Date:   Fri, 18 Jun 2021 11:14:16 -0400
From:   Zi Yan <ziy@...dia.com>
To:     Huang Ying <ying.huang@...el.com>
Cc:     linux-mm@...ck.org, linux-kernel@...r.kernel.org,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Yang Shi <shy828301@...il.com>, 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>,
        osalvador <osalvador@...e.de>
Subject: Re: [PATCH -V8 02/10] mm/numa: automatically generate node migration order

On 18 Jun 2021, at 2:15, Huang Ying wrote:

> From: Dave Hansen <dave.hansen@...ux.intel.com>
>
> When memory fills up on a node, memory contents can be
> automatically migrated to another node.  The biggest problems are
> knowing when to migrate and to where the migration should be
> targeted.
>
> The most straightforward way to generate the "to where" list would
> be to follow the page allocator fallback lists.  Those lists
> already tell us if memory is full where to look next.  It would
> also be logical to move memory in that order.
>
> But, the allocator fallback lists have a fatal flaw: most nodes
> appear in all the lists.  This would potentially lead to migration
> cycles (A->B, B->A, A->B, ...).
>
> Instead of using the allocator fallback lists directly, keep a
> separate node migration ordering.  But, reuse the same data used
> to generate page allocator fallback in the first place:
> find_next_best_node().
>
> This means that the firmware data used to populate node distances
> essentially dictates the ordering for now.  It should also be
> architecture-neutral since all NUMA architectures have a working
> find_next_best_node().
>
> The protocol for node_demotion[] access and writing is not
> standard.  It has no specific locking and is intended to be read
> locklessly.  Readers must take care to avoid observing changes
> that appear incoherent.  This was done so that node_demotion[]
> locking has no chance of becoming a bottleneck on large systems
> with lots of CPUs in direct reclaim.
>
> This code is unused for now.  It will be called later in the
> series.
>
> Signed-off-by: Dave Hansen <dave.hansen@...ux.intel.com>
> Signed-off-by: "Huang, Ying" <ying.huang@...el.com>
> Reviewed-by: Yang Shi <shy828301@...il.com>
> 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: osalvador <osalvador@...e.de>
>
> --
>
> Changes from 20200122:
>  * Add big node_demotion[] comment
> Changes from 20210302:
>  * Fix typo in node_demotion[] comment
> ---
>  mm/internal.h   |   5 ++
>  mm/migrate.c    | 175 +++++++++++++++++++++++++++++++++++++++++++++++-
>  mm/page_alloc.c |   2 +-
>  3 files changed, 180 insertions(+), 2 deletions(-)
>
> diff --git a/mm/internal.h b/mm/internal.h
> index 2f1182948aa6..0344cd78e170 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -522,12 +522,17 @@ static inline void mminit_validate_memmodel_limits(unsigned long *start_pfn,
>
>  #ifdef CONFIG_NUMA
>  extern int node_reclaim(struct pglist_data *, gfp_t, unsigned int);
> +extern int find_next_best_node(int node, nodemask_t *used_node_mask);
>  #else
>  static inline int node_reclaim(struct pglist_data *pgdat, gfp_t mask,
>  				unsigned int order)
>  {
>  	return NODE_RECLAIM_NOSCAN;
>  }
> +static inline int find_next_best_node(int node, nodemask_t *used_node_mask)
> +{
> +	return NUMA_NO_NODE;
> +}
>  #endif
>
>  extern int hwpoison_filter(struct page *p);
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 6cab668132f9..111f8565f75d 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1136,6 +1136,44 @@ 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:
> + *
> + *	{  1, // Node 0 migrates to 1
> + *	   2, // Node 1 migrates to 2
> + *	  -1, // Node 2 does not migrate
> + *	   4, // Node 3 migrates to 4
> + *	   5, // Node 4 migrates to 5
> + *	  -1} // Node 5 does not migrate
> + */
> +
> +/*
> + * Writes to this array occur without locking.  READ_ONCE()
> + * is recommended for readers to ensure consistent reads.
> + */
>  static int node_demotion[MAX_NUMNODES] __read_mostly =
>  	{[0 ...  MAX_NUMNODES - 1] = NUMA_NO_NODE};
>
> @@ -1150,7 +1188,13 @@ static int node_demotion[MAX_NUMNODES] __read_mostly =
>   */
>  int next_demotion_node(int node)
>  {
> -	return node_demotion[node];
> +	/*
> +	 * node_demotion[] is updated without excluding
> +	 * this function from running.  READ_ONCE() avoids
> +	 * reading multiple, inconsistent 'node' values
> +	 * during an update.
> +	 */
> +	return READ_ONCE(node_demotion[node]);
>  }

Is it necessary to have two separate patches to add node_demotion and
next_demotion_node() then modify it immediately? Maybe merge Patch 1 into 2?

Hmm, I just checked Patch 3 and it changes node_demotion again and uses RCU.
I guess it might be much simpler to just introduce node_demotion with RCU
in this patch and Patch 3 only takes care of hotplug events.

