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

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Date:   Thu, 19 Jul 2018 22:52:35 +0200
From:   Oscar Salvador <osalvador@...hadventures.net>
To:     Michal Hocko <mhocko@...nel.org>
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 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.

> Btw. are you sure this change is correct? E.g.
> 		/*
> 		 * Set an approximate value for lowmem here, it will be adjusted
> 		 * when the bootmem allocator frees pages into the buddy system.
> 		 * And all highmem pages will be managed by the buddy system.
> 		 */
> 		zone->managed_pages = is_highmem_idx(j) ? realsize : freesize;
> 
> expects freesize to be calculated properly and just from quick reading
> the code I do not see why skipping other adjustments is ok for size > 0.
> Maybe this is OK, I dunno and my brain is already heading few days off
> but a real cleanup wouldn't even make me think what the heck is going on
> here.

This changed in commit e69438596bb3e97809e76be315e54a4a444f4797.
Current code does not have "realsize" anymore.

Thanks
-- 
Oscar Salvador
SUSE L3