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

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Message-ID: <YU3aT7i2vBNxewam@dhcp22.suse.cz>
Date:   Fri, 24 Sep 2021 16:01:51 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     brookxu <brookxu.cn@...il.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

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?
-- 
Michal Hocko
SUSE Labs