linux-kernel - Re: [PATCH 3/5] mm/vmalloc.c: correct lazy_max

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Date:   Wed, 21 Sep 2016 14:21:35 -0700 (PDT)
From:   David Rientjes <rientjes@...gle.com>
To:     zijun_hu <zijun_hu@...o.com>
cc:     Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, zijun_hu@....com, tj@...nel.org,
        mingo@...nel.org, iamjoonsoo.kim@....com,
        mgorman@...hsingularity.net
Subject: Re: [PATCH 3/5] mm/vmalloc.c: correct lazy_max_pages() return
 value

On Wed, 21 Sep 2016, zijun_hu wrote:

> From: zijun_hu <zijun_hu@....com>
> 
> correct lazy_max_pages() return value if the number of online
> CPUs is power of 2
> 
> Signed-off-by: zijun_hu <zijun_hu@....com>
> ---
>  mm/vmalloc.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)
> 
> diff --git a/mm/vmalloc.c b/mm/vmalloc.c
> index a125ae8..2804224 100644
> --- a/mm/vmalloc.c
> +++ b/mm/vmalloc.c
> @@ -594,7 +594,9 @@ static unsigned long lazy_max_pages(void)
>  {
>  	unsigned int log;
>  
> -	log = fls(num_online_cpus());
> +	log = num_online_cpus();
> +	if (log > 1)
> +		log = (unsigned int)get_count_order(log);
>  
>  	return log * (32UL * 1024 * 1024 / PAGE_SIZE);
>  }

The implementation of lazy_max_pages() is somewhat arbitrarily defined, 
the existing approximation has been around for eight years and 
num_online_cpus() isn't intended to be rounded up to the next power of 2.  
I'd be inclined to just leave it as it is.