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Date: Mon, 6 Dec 2021 13:42:33 +0800
From: Baolin Wang <baolin.wang@...ux.alibaba.com>
To: Huang Ying <ying.huang@...el.com>,
Andrew Morton <akpm@...ux-foundation.org>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org,
Dave Hansen <dave.hansen@...ux.intel.com>,
Yang Shi <shy828301@...il.com>, Zi Yan <ziy@...dia.com>,
Oscar Salvador <osalvador@...e.de>,
Michal Hocko <mhocko@...e.com>, Wei Xu <weixugc@...gle.com>,
David Rientjes <rientjes@...gle.com>,
Dan Williams <dan.j.williams@...el.com>,
David Hildenbrand <david@...hat.com>,
Greg Thelen <gthelen@...gle.com>,
Keith Busch <kbusch@...nel.org>,
Yang Shi <yang.shi@...ux.alibaba.com>
Subject: Re: [PATCH] mm/migrate: move node demotion code to near its user
On 2021/12/6 11:12, Huang Ying wrote:
> Now, node_demotion and next_demtion_node() is placed between
> __unmap_and_move() and unmap_and_move(). This hurts the code
> readability. So, move it to near its user in the file. There's no
> any functionality change in this patch.
>
> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
> Cc: Dave Hansen <dave.hansen@...ux.intel.com>
> Cc: Yang Shi <shy828301@...il.com>
> Cc: Zi Yan <ziy@...dia.com>
> Cc: Oscar Salvador <osalvador@...e.de>
> Cc: Michal Hocko <mhocko@...e.com>
> Cc: Wei Xu <weixugc@...gle.com>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: Greg Thelen <gthelen@...gle.com>
> Cc: Keith Busch <kbusch@...nel.org>
> Cc: Yang Shi <yang.shi@...ux.alibaba.com>
> Cc: Baolin Wang <baolin.wang@...ux.alibaba.com>
LGTM.
Reviewed-by: Baolin Wang <baolin.wang@...ux.alibaba.com>
> ---
> mm/migrate.c | 265 +++++++++++++++++++++++++--------------------------
> 1 file changed, 132 insertions(+), 133 deletions(-)
>
> diff --git a/mm/migrate.c b/mm/migrate.c
> index c503ef1f4360..d487a399253b 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1083,139 +1083,6 @@ static int __unmap_and_move(struct page *page, struct page *newpage,
> return rc;
> }
>
> -
> -/*
> - * node_demotion[] example:
> - *
> - * Consider a system with two sockets. Each socket has
> - * three classes of memory attached: fast, medium and slow.
> - * Each memory class is placed in its own NUMA node. The
> - * CPUs are placed in the node with the "fast" memory. The
> - * 6 NUMA nodes (0-5) might be split among the sockets like
> - * this:
> - *
> - * Socket A: 0, 1, 2
> - * Socket B: 3, 4, 5
> - *
> - * When Node 0 fills up, its memory should be migrated to
> - * Node 1. When Node 1 fills up, it should be migrated to
> - * Node 2. The migration path start on the nodes with the
> - * processors (since allocations default to this node) and
> - * fast memory, progress through medium and end with the
> - * slow memory:
> - *
> - * 0 -> 1 -> 2 -> stop
> - * 3 -> 4 -> 5 -> stop
> - *
> - * This is represented in the node_demotion[] like this:
> - *
> - * { nr=1, nodes[0]=1 }, // Node 0 migrates to 1
> - * { nr=1, nodes[0]=2 }, // Node 1 migrates to 2
> - * { nr=0, nodes[0]=-1 }, // Node 2 does not migrate
> - * { nr=1, nodes[0]=4 }, // Node 3 migrates to 4
> - * { nr=1, nodes[0]=5 }, // Node 4 migrates to 5
> - * { nr=0, nodes[0]=-1 }, // Node 5 does not migrate
> - *
> - * Moreover some systems may have multiple slow memory nodes.
