On Sat, Jun 19, 2021 at 1:19 AM Huang, Ying <ying.huang@xxxxxxxxx> wrote:
>
> Zi Yan <ziy@xxxxxxxxxx> writes:
>
> > On 18 Jun 2021, at 2:15, Huang Ying wrote:
> >
> >> From: Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx>
> >>
> >> 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@xxxxxxxxxxxxxxx>
> >> Signed-off-by: "Huang, Ying" <ying.huang@xxxxxxxxx>
> >> Reviewed-by: Yang Shi <shy828301@xxxxxxxxx>
> >> Cc: Michal Hocko <mhocko@xxxxxxxx>
> >> Cc: Wei Xu <weixugc@xxxxxxxxxx>
> >> Cc: David Rientjes <rientjes@xxxxxxxxxx>
> >> Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
> >> Cc: David Hildenbrand <david@xxxxxxxxxx>
> >> Cc: osalvador <osalvador@xxxxxxx>
> >>
> >> --
> >>
> >> 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.
>
> Hi, Dave,
>
> What do you think about this?
Squashing patch #1 and #2 had been mentioned in the previous review
and it seems Dave agreed.
https://lore.kernel.org/linux-mm/4573cb9a-31ca-3b3d-96bc-5d94876b9709@xxxxxxxxx/
>
> >>
> >> /*
> >> @@ -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.
>
> I don't think that's necessary at least for now. Do you know any real
> world use case for this?
>
> Best Regards,
> Huang, Ying
>
> [snip]
