Yang Shi <shy828301@xxxxxxxxx> writes:
> 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/
Thanks a lot for your information!
Best Regards,
Huang, Ying
[snip]
