Re: [PATCH v2] powerpc/mm: Update default hugetlb size early

[David Hildenbrand <david@redhat.com>]

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.

-- 
Thanks,

David / dhildenb