On 2/11/22 14:00, David Hildenbrand wrote:
On 11.02.22 07:52, Aneesh Kumar K.V wrote:
commit: d9c234005227 ("Do not depend on MAX_ORDER when grouping pages by mobility")
introduced pageblock_order which will be used to group pages better.
The kernel now groups pages based on the value of HPAGE_SHIFT. Hence HPAGE_SHIFT
should be set before we call set_pageblock_order.
set_pageblock_order happens early in the boot and default hugetlb page size
should be initialized before that to compute the right pageblock_order value.
Currently, default hugetlbe page size is set via arch_initcalls which happens
late in the boot as shown via the below callstack:
[c000000007383b10] [c000000001289328] hugetlbpage_init+0x2b8/0x2f8
[c000000007383bc0] [c0000000012749e4] do_one_initcall+0x14c/0x320
[c000000007383c90] [c00000000127505c] kernel_init_freeable+0x410/0x4e8
[c000000007383da0] [c000000000012664] kernel_init+0x30/0x15c
[c000000007383e10] [c00000000000cf14] ret_from_kernel_thread+0x5c/0x64
and the pageblock_order initialization is done early during the boot.
[c0000000018bfc80] [c0000000012ae120] set_pageblock_order+0x50/0x64
[c0000000018bfca0] [c0000000012b3d94] sparse_init+0x188/0x268
[c0000000018bfd60] [c000000001288bfc] initmem_init+0x28c/0x328
[c0000000018bfe50] [c00000000127b370] setup_arch+0x410/0x480
[c0000000018bfed0] [c00000000127401c] start_kernel+0xb8/0x934
[c0000000018bff90] [c00000000000d984] start_here_common+0x1c/0x98
delaying default hugetlb page size initialization implies the kernel will
initialize pageblock_order to (MAX_ORDER - 1) which is not an optimal
value for mobility grouping. IIUC we always had this issue. But it was not
a problem for hash translation mode because (MAX_ORDER - 1) is the same as
HUGETLB_PAGE_ORDER (8) in the case of hash (16MB). With radix,
HUGETLB_PAGE_ORDER will be 5 (2M size) and hence pageblock_order should be
5 instead of 8.
A related question: Can we on ppc still have pageblock_order > MAX_ORDER
- 1? We have some code for that and I am not so sure if we really need that.
I also have been wondering about the same. On book3s64 I don't think we
need that support for both 64K and 4K page size because with hash
hugetlb size is MAX_ORDER -1. (16MB hugepage size)
I am not sure about the 256K page support. Christophe may be able to
answer that.
For the gigantic hugepage support we depend on cma based allocation or
firmware reservation. So I am not sure why we ever considered pageblock
> MAX_ORDER -1 scenario. If you have pointers w.r.t why that was ever
needed, I could double-check whether ppc64 is still dependent on that.
-aneesh