>
>  /*
> @@ -3144,3 +3188,132 @@ void migrate_vma_finalize(struct migrate_vma *migrate)
>  }
>  EXPORT_SYMBOL(migrate_vma_finalize);
>  #endif /* CONFIG_DEVICE_PRIVATE */
> +
> +/* Disable reclaim-based migration. */
> +static void disable_all_migrate_targets(void)
> +{
> +	int node;
> +
> +	for_each_online_node(node)
> +		node_demotion[node] = NUMA_NO_NODE;
> +}
> +
> +/*
> + * Find an automatic demotion target for 'node'.
> + * Failing here is OK.  It might just indicate
> + * being at the end of a chain.
> + */
> +static int establish_migrate_target(int node, nodemask_t *used)
> +{
> +	int migration_target;
> +
> +	/*
> +	 * Can not set a migration target on a
> +	 * node with it already set.
> +	 *
> +	 * No need for READ_ONCE() here since this
> +	 * in the write path for node_demotion[].
> +	 * This should be the only thread writing.
> +	 */
> +	if (node_demotion[node] != NUMA_NO_NODE)
> +		return NUMA_NO_NODE;
> +
> +	migration_target = find_next_best_node(node, used);
> +	if (migration_target == NUMA_NO_NODE)
> +		return NUMA_NO_NODE;
> +
> +	node_demotion[node] = migration_target;
> +
> +	return migration_target;
> +}
> +
> +/*
> + * When memory fills up on a node, memory contents can be
> + * automatically migrated to another node instead of
> + * discarded at reclaim.
> + *
> + * Establish a "migration path" which will start at nodes
> + * with CPUs and will follow the priorities used to build the
> + * page allocator zonelists.
> + *
> + * The difference here is that cycles must be avoided.  If
> + * node0 migrates to node1, then neither node1, nor anything
> + * node1 migrates to can migrate to node0.
> + *
> + * This function can run simultaneously with readers of
> + * node_demotion[].  However, it can not run simultaneously
> + * with itself.  Exclusion is provided by memory hotplug events
> + * being single-threaded.
> + */
> +static void __set_migration_target_nodes(void)
> +{
> +	nodemask_t next_pass	= NODE_MASK_NONE;
> +	nodemask_t this_pass	= NODE_MASK_NONE;
> +	nodemask_t used_targets = NODE_MASK_NONE;
> +	int node;
> +
> +	/*
> +	 * Avoid any oddities like cycles that could occur
> +	 * from changes in the topology.  This will leave
> +	 * a momentary gap when migration is disabled.
> +	 */
> +	disable_all_migrate_targets();
> +
> +	/*
> +	 * Ensure that the "disable" is visible across the system.
> +	 * Readers will see either a combination of before+disable
> +	 * state or disable+after.  They will never see before and
> +	 * after state together.
> +	 *
> +	 * The before+after state together might have cycles and
> +	 * could cause readers to do things like loop until this
> +	 * function finishes.  This ensures they can only see a
> +	 * single "bad" read and would, for instance, only loop
> +	 * once.
> +	 */
> +	smp_wmb();
> +
> +	/*
> +	 * Allocations go close to CPUs, first.  Assume that
> +	 * the migration path starts at the nodes with CPUs.
> +	 */
> +	next_pass = node_states[N_CPU];

Is there a plan of allowing user to change where the migration
path starts? Or maybe one step further providing an interface
to allow user to specify the demotion path. Something like
/sys/devices/system/node/node*/node_demotion.


> +again:
> +	this_pass = next_pass;
> +	next_pass = NODE_MASK_NONE;
> +	/*
> +	 * To avoid cycles in the migration "graph", ensure
> +	 * that migration sources are not future targets by
> +	 * setting them in 'used_targets'.  Do this only
> +	 * once per pass so that multiple source nodes can
> +	 * share a target node.
> +	 *
> +	 * 'used_targets' will become unavailable in future
> +	 * passes.  This limits some opportunities for
> +	 * multiple source nodes to share a destination.
> +	 */
> +	nodes_or(used_targets, used_targets, this_pass);
> +	for_each_node_mask(node, this_pass) {
> +		int target_node = establish_migrate_target(node, &used_targets);
> +
> +		if (target_node == NUMA_NO_NODE)
> +			continue;
> +
> +		/* Visit targets from this pass in the next pass: */
> +		node_set(target_node, next_pass);
> +	}
> +	/* Is another pass necessary? */
> +	if (!nodes_empty(next_pass))
> +		goto again;
> +}
> +
> +/*
> + * For callers that do not hold get_online_mems() already.
> + */
> +__maybe_unused // <- temporay to prevent warnings during bisects
> +static void set_migration_target_nodes(void)
> +{
> +	get_online_mems();
> +	__set_migration_target_nodes();
> +	put_online_mems();
> +}
> diff --git a/mm/page_alloc.c b/mm/page_alloc.c
> index d1f5de1c1283..e033ae2e8bce 100644
> --- a/mm/page_alloc.c
> +++ b/mm/page_alloc.c
> @@ -5973,7 +5973,7 @@ static int node_load[MAX_NUMNODES];
>   *
>   * Return: node id of the found node or %NUMA_NO_NODE if no node is found.
>   */
> -static int find_next_best_node(int node, nodemask_t *used_node_mask)
> +int find_next_best_node(int node, nodemask_t *used_node_mask)
>  {
>  	int n, val;
>  	int min_val = INT_MAX;
> -- 
> 2.30.2


—
Best Regards,
Yan, Zi

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