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From:   Yang Shi <shy828301@gmail.com>
Date:   Mon, 8 Mar 2021 15:58:56 -0800
Message-ID: <CAHbLzkqee9w6m4NQ4+=Dmp+E10=DVYGYP4FzA8hMJNvBAMMQ0w@mail.gmail.com>
Subject: Re: [PATCH 01/10] mm/numa: node demotion data structure and lookup
To:     Dave Hansen <dave.hansen@linux.intel.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Linux MM <linux-mm@kvack.org>,
        Yang Shi <yang.shi@linux.alibaba.com>,
        David Rientjes <rientjes@google.com>,
        Huang Ying <ying.huang@intel.com>,
        Dan Williams <dan.j.williams@intel.com>,
        David Hildenbrand <david@redhat.com>,
        Oscar Salvador <osalvador@suse.de>
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On Thu, Mar 4, 2021 at 4:00 PM Dave Hansen <dave.hansen@linux.intel.com> wrote:
>
>
> From: Dave Hansen <dave.hansen@linux.intel.com>
>
> Prepare for the kernel to auto-migrate pages to other memory nodes
> with a user defined node migration table. This allows creating single
> migration target for each NUMA node to enable the kernel to do NUMA
> page migrations instead of simply reclaiming colder pages. A node
> with no target is a "terminal node", so reclaim acts normally there.
> The migration target does not fundamentally _need_ to be a single node,
> but this implementation starts there to limit complexity.
>
> If you consider the migration path as a graph, cycles (loops) in the
> graph are disallowed.  This avoids wasting resources by constantly
> migrating (A->B, B->A, A->B ...).  The expectation is that cycles will
> never be allowed.

Reviewed-by: Yang Shi <shy828301@gmail.com>

>
> Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
> Cc: Yang Shi <yang.shi@linux.alibaba.com>
> Cc: David Rientjes <rientjes@google.com>
> Cc: Huang Ying <ying.huang@intel.com>
> Cc: Dan Williams <dan.j.williams@intel.com>
> Cc: David Hildenbrand <david@redhat.com>
> Cc: osalvador <osalvador@suse.de>
>
> --
>
> changes since 20200122:
>  * Make node_demotion[] __read_mostly
>
> changes in July 2020:
>  - Remove loop from next_demotion_node() and get_online_mems().
>    This means that the node returned by next_demotion_node()
>    might now be offline, but the worst case is that the
>    allocation fails.  That's fine since it is transient.
> ---
>
>  b/mm/migrate.c |   17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
>
> diff -puN mm/migrate.c~0006-node-Define-and-export-memory-migration-path mm/migrate.c
> --- a/mm/migrate.c~0006-node-Define-and-export-memory-migration-path    2021-03-04 15:35:51.353806441 -0800
> +++ b/mm/migrate.c      2021-03-04 15:35:51.359806441 -0800
> @@ -1157,6 +1157,23 @@ out:
>         return rc;
>  }
>
> +static int node_demotion[MAX_NUMNODES] __read_mostly =
> +       {[0 ...  MAX_NUMNODES - 1] = NUMA_NO_NODE};
> +
> +/**
> + * next_demotion_node() - Get the next node in the demotion path
> + * @node: The starting node to lookup the next node
> + *
> + * @returns: node id for next memory node in the demotion path hierarchy
> + * from @node; NUMA_NO_NODE if @node is terminal.  This does not keep
> + * @node online or guarantee that it *continues* to be the next demotion
> + * target.
> + */
> +int next_demotion_node(int node)
> +{
> +       return node_demotion[node];
> +}
> +
>  /*
>   * Obtain the lock on page, remove all ptes and migrate the page
>   * to the newly allocated page in newpage.
> _
>