> - * Suppose a system has one socket with 3 memory nodes, node 0
> - * is fast memory type, and node 1/2 both are slow memory
> - * type, and the distance between fast memory node and slow
> - * memory node is same. So the migration path should be:
> - *
> - * 0 -> 1/2 -> stop
> - *
> - * This is represented in the node_demotion[] like this:
> - * { nr=2, {nodes[0]=1, nodes[1]=2} }, // Node 0 migrates to node 1 and node 2
> - * { nr=0, nodes[0]=-1, }, // Node 1 dose not migrate
> - * { nr=0, nodes[0]=-1, }, // Node 2 does not migrate
> - */
> -
> -/*
> - * Writes to this array occur without locking. Cycles are
> - * not allowed: Node X demotes to Y which demotes to X...
> - *
> - * If multiple reads are performed, a single rcu_read_lock()
> - * must be held over all reads to ensure that no cycles are
> - * observed.
> - */
> -#define DEFAULT_DEMOTION_TARGET_NODES 15
> -
> -#if MAX_NUMNODES < DEFAULT_DEMOTION_TARGET_NODES
> -#define DEMOTION_TARGET_NODES (MAX_NUMNODES - 1)
> -#else
> -#define DEMOTION_TARGET_NODES DEFAULT_DEMOTION_TARGET_NODES
> -#endif
> -
> -struct demotion_nodes {
> - unsigned short nr;
> - short nodes[DEMOTION_TARGET_NODES];
> -};
> -
> -static struct demotion_nodes *node_demotion __read_mostly;
> -
> -/**
> - * next_demotion_node() - Get the next node in the demotion path
> - * @node: The starting node to lookup the next node
> - *
> - * Return: 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)
> -{
> - struct demotion_nodes *nd;
> - unsigned short target_nr, index;
> - int target;
> -
> - if (!node_demotion)
> - return NUMA_NO_NODE;
> -
> - nd = &node_demotion[node];
> -
> - /*
> - * node_demotion[] is updated without excluding this
> - * function from running. RCU doesn't provide any
> - * compiler barriers, so the READ_ONCE() is required
> - * to avoid compiler reordering or read merging.
> - *
> - * Make sure to use RCU over entire code blocks if
> - * node_demotion[] reads need to be consistent.
> - */
> - rcu_read_lock();
> - target_nr = READ_ONCE(nd->nr);
> -
> - switch (target_nr) {
> - case 0:
> - target = NUMA_NO_NODE;
> - goto out;
> - case 1:
> - index = 0;
> - break;
> - default:
> - /*
> - * If there are multiple target nodes, just select one
> - * target node randomly.
> - *
> - * In addition, we can also use round-robin to select
> - * target node, but we should introduce another variable
> - * for node_demotion[] to record last selected target node,
> - * that may cause cache ping-pong due to the changing of
> - * last target node. Or introducing per-cpu data to avoid
> - * caching issue, which seems more complicated. So selecting
> - * target node randomly seems better until now.
> - */
> - index = get_random_int() % target_nr;
> - break;
> - }
> -
> - target = READ_ONCE(nd->nodes[index]);
> -
> -out:
> - rcu_read_unlock();
> - return target;
> -}
> -
> /*
> * Obtain the lock on page, remove all ptes and migrate the page
> * to the newly allocated page in newpage.
> @@ -3035,6 +2902,138 @@ void migrate_vma_finalize(struct migrate_vma *migrate)
> EXPORT_SYMBOL(migrate_vma_finalize);
> #endif /* CONFIG_DEVICE_PRIVATE */
>
> +/*
> + * node_demotion[] example:
> + *
> + * Consider a system with two sockets. Each socket has
> + * three classes of memory attached: fast, medium and slow.
