Re: [PATCH 2/2] mm/page_alloc: avoid counting event if no successful allocation

From: Xu, Yanfei
Date: Sat Jul 10 2021 - 07:32:34 EST



On 7/9/21 8:26 PM, Mel Gorman wrote:
[Please note: This e-mail is from an EXTERNAL e-mail address]

On Fri, Jul 09, 2021 at 06:28:55PM +0800, Yanfei Xu wrote:
While the nr_populated is non-zero, however the nr_account might be
zero if allocating fails. In this case, not to count event can save
some cycles.


The much more likely path is that nr_account is positive so we avoid a
branch in the common case.


Got it.

And this commit extract the check of "page_array" from a while
statement to avoid unnecessary checks for it.


I'm surprised the compiler does not catch that page_array is invariant
for the loop. Did you check if gcc generates different code is page_array
is explicitly checked once instead of putting it in the loop?


Hm.. In fact, the page_array always doesn't appear in the loop due to the compiler's optimization. And I just confirmed the assembly.

not apply my patch:

ffffffff81267100 <__alloc_pages_bulk>:
ffffffff81267100: e8 0b 73 df ff callq ffffffff8105e410 <__fentry__>
ffffffff81267105: 55 push %rbp
ffffffff81267106: 48 89 e5 mov %rsp,%rbp
ffffffff81267109: 41 57 push %r15
ffffffff8126710b: 41 56 push %r14
ffffffff8126710d: 41 55 push %r13
ffffffff8126710f: 41 54 push %r12
ffffffff81267111: 53 push %rbx
ffffffff81267112: 48 83 ec 58 sub $0x58,%rsp
ffffffff81267116: 85 c9 test %ecx,%ecx
ffffffff81267118: 89 7d c4 mov %edi,-0x3c(%rbp)
ffffffff8126711b: 89 75 9c mov %esi,-0x64(%rbp)
ffffffff8126711e: 48 89 55 a0 mov %rdx,-0x60(%rbp)
ffffffff81267122: 89 4d d4 mov %ecx,-0x2c(%rbp)
ffffffff81267125: 4c 89 45 a8 mov %r8,-0x58(%rbp)
ffffffff81267129: 4c 89 4d c8 mov %r9,-0x38(%rbp)
ffffffff8126712d: 0f 8e 7c 05 00 00 jle ffffffff812676af <__alloc_pages_bulk+0x5af>
ffffffff81267133: 4d 85 c9 test %r9,%r9
ffffffff81267136: 0f 84 84 05 00 00 je ffffffff812676c0 <__alloc_pages_bulk+0x5c0>
ffffffff8126713c: 49 83 39 00 cmpq $0x0,(%r9)
ffffffff81267140: 0f 84 7a 05 00 00 je ffffffff812676c0 <__alloc_pages_bulk+0x5c0>
ffffffff81267146: 49 8d 41 08 lea 0x8(%r9),%rax
ffffffff8126714a: 89 ca mov %ecx,%edx
ffffffff8126714c: 45 31 e4 xor %r12d,%r12d
ffffffff8126714f: eb 0b jmp ffffffff8126715c <__alloc_pages_bulk+0x5c>
ffffffff81267151: 48 83 c0 08 add $0x8,%rax
ffffffff81267155: 48 83 78 f8 00 cmpq $0x0,-0x8(%rax)
ffffffff8126715a: 74 1c je ffffffff81267178 <__alloc_pages_bulk+0x78>
ffffffff8126715c: 41 83 c4 01 add $0x1,%r12d
ffffffff81267160: 44 39 e2 cmp %r12d,%edx
ffffffff81267163: 75 ec jne ffffffff81267151 <__alloc_pages_bulk+0x51>
ffffffff81267165: 48 63 45 d4 movslq -0x2c(%rbp),%rax
ffffffff81267169: 48 83 c4 58 add $0x58,%rsp
ffffffff8126716d: 5b pop %rbx
ffffffff8126716e: 41 5c pop %r12
ffffffff81267170: 41 5d pop %r13
ffffffff81267172: 41 5e pop %r14
ffffffff81267174: 41 5f pop %r15
ffffffff81267176: 5d pop %rbp
ffffffff81267177: c3 retq
ffffffff81267178: 8b 45 d4 mov -0x2c(%rbp),%eax
ffffffff8126717b: 44 29 e0 sub %r12d,%eax
ffffffff8126717e: 83 f8 01 cmp $0x1,%eax
ffffffff81267181: 0f 84 47 04 00 00 je ffffffff812675ce <__alloc_pages_bulk+0x4ce>
ffffffff81267187: 8b 0d 53 22 a4 01 mov 0x1a42253(%rip),%ecx # ffffffff82ca93e0 <page_group_by_mobility_disabled>
ffffffff8126718d: 48 63 45 9c movslq -0x64(%rbp),%rax
ffffffff81267191: 44 8b 7d c4 mov -0x3c(%rbp),%r15d
ffffffff81267195: 44 23 3d 58 22 a4 01 and 0x1a42258(%rip),%r15d # ffffffff82ca93f4 <gfp_allowed_mask>


