linux-kernel - Re: [PATCH 1/1] mm: vmalloc: Optimize vmap_lazy_nr arithmetic when purging each vmap

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Message-ID: <ea017b05-7a42-4d3a-a1f2-9bceb56966e3@wanadoo.fr>
Date: Sat, 31 Aug 2024 09:03:56 +0200
From: Christophe JAILLET <christophe.jaillet@...adoo.fr>
To: Uladzislau Rezki <urezki@...il.com>,
 Adrian Huang <adrianhuang0701@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
 Christoph Hellwig <hch@...radead.org>, linux-mm@...ck.org,
 linux-kernel@...r.kernel.org, Adrian Huang <ahuang12@...ovo.com>
Subject: Re: [PATCH 1/1] mm: vmalloc: Optimize vmap_lazy_nr arithmetic when
 purging each vmap_area

Le 30/08/2024 à 18:26, Uladzislau Rezki a écrit :

> At least in my case, HW, i do not see that atomic_long_add_return() is a
> top when it comes to CPU cycles. Below one is the hottest instead:
> 
> static bool
> is_vn_id_valid(unsigned int node_id)
> {
> 	if (node_id < nr_vmap_nodes)
> 		return true;
> 
> 	return false;
> }
> 
> access to "nr_vmap_nodes" which is read-only and globally defined:
> 
> static __read_mostly unsigned int nr_vmap_nodes = 1;
> 
> Any thoughts?
> 
> --
> Uladzislau Rezki
> 

Hi,

unrelated to your use case, but something that coud easily save a few 
cycles on some system, IMHO.

Maybe:

#if NR_CPUS > 1
static __read_mostly unsigned int nr_vmap_nodes = 1;
static __read_mostly unsigned int vmap_zone_size = 1;
#else
#define nr_vmap_nodes	1
#define vmap_zone_size	1
#endif

So that the compiler can do a better job because some loops can be 
optimized away and there is no need to access some memory to get theses 
values.

Not sure if such a use case can exist or is of any interest.

This is valide because of [1] and the #ifdef around the 
num_possible_cpus() declaration [2, 3].


Just my 2c.

CJ

[1]: https://elixir.bootlin.com/linux/v6.11-rc5/source/mm/vmalloc.c#L5026

[2]: 
https://elixir.bootlin.com/linux/v6.11-rc5/source/include/linux/cpumask.h#L1083
[3]: 
https://elixir.bootlin.com/linux/v6.11-rc5/source/include/linux/cpumask.h#L1136