linux-kernel - Re: [PATCH v15 10/10] lib/nodemask: Optimize node_random for nodemask with single NUMA node

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Date:   Mon, 26 Sep 2022 13:17:53 -0700
From:   Yury Norov <yury.norov@...il.com>
To:     "Aneesh Kumar K.V" <aneesh.kumar@...ux.ibm.com>
Cc:     linux-mm@...ck.org, akpm@...ux-foundation.org,
        Wei Xu <weixugc@...gle.com>, Huang Ying <ying.huang@...el.com>,
        Yang Shi <shy828301@...il.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Tim C Chen <tim.c.chen@...el.com>,
        Michal Hocko <mhocko@...nel.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hesham Almatary <hesham.almatary@...wei.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Jonathan Cameron <Jonathan.Cameron@...wei.com>,
        Alistair Popple <apopple@...dia.com>,
        Dan Williams <dan.j.williams@...el.com>,
        Johannes Weiner <hannes@...xchg.org>, jvgediya.oss@...il.com,
        Bharata B Rao <bharata@....com>
Subject: Re: [PATCH v15 10/10] lib/nodemask: Optimize node_random for
 nodemask with single NUMA node

Hi Aneesh,

Please CC maintainers in your recipient list.

On Thu, Aug 18, 2022 at 06:40:42PM +0530, Aneesh Kumar K.V wrote:
> The most common case for certain node_random usage (demotion nodemask) is with
> nodemask weight 1. We can avoid calling get_random_init() in that case and
> always return the only node set in the nodemask.

Can you move the comment about get_random_int() to the code?.
 
> A simple test as below
>   before = rdtsc_ordered();
>   for (i= 0; i < 100; i++) {
>       rand = node_random(&nmask);
>   }
>   after = rdtsc_ordered();
> 
> Without fix after - before : 16438
> With fix after - before : 816
> 
> Reviewed-by: "Huang, Ying" <ying.huang@...el.com>
> Signed-off-by: Aneesh Kumar K.V <aneesh.kumar@...ux.ibm.com>
> ---
>  include/linux/nodemask.h | 15 ++++++++++++---
>  1 file changed, 12 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/nodemask.h b/include/linux/nodemask.h
> index 4b71a96190a8..ac5b6a371be5 100644
> --- a/include/linux/nodemask.h
> +++ b/include/linux/nodemask.h
> @@ -504,12 +504,21 @@ static inline int num_node_state(enum node_states state)
>  static inline int node_random(const nodemask_t *maskp)
>  {
>  #if defined(CONFIG_NUMA) && (MAX_NUMNODES > 1)
> -	int w, bit = NUMA_NO_NODE;
> +	int w, bit;
>  
>  	w = nodes_weight(*maskp);
> -	if (w)
> +	switch (w) {
> +	case 0:
> +		bit = NUMA_NO_NODE;
> +		break;

Why not 'return NUMA_NO_NODE' instead of break thing?

> +	case 1:
> +		bit = first_node(*maskp);
> +		break;
> +	default:
>  		bit = bitmap_ord_to_pos(maskp->bits,
> -			get_random_int() % w, MAX_NUMNODES);
> +					get_random_int() % w, MAX_NUMNODES);

Don't fix tabs - it trashes the history.

> +		break;
> +	}
>  	return bit;
>  #else
>  	return 0;
> -- 
> 2.37.2
> 
>