linux-kernel - Re: [PATCH v2 2/4] mm/sparse: Optimize sparse_add_one

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Message-ID: <20190326225716.GY3659@MiWiFi-R3L-srv>
Date:   Wed, 27 Mar 2019 06:57:16 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        akpm@...ux-foundation.org, rppt@...ux.ibm.com, osalvador@...e.de,
        willy@...radead.org, william.kucharski@...cle.com
Subject: Re: [PATCH v2 2/4] mm/sparse: Optimize sparse_add_one_section()

Hi Michal,

On 03/26/19 at 03:31pm, Michal Hocko wrote:
> > > > OK, I am fine to drop it. Or only put the section existence checking
> > > > earlier to avoid unnecessary usemap/memmap allocation?
> > > 
> > > DO you have any data on how often that happens? Should basically never
> > > happening, right?
> > 
> > Oh, you think about it in this aspect. Yes, it rarely happens.
> > Always allocating firstly can increase efficiency. Then I will just drop
> > it.
> 
> OK, let me try once more. Doing a check early is something that makes
> sense in general. Another question is whether the check is needed at
> all. So rather than fiddling with its placement I would go whether it is
> actually failing at all. I suspect it doesn't because the memory hotplug
> is currently enforced to be section aligned. There are people who would
> like to allow subsection or section unaligned aware hotplug and then
> this would be much more relevant but without any solid justification
> such a patch is not really helpful because it might cause code conflicts
> with other work or obscure the git blame tracking by an additional hop.
> 
> In short, if you want to optimize something then make sure you describe
> what you are optimizing how it helps.

I must be dizzy last night when thinking and replying mails, I thought
about it a while, got a point you may mean. Now when I check mail and
rethink about it, that reply may make misunderstanding. It doesn't
actually makes sense to optimize, just a little code block moving. I now
agree with you that it doesn't optimize anything and may impact people's
code change. Sorry about that.

Thanks
Baoquan