Re: [RFC 2/2] x86_64: expand kernel stack to 16K

Minchan Kim
2014-05-29T03:45:56Z

On Wed, May 28, 2014 at 09:09:23AM -0700, Linus Torvalds wrote:
> On Tue, May 27, 2014 at 11:53 PM, Minchan Kim <minchan@kernel.org> wrote:
> >
> > So, my stupid idea is just let's expand stack size and keep an eye
> > toward stack consumption on each kernel functions via stacktrace of ftrace.
>
> We probably have to do this at some point, but that point is not -rc7.
>
> And quite frankly, from the backtrace, I can only say: there is some
> bad shit there. The current VM stands out as a bloated pig:
>
> > [ 1065.604404] kworker/-5766 0d..2 1071625991us : stack_trace_call: 0) 7696 16 lookup_address+0x28/0x30
> > [ 1065.604404] kworker/-5766 0d..2 1071625991us : stack_trace_call: 1) 7680 16 _lookup_address_cpa.isra.3+0x3b/0x40
> > [ 1065.604404] kworker/-5766 0d..2 1071625991us : stack_trace_call: 2) 7664 24 __change_page_attr_set_clr+0xe0/0xb50
> > [ 1065.604404] kworker/-5766 0d..2 1071625991us : stack_trace_call: 3) 7640 392 kernel_map_pages+0x6c/0x120
> > [ 1065.604404] kworker/-5766 0d..2 1071625992us : stack_trace_call: 4) 7248 256 get_page_from_freelist+0x489/0x920
> > [ 1065.604404] kworker/-5766 0d..2 1071625992us : stack_trace_call: 5) 6992 352 __alloc_pages_nodemask+0x5e1/0xb20
>
> > [ 1065.604404] kworker/-5766 0d..2 1071625995us : stack_trace_call: 23) 4672 160 __swap_writepage+0x150/0x230
> > [ 1065.604404] kworker/-5766 0d..2 1071625996us : stack_trace_call: 24) 4512 32 swap_writepage+0x42/0x90
> > [ 1065.604404] kworker/-5766 0d..2 1071625996us : stack_trace_call: 25) 4480 320 shrink_page_list+0x676/0xa80
> > [ 1065.604404] kworker/-5766 0d..2 1071625996us : stack_trace_call: 26) 4160 208 shrink_inactive_list+0x262/0x4e0
> > [ 1065.604404] kworker/-5766 0d..2 1071625996us : stack_trace_call: 27) 3952 304 shrink_lruvec+0x3e1/0x6a0
> > [ 1065.604404] kworker/-5766 0d..2 1071625996us : stack_trace_call: 28) 3648 80 shrink_zone+0x3f/0x110
> > [ 1065.604404] kworker/-5766 0d..2 1071625997us : stack_trace_call: 29) 3568 128 do_try_to_free_pages+0x156/0x4c0
> > [ 1065.604404] kworker/-5766 0d..2 1071625997us : stack_trace_call: 30) 3440 208 try_to_free_pages+0xf7/0x1e0
> > [ 1065.604404] kworker/-5766 0d..2 1071625997us : stack_trace_call: 31) 3232 352 __alloc_pages_nodemask+0x783/0xb20
> > [ 1065.604404] kworker/-5766 0d..2 1071625997us : stack_trace_call: 32) 2880 8 alloc_pages_current+0x10f/0x1f0
> > [ 1065.604404] kworker/-5766 0d..2 1071625997us : stack_trace_call: 33) 2872 200 __page_cache_alloc+0x13f/0x160
>
> That __alloc_pages_nodemask() thing in particular looks bad. It
> actually seems not to be the usual "let's just allocate some
> structures on the stack" disease, it looks more like "lots of
> inlining, horrible calling conventions, and lots of random stupid
> variables".

Yes. For example, with mark __alloc_pages_slowpath noinline_for_stack,
we can reduce 176byte. And there are more places we could reduce stack
consumption but I thought it was bandaid although reducing stack itself
is desireable.

before

ffffffff81150600 <__alloc_pages_nodemask>:
ffffffff81150600: e8 fb f6 59 00 callq ffffffff816efd00 <__entry_text_start>
ffffffff81150605: 55 push %rbp
ffffffff81150606: b8 e8 e8 00 00 mov $0xe8e8,%eax
ffffffff8115060b: 48 89 e5 mov %rsp,%rbp
ffffffff8115060e: 41 57 push %r15
ffffffff81150610: 41 56 push %r14
ffffffff81150612: 41 be 22 01 32 01 mov $0x1320122,%r14d
ffffffff81150618: 41 55 push %r13
ffffffff8115061a: 41 54 push %r12
ffffffff8115061c: 41 89 fc mov %edi,%r12d
ffffffff8115061f: 53 push %rbx
ffffffff81150620: 48 81 ec 28 01 00 00 sub $0x128,%rsp
ffffffff81150627: 48 89 55 88 mov %rdx,-0x78(%rbp)
ffffffff8115062b: 89 fa mov %edi,%edx
ffffffff8115062d: 83 e2 0f and $0xf,%edx
ffffffff81150630: 48 89 4d 90 mov %rcx,-0x70(%rbp)

after:

ffffffff81150600 <__alloc_pages_nodemask>:
ffffffff81150600: e8 7b f6 59 00 callq ffffffff816efc80 <__entry_text_start>
ffffffff81150605: 55 push %rbp
ffffffff81150606: b8 e8 e8 00 00 mov $0xe8e8,%eax
ffffffff8115060b: 48 89 e5 mov %rsp,%rbp
ffffffff8115060e: 41 57 push %r15
ffffffff81150610: 41 bf 22 01 32 01 mov $0x1320122,%r15d
ffffffff81150616: 41 56 push %r14
ffffffff81150618: 41 55 push %r13
ffffffff8115061a: 41 54 push %r12
ffffffff8115061c: 41 89 fc mov %edi,%r12d
ffffffff8115061f: 53 push %rbx
ffffffff81150620: 48 83 ec 78 sub $0x78,%rsp
ffffffff81150624: 48 89 55 a8 mov %rdx,-0x58(%rbp)
ffffffff81150628: 89 fa mov %edi,%edx
ffffffff8115062a: 83 e2 0f and $0xf,%edx
ffffffff8115062d: 48 89 4d b0 mov %rcx,-0x50(%rbp)

>
> >From a quick glance at the frame usage, some of it seems to be gcc
> being rather bad at stack allocation, but lots of it is just nasty
> spilling around the disgusting call-sites with tons or arguments. A
> _lot_ of the stack slots are marked as "%sfp" (which is gcc'ese for
> "spill frame pointer", afaik).
>
> Avoiding some inlining, and using a single flag value rather than the
> collection of "bool"s would probably help. But nothing really
> trivially obvious stands out.
>
> But what *does* stand out (once again) is that we probably shouldn't
> do swap-out in direct reclaim. This came up the last time we had stack
> issues (XFS) too. I really do suspect that direct reclaim should only
> do the kind of reclaim that does not need any IO at all.
>
> I think we _do_ generally avoid IO in direct reclaim, but swap is
> special. And not for a good reason, afaik. DaveC, remind me, I think
> you said something about the swap case the last time this came up..
>
> Linus
>
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--
Kind regards,
Minchan Kim