applied my patch:

ffffffff81267100 <__alloc_pages_bulk>:
ffffffff81267100: e8 0b 73 df ff callq ffffffff8105e410 <__fentry__>
ffffffff81267105: 55 push %rbp
ffffffff81267106: 48 89 e5 mov %rsp,%rbp
ffffffff81267109: 41 57 push %r15
ffffffff8126710b: 41 56 push %r14
ffffffff8126710d: 41 55 push %r13
ffffffff8126710f: 41 54 push %r12
ffffffff81267111: 53 push %rbx
ffffffff81267112: 48 83 ec 60 sub $0x60,%rsp
ffffffff81267116: 85 c9 test %ecx,%ecx
ffffffff81267118: 89 7d c8 mov %edi,-0x38(%rbp)
ffffffff8126711b: 89 75 9c mov %esi,-0x64(%rbp)
ffffffff8126711e: 48 89 55 a0 mov %rdx,-0x60(%rbp)
ffffffff81267122: 89 4d d0 mov %ecx,-0x30(%rbp)
ffffffff81267125: 4c 89 45 a8 mov %r8,-0x58(%rbp)
ffffffff81267129: 0f 8e 91 05 00 00 jle ffffffff812676c0 <__alloc_pages_bulk+0x5c0>
ffffffff8126712f: 4d 85 c9 test %r9,%r9
ffffffff81267132: 4d 89 cf mov %r9,%r15
ffffffff81267135: 0f 84 4d 05 00 00 je ffffffff81267688 <__alloc_pages_bulk+0x588>
ffffffff8126713b: 8d 49 ff lea -0x1(%rcx),%ecx
ffffffff8126713e: 31 c0 xor %eax,%eax
ffffffff81267140: eb 03 jmp ffffffff81267145 <__alloc_pages_bulk+0x45>
ffffffff81267142: 48 89 d0 mov %rdx,%rax
ffffffff81267145: 49 83 3c c7 00 cmpq $0x0,(%r15,%rax,8)
ffffffff8126714a: 41 89 c4 mov %eax,%r12d
ffffffff8126714d: 74 0d je ffffffff8126715c <__alloc_pages_bulk+0x5c>
ffffffff8126714f: 44 8d 60 01 lea 0x1(%rax),%r12d
ffffffff81267153: 48 39 c1 cmp %rax,%rcx
ffffffff81267156: 48 8d 50 01 lea 0x1(%rax),%rdx
ffffffff8126715a: 75 e6 jne ffffffff81267142 <__alloc_pages_bulk+0x42>
ffffffff8126715c: 8b 45 d0 mov -0x30(%rbp),%eax
ffffffff8126715f: 41 39 c4 cmp %eax,%r12d
ffffffff81267162: 0f 84 19 04 00 00 je ffffffff81267581 <__alloc_pages_bulk+0x481>
ffffffff81267168: 44 29 e0 sub %r12d,%eax
ffffffff8126716b: 83 f8 01 cmp $0x1,%eax
ffffffff8126716e: 0f 84 66 04 00 00 je ffffffff812675da <__alloc_pages_bulk+0x4da>
ffffffff81267174: 48 63 45 9c movslq -0x64(%rbp),%rax
ffffffff81267178: 8b 0d 62 22 a4 01 mov 0x1a42262(%rip),%ecx # ffffffff82ca93e0 <page_group_by_mobility_disabled>
ffffffff8126717e: 8b 5d c8 mov -0x38(%rbp),%ebx
ffffffff81267181: 23 1d 6d 22 a4 01 and 0x1a4226d(%rip),%ebx # ffffffff82ca93f4 <gfp_allowed_mask>


Thanks,
Yanfei

--
Mel Gorman
SUSE Labs