Andi Kleen a écrit :
Is it possible virt_to_slab(objp)->nodeid being different from pfn_to_nid(objp)
?
It is possible the page allocator falls back to another node than
requested. We would need to check that this never occurs.
The only way to ensure that would be to set a strict mempolicy.
But I'm not sure that's a good idea -- after all you don't want
to fail an allocation in this case.
But pfn_to_nid on the object like proposed by Eric should work anyways.
But I'm not sure the tables used for that will be more often cache hot
than the slab.
pfn_to_nid() on most x86_64 machines access one cache line (struct memnode).
Node 0 MemBase 0000000000000000 Limit 0000000280000000
Node 1 MemBase 0000000280000000 Limit 0000000480000000
NUMA: Using 31 for the hash shift.
On this example, we use only 8 bytes of memnode.embedded_map[] to find nid of
all 16 GB of ram. On profiles I have, memnode is always hot (no cache miss on it).
While virt_to_slab() has to access :
1) struct page -> page_get_slab() (page->lru.prev) (one cache miss)
2) struct slab -> nodeid (one other cache miss)
So using pfn_to_nid() would avoid 2 cache misses.
I understand we want to do special things (fallback and such tricks) at
allocation time, but I believe that we can just trust the real nid of memory
at free time.
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