linux-kernel - Re: [PATCH -V6 RESEND 2/3] NOT kernel/man-pages: man2/set_mempolicy.2: Add mode flag MPOL_F_NUMA

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Date:   Wed, 2 Dec 2020 11:43:57 +0000
From:   Mel Gorman <mgorman@...e.de>
To:     Huang Ying <ying.huang@...el.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        "Matthew Wilcox (Oracle)" <willy@...radead.org>,
        Rafael Aquini <aquini@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...hat.com>,
        Rik van Riel <riel@...riel.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Dave Hansen <dave.hansen@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Michal Hocko <mhocko@...e.com>,
        David Rientjes <rientjes@...gle.com>, linux-api@...r.kernel.org
Subject: Re: [PATCH -V6 RESEND 2/3] NOT kernel/man-pages:
 man2/set_mempolicy.2: Add mode flag MPOL_F_NUMA_BALANCING

On Wed, Dec 02, 2020 at 04:42:33PM +0800, Huang Ying wrote:
> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
> ---
>  man2/set_mempolicy.2 | 9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/man2/set_mempolicy.2 b/man2/set_mempolicy.2
> index 68011eecb..3754b3e12 100644
> --- a/man2/set_mempolicy.2
> +++ b/man2/set_mempolicy.2
> @@ -113,6 +113,12 @@ A nonempty
>  .I nodemask
>  specifies node IDs that are relative to the set of
>  node IDs allowed by the process's current cpuset.
> +.TP
> +.BR MPOL_F_NUMA_BALANCING " (since Linux 5.11)"
> +Enable the Linux kernel NUMA balancing for the task if it is supported
> +by kernel.
> +If the flag isn't supported by Linux kernel, return -1 and errno is
> +set to EINVAL.
>  .PP
>  .I nodemask
>  points to a bit mask of node IDs that contains up to
> @@ -293,6 +299,9 @@ argument specified both

Should this be expanded more to clarify it applies to MPOL_BIND
specifically?

Maybe the first patch should be expanded more and explictly fail if
MPOL_F_NUMA_BALANCING is used with anything other than MPOL_BIND?

>  .B MPOL_F_STATIC_NODES
>  and
>  .BR MPOL_F_RELATIVE_NODES .
> +Or, the
> +.B MPOL_F_NUMA_BALANCING
> +isn't supported by the Linux kernel.

This will be difficult for an app to distinguish but we can't go back in
time and make this ENOSYS :(

The linux-api people might have more guidance but it may go to the
extent of including a small test program in the manual page for a
sequence that tests whether MPOL_F_NUMA_BALANCING works. They might have
a better recommendation on how it should be handled.

-- 
Mel Gorman
SUSE Labs