Messages in this thread | ![/](/images/icornerl.gif) | | From | "Huang, Ying" <> | Subject | Re: [PATCH -V8 02/10] mm/numa: automatically generate node migration order | Date | Sat, 19 Jun 2021 16:18:55 +0800 |
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Zi Yan <ziy@nvidia.com> writes:
> On 18 Jun 2021, at 2:15, Huang Ying wrote: > >> From: Dave Hansen <dave.hansen@linux.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@linux.intel.com> >> Signed-off-by: "Huang, Ying" <ying.huang@intel.com> >> Reviewed-by: Yang Shi <shy828301@gmail.com> >> Cc: Michal Hocko <mhocko@suse.com> >> Cc: Wei Xu <weixugc@google.com> >> Cc: David Rientjes <rientjes@google.com> >> Cc: Dan Williams <dan.j.williams@intel.com> >> Cc: David Hildenbrand <david@redhat.com> >> Cc: osalvador <osalvador@suse.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.
Hi, Dave,
What do you think about this?
>> >> /* >> @@ -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]
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