linux-kernel - Re: [PATCH 1/2] mm/page_alloc: add same penalty is enough to get round-robin order

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Date:   Thu, 7 Apr 2022 11:53:19 +0200
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Wei Yang <richard.weiyang@...il.com>
Cc:     akpm@...ux-foundation.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, Oscar Salvador <osalvador@...e.de>,
        David Hildenbrand <david@...hat.com>
Subject: Re: [PATCH 1/2] mm/page_alloc: add same penalty is enough to get
 round-robin order

On 4/7/22 01:47, Wei Yang wrote:
> On Tue, Apr 05, 2022 at 07:11:12PM +0200, Vlastimil Babka wrote:
>>On 1/23/22 02:35, Wei Yang wrote:
>>> To make node order in round-robin in the same distance group, we add a
>>> penalty to the first node we got in each round.
>>> 
>>> To get a round-robin order in the same distance group, we don't need to
>>> decrease the penalty since:
>>> 
>>>   * find_next_best_node() always iterates node in the same order
>>>   * distance matters more then penalty in find_next_best_node()
>>>   * in nodes with the same distance, the first one would be picked up
>>> 
>>> So it is fine to increase same penalty when we get the first node in the
>>> same distance group.
>>
>>With that logic I'm not even sure if we need nr_online_nodes as penalty or
>>it could be just 1. Would you know?
> 
> Yes, it has the same effect.

Good.

> [    0.031849] Fallback order for Node 0: 0 1 2 3 4 5 6 7
> [    0.031854] Fallback order for Node 1: 1 2 3 0 5 6 7 4
> [    0.031857] Fallback order for Node 2: 2 3 0 1 6 7 4 5
> [    0.031860] Fallback order for Node 3: 3 0 1 2 7 4 5 6
> [    0.031864] Fallback order for Node 4: 4 5 6 7 0 1 2 3
> [    0.031867] Fallback order for Node 5: 5 6 7 4 1 2 3 0
> [    0.031870] Fallback order for Node 6: 6 7 4 5 2 3 0 1
> [    0.031873] Fallback order for Node 7: 7 4 5 6 3 0 1 2
> 
> Do you prefer to set it to 1?

Yeah I think it's worth simplyfing as much as feasible, so the code is more
obvious. I think we can also then remove the MAX_NODE_LOAD #define and usage.

Also please Cc at least Oscar and David (added to Cc now) on v2 as they have
been active in memory hotplug area recently.

Thanks,
Vlastimil