linux-kernel - Re: [PATCH v2 3/5] mm/page_alloc: Optimize free_area_init

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Message-ID: <20180723083519.GG17905@dhcp22.suse.cz>
Date:   Mon, 23 Jul 2018 10:35:19 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Oscar Salvador <osalvador@...hadventures.net>
Cc:     akpm@...ux-foundation.org, pasha.tatashin@...cle.com,
        vbabka@...e.cz, aaron.lu@...el.com, iamjoonsoo.kim@....com,
        linux-kernel@...r.kernel.org, linux-mm@...ck.org,
        Oscar Salvador <osalvador@...e.de>
Subject: Re: [PATCH v2 3/5] mm/page_alloc: Optimize free_area_init_core

On Fri 20-07-18 12:03:27, Oscar Salvador wrote:
> On Thu, Jul 19, 2018 at 10:52:35PM +0200, Oscar Salvador wrote:
> > On Thu, Jul 19, 2018 at 05:15:55PM +0200, Michal Hocko wrote:
> > > Your changelog doesn't really explain the motivation. Does the change
> > > help performance? Is this a pure cleanup?
> > 
> > Hi Michal,
> > 
> > Sorry to not have explained this better from the very beginning.
> > 
> > It should help a bit in performance terms as we would be skipping those
> > condition checks and assignations for zones that do not have any pages.
> > It is not a huge win, but I think that skipping code we do not really need to run
> > is worh to have.
> > 
> > > The function is certainly not an example of beauty. It is more an
> > > example of changes done on top of older ones without much thinking. But
> > > I do not see your change would make it so much better. I would consider
> > > it a much nicer cleanup if it was split into logical units each doing
> > > one specific thing.
> > 
> > About the cleanup, I thought that moving that block of code to a separate function
> > would make the code easier to follow.
> > If you think that this is still not enough, I can try to split it and see the outcome.
> 
> I tried to split it innto three logical blocks:
> 
> - Substract memmap pages
> - Substract dma reserves
> - Account kernel pages (nr_kernel_pages and nr_total_pages)

No, I do not think this is much better. Why do we need to separate those
functions out? I think you are too focused on the current function
without a broader context. Think about it. We have two code paths.
Early initialization and the hotplug. The two are subtly different in
some aspects. Maybe reusing free_area_init_core is the wrong thing and
we should have a dedicated subset of this function. This would make the
code more clear probably. You wouldn't have to think which part of
free_area_init_core is special and what has to be done if this function
was to be used in a different context. See my point?
-- 
Michal Hocko
SUSE Labs