> + * Each memory class is placed in its own NUMA node. The
> + * CPUs are placed in the node with the "fast" memory. The
> + * 6 NUMA nodes (0-5) might be split among the sockets like
> + * this:
> + *
> + * Socket A: 0, 1, 2
> + * Socket B: 3, 4, 5
> + *
> + * When Node 0 fills up, its memory should be migrated to
> + * Node 1. When Node 1 fills up, it should be migrated to
> + * Node 2. The migration path start on the nodes with the
> + * processors (since allocations default to this node) and
> + * fast memory, progress through medium and end with the
> + * slow memory:
> + *
> + * 0 -> 1 -> 2 -> stop
> + * 3 -> 4 -> 5 -> stop
> + *
> + * This is represented in the node_demotion[] like this:
> + *
> + * { nr=1, nodes[0]=1 }, // Node 0 migrates to 1
> + * { nr=1, nodes[0]=2 }, // Node 1 migrates to 2
> + * { nr=0, nodes[0]=-1 }, // Node 2 does not migrate
> + * { nr=1, nodes[0]=4 }, // Node 3 migrates to 4
> + * { nr=1, nodes[0]=5 }, // Node 4 migrates to 5
> + * { nr=0, nodes[0]=-1 }, // Node 5 does not migrate
> + *
> + * Moreover some systems may have multiple slow memory nodes.
> + * Suppose a system has one socket with 3 memory nodes, node 0
> + * is fast memory type, and node 1/2 both are slow memory
> + * type, and the distance between fast memory node and slow
> + * memory node is same. So the migration path should be:
> + *
> + * 0 -> 1/2 -> stop
> + *
> + * This is represented in the node_demotion[] like this:
> + * { nr=2, {nodes[0]=1, nodes[1]=2} }, // Node 0 migrates to node 1 and node 2
> + * { nr=0, nodes[0]=-1, }, // Node 1 dose not migrate
> + * { nr=0, nodes[0]=-1, }, // Node 2 does not migrate
> + */
> +
> +/*
> + * Writes to this array occur without locking. Cycles are
> + * not allowed: Node X demotes to Y which demotes to X...
> + *
> + * If multiple reads are performed, a single rcu_read_lock()
> + * must be held over all reads to ensure that no cycles are
> + * observed.
> + */
> +#define DEFAULT_DEMOTION_TARGET_NODES 15
> +
> +#if MAX_NUMNODES < DEFAULT_DEMOTION_TARGET_NODES
> +#define DEMOTION_TARGET_NODES (MAX_NUMNODES - 1)
> +#else
> +#define DEMOTION_TARGET_NODES DEFAULT_DEMOTION_TARGET_NODES
> +#endif
> +
> +struct demotion_nodes {
> + unsigned short nr;
> + short nodes[DEMOTION_TARGET_NODES];
> +};
> +
> +static struct demotion_nodes *node_demotion __read_mostly;
> +
> +/**
> + * next_demotion_node() - Get the next node in the demotion path
> + * @node: The starting node to lookup the next node
> + *
> + * Return: 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)
> +{
> + struct demotion_nodes *nd;
> + unsigned short target_nr, index;
> + int target;
> +
> + if (!node_demotion)
> + return NUMA_NO_NODE;
> +
> + nd = &node_demotion[node];
> +
> + /*
> + * node_demotion[] is updated without excluding this
> + * function from running. RCU doesn't provide any
> + * compiler barriers, so the READ_ONCE() is required
> + * to avoid compiler reordering or read merging.
> + *
> + * Make sure to use RCU over entire code blocks if
> + * node_demotion[] reads need to be consistent.
> + */
> + rcu_read_lock();
> + target_nr = READ_ONCE(nd->nr);
> +
> + switch (target_nr) {
> + case 0:
> + target = NUMA_NO_NODE;
> + goto out;
> + case 1:
> + index = 0;
> + break;
> + default:
> + /*
> + * If there are multiple target nodes, just select one
> + * target node randomly.
> + *
> + * In addition, we can also use round-robin to select
> + * target node, but we should introduce another variable
> + * for node_demotion[] to record last selected target node,
> + * that may cause cache ping-pong due to the changing of
> + * last target node. Or introducing per-cpu data to avoid
> + * caching issue, which seems more complicated. So selecting
> + * target node randomly seems better until now.
> + */
> + index = get_random_int() % target_nr;
> + break;
> + }
> +
> + target = READ_ONCE(nd->nodes[index]);
> +
> +out:
> + rcu_read_unlock();
> + return target;
> +}
> +
> #if defined(CONFIG_HOTPLUG_CPU)
> /* Disable reclaim-based migration. */
> static void __disable_all_migrate_targets(void)
>
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