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Date: Fri, 29 Jan 2021 12:46:35 -0800
From: Yang Shi <shy828301@...il.com>
To: Dave Hansen <dave.hansen@...ux.intel.com>
Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
Linux MM <linux-mm@...ck.org>,
Yang Shi <yang.shi@...ux.alibaba.com>,
David Rientjes <rientjes@...gle.com>,
Huang Ying <ying.huang@...el.com>,
Dan Williams <dan.j.williams@...el.com>,
David Hildenbrand <david@...hat.com>,
Oscar Salvador <osalvador@...e.de>
Subject: Re: [RFC][PATCH 05/13] mm/numa: automatically generate node migration order
On Mon, Jan 25, 2021 at 4:41 PM Dave Hansen <dave.hansen@...ux.intel.com> 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>
> Cc: Yang Shi <yang.shi@...ux.alibaba.com>
> Cc: David Rientjes <rientjes@...gle.com>
> Cc: Huang Ying <ying.huang@...el.com>
> Cc: Dan Williams <dan.j.williams@...el.com>
> Cc: David Hildenbrand <david@...hat.com>
> Cc: osalvador <osalvador@...e.de>
> ---
>
> b/mm/internal.h | 5 +
> b/mm/migrate.c | 137 +++++++++++++++++++++++++++++++++++++++++++++++++++++-
> b/mm/page_alloc.c | 2
> 3 files changed, 142 insertions(+), 2 deletions(-)
>
> diff -puN mm/internal.h~auto-setup-default-migration-path-from-firmware mm/internal.h
> --- a/mm/internal.h~auto-setup-default-migration-path-from-firmware 2021-01-25 16:23:10.607866706 -0800
> +++ b/mm/internal.h 2021-01-25 16:23:10.616866706 -0800
> @@ -515,12 +515,17 @@ static inline void mminit_validate_memmo
>
> #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 -puN mm/migrate.c~auto-setup-default-migration-path-from-firmware mm/migrate.c
> --- a/mm/migrate.c~auto-setup-default-migration-path-from-firmware 2021-01-25 16:23:10.609866706 -0800
> +++ b/mm/migrate.c 2021-01-25 16:23:10.617866706 -0800
> @@ -1161,6 +1161,10 @@ out:
> return rc;
> }
>
> +/*
> + * Writes to this array occur without locking. READ_ONCE()
> + * is recommended for readers to ensure consistent reads.
> + */
> static int node_demotion[MAX_NUMNODES] = {[0 ... MAX_NUMNODES - 1] = NUMA_NO_NODE};
>
> /**
> @@ -1174,7 +1178,13 @@ static int node_demotion[MAX_NUMNODES] =
> */
> 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.
> + */
Don't we need a smp_rmb() here? The single write barrier might be not
enough in migration target set. Typically a write barrier should be
used in pairs with a read barrier.
> + return READ_ONCE(node_demotion[node]);
Why not consolidate the patch #4 in this patch? The patch #4 just add
the definition of node_demotion and the function, then the function is
changed in this patch, and the function is not used by anyone between
the adding and changing.
> }
>
> /*
> @@ -3124,3 +3134,128 @@ void migrate_vma_finalize(struct 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.
Maybe an example diagram for the physical topology and how the
migration target is generated in the comment seems helpful to
understand the code.
> + */
> +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];
> +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 desintation.
s/desination/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;
> +}
> +
> +void set_migration_target_nodes(void)
It seems this function is not called outside migrate.c, so it should be static.
> +{
> + get_online_mems();
> + __set_migration_target_nodes();
> + put_online_mems();
> +}
> diff -puN mm/page_alloc.c~auto-setup-default-migration-path-from-firmware mm/page_alloc.c
> --- a/mm/page_alloc.c~auto-setup-default-migration-path-from-firmware 2021-01-25 16:23:10.612866706 -0800
> +++ b/mm/page_alloc.c 2021-01-25 16:23:10.619866706 -0800
> @@ -5704,7 +5704,7 @@ 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;
> _
>
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