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SubjectRe: Large stack usage in fs code (especially for PPC64)

On Mon, 17 Nov 2008, Linus Torvalds wrote:
>
> I do wonder just _what_ it is that causes the stack frames to be so
> horrid. For example, you have
>
> 18) 8896 160 .kmem_cache_alloc+0xfc/0x140
>
> and I'm looking at my x86-64 compile, and it has a stack frame of just 8
> bytes (!) for local variables plus the save/restore area (which looks like
> three registers plus frame pointer plus return address). IOW, if I'm
> looking at the code right (so big caveat: I did _not_ do a real stack
> dump!) the x86-64 stack cost for that same function is on the order of 48
> bytes. Not 160.

Out of curiosity, I just ran stack_trace on the latest version of git
(pulled sometime today) and ran it on my x86_64.

I have SLUB and SLUB debug defined, and here's what I found:

11) 3592 64 kmem_cache_alloc+0x64/0xa3

64 bytes, still much lower than the 160 of PPC64.

Just to see where it got that number, I ran objdump on slub.o.

000000000000300e <kmem_cache_alloc_node>:
300e: 55 push %rbp
300f: 48 89 e5 mov %rsp,%rbp
3012: 41 57 push %r15
3014: 41 56 push %r14
3016: 41 55 push %r13
3018: 41 54 push %r12
301a: 53 push %rbx
301b: 48 83 ec 08 sub $0x8,%rsp
Six words pushed, plus the 8 byte stack frame.

6*8+8 = 56

But we also add the push of the function return address which is another 8
bytes which gives us 64 bytes.

Here's the complete dump:

[root@bxrhel51 linux-compile.git]# cat /debug/tracing/stack_trace
Depth Size Location (54 entries)
----- ---- --------
0) 4504 48 __mod_zone_page_state+0x59/0x68
1) 4456 64 __rmqueue_smallest+0xa0/0xd9
2) 4392 80 __rmqueue+0x24/0x172
3) 4312 96 rmqueue_bulk+0x57/0xa3
4) 4216 224 get_page_from_freelist+0x371/0x6e1
5) 3992 160 __alloc_pages_internal+0xe0/0x3f8
6) 3832 16 __alloc_pages_nodemask+0xe/0x10
7) 3816 48 alloc_pages_current+0xbe/0xc7
8) 3768 16 alloc_slab_page+0x28/0x34
9) 3752 64 new_slab+0x4a/0x1bb
10) 3688 96 __slab_alloc+0x203/0x364
11) 3592 64 kmem_cache_alloc+0x64/0xa3
12) 3528 48 alloc_buffer_head+0x22/0x9d
13) 3480 64 alloc_page_buffers+0x2f/0xd1
14) 3416 48 create_empty_buffers+0x22/0xb5
15) 3368 176 block_read_full_page+0x6b/0x25c
16) 3192 16 blkdev_readpage+0x18/0x1a
17) 3176 64 read_cache_page_async+0x85/0x11a
18) 3112 32 read_cache_page+0x13/0x48
19) 3080 48 read_dev_sector+0x36/0xaf
20) 3032 96 read_lba+0x51/0xb0
21) 2936 176 efi_partition+0x92/0x585
22) 2760 128 rescan_partitions+0x173/0x308
23) 2632 96 __blkdev_get+0x22a/0x2ea
24) 2536 16 blkdev_get+0x10/0x12
25) 2520 80 register_disk+0xe5/0x14a
26) 2440 48 add_disk+0xbd/0x11f
27) 2392 96 sd_probe+0x2c5/0x3ad
28) 2296 48 driver_probe_device+0xc5/0x14c
29) 2248 16 __device_attach+0xe/0x10
30) 2232 64 bus_for_each_drv+0x56/0x8d
31) 2168 48 device_attach+0x68/0x7f
32) 2120 32 bus_attach_device+0x2d/0x5e
33) 2088 112 device_add+0x45f/0x5d3
34) 1976 64 scsi_sysfs_add_sdev+0xb7/0x246
35) 1912 336 scsi_probe_and_add_lun+0x9f9/0xad7
36) 1576 80 __scsi_add_device+0xb6/0xe5
37) 1496 80 ata_scsi_scan_host+0x9e/0x1c6
38) 1416 64 ata_host_register+0x238/0x252
39) 1352 96 ata_pci_sff_activate_host+0x1a1/0x1ce
40) 1256 256 piix_init_one+0x646/0x6b2
41) 1000 144 pci_device_probe+0xc9/0x120
42) 856 48 driver_probe_device+0xc5/0x14c
43) 808 48 __driver_attach+0x67/0x91
44) 760 64 bus_for_each_dev+0x54/0x84
45) 696 16 driver_attach+0x21/0x23
46) 680 64 bus_add_driver+0xba/0x204
47) 616 64 driver_register+0x98/0x110
48) 552 48 __pci_register_driver+0x6b/0xa4
49) 504 16 piix_init+0x19/0x2c
50) 488 272 _stext+0x5d/0x145
51) 216 32 kernel_init+0x127/0x17a
52) 184 184 child_rip+0xa/0x11
-- Steve



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