thanks for your time.
>>
>> Michal Hocko wrote on 2021/9/24 17:34:
>>> On Fri 24-09-21 14:46:22, brookxu wrote:
>>>> From: Chunguang Xu <brookxu@...cent.com>
>>>>
>>>> In order to track inflight foreign writeback, we init
>>>> wb_completion.cnt to 1. For normal writeback, this cause
>>>> wb_wait_for_completion() to perform meaningless atomic
>>>> operations. Since foreign writebacks rarely occur in most
>>>> scenarios, we can init wb_completion.cnt to 0 and set
>>>> frn.done.cnt to 1. In this way we can avoid unnecessary
>>>> atomic operations.
>>>
>>> Does this lead to any measurable differences?
>>
>> I created multiple cgroups that performed IO on multiple disks,
>> then flushed the cache with sync command, and no measurable
>> differences have been observed so far.
> 
> OK, so why do we want to optimize this code?

Just a optimization point discovered during the diagnosis, no
behavior change.

>