linux-kernel - Re: [PATCH] mm/numa_balancing: Teach mpol_to

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Date: Thu, 27 Jun 2024 14:37:19 -0700
From: Andrew Morton <akpm@...ux-foundation.org>
To: Tvrtko Ursulin <tursulin@...lia.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org, kernel-dev@...lia.com,
 Tvrtko Ursulin <tvrtko.ursulin@...lia.com>, Huang Ying
 <ying.huang@...el.com>, Mel Gorman <mgorman@...e.de>, Peter Zijlstra
 <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, Rik van Riel
 <riel@...riel.com>, Johannes Weiner <hannes@...xchg.org>,
 "Matthew Wilcox (Oracle)" <willy@...radead.org>, Dave Hansen
 <dave.hansen@...el.com>, Andi Kleen <ak@...ux.intel.com>, Michal Hocko
 <mhocko@...e.com>, David Rientjes <rientjes@...gle.com>
Subject: Re: [PATCH] mm/numa_balancing: Teach mpol_to_str about the
 balancing mode

On Tue, 25 Jun 2024 14:26:05 +0100 Tvrtko Ursulin <tursulin@...lia.com> wrote:

> From: Tvrtko Ursulin <tvrtko.ursulin@...lia.com>
> 
> If a task has had MPOL_F_NUMA_BALANCING set it is useful to show that in
> procfs. Teach the mpol_to_str helper about its existance and while at it
> update the comment to account for "weighted interleave" when suggesting
> a recommended buffer size.
> 
> ...
>
> --- a/mm/mempolicy.c
> +++ b/mm/mempolicy.c
> @@ -3293,8 +3293,9 @@ int mpol_parse_str(char *str, struct mempolicy **mpol)
>   * @pol:  pointer to mempolicy to be formatted
>   *
>   * Convert @pol into a string.  If @buffer is too short, truncate the string.
> - * Recommend a @maxlen of at least 32 for the longest mode, "interleave", the
> - * longest flag, "relative", and to display at least a few node ids.
> + * Recommend a @maxlen of at least 42 for the longest mode, "weighted
> + * interleave", the longest flag, "balancing", and to display at least a few
> + * node ids.
>   */
>  void mpol_to_str(char *buffer, int maxlen, struct mempolicy *pol)
>  {
> @@ -3331,12 +3332,15 @@ void mpol_to_str(char *buffer, int maxlen, struct mempolicy *pol)
>  		p += snprintf(p, buffer + maxlen - p, "=");
>  
>  		/*
> -		 * Currently, the only defined flags are mutually exclusive
> +		 * The below two flags are mutually exclusive:
>  		 */
>  		if (flags & MPOL_F_STATIC_NODES)
>  			p += snprintf(p, buffer + maxlen - p, "static");
>  		else if (flags & MPOL_F_RELATIVE_NODES)
>  			p += snprintf(p, buffer + maxlen - p, "relative");
> +
> +		if (flags & MPOL_F_NUMA_BALANCING)
> +			p += snprintf(p, buffer + maxlen - p, "balancing");
>  	}
>  
>  	if (!nodes_empty(nodes))

Is it strange to report this via mount options?  `static' and
`relative' can be set via mount options but afaict `balancing' cannot? 
I guess not...

Documentation/filesystems/tmpfs.rst appears to be a suitable place to
document this new userspace API please.