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:   Thu, 03 Dec 2020 09:49:02 +0800
From:   "Huang\, Ying" <ying.huang@...el.com>
To:     Mel Gorman <mgorman@...e.de>
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

Mel Gorman <mgorman@...e.de> writes:

> 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?

For MPOL_PREFERRED, why could we not use NUMA balancing to migrate pages
to the accessing local node if it is same as the preferred node?  We
have a way to turn off NUMA balancing already, why could we not provide
a way to enable it if that's intended?

Even for MPOL_INTERLEAVE, if the target node is the same as the
accessing local node, can we use NUMA balancing to migrate pages?

So, I prefer to make MPOL_F_NUMA_BALANCING to be

  Optimizing with NUMA balancing if possible, and we may add more
  optimization in the future.

Do you agree?

Best Regards,
Huang, Ying

>>  .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.