Re: Random panic in load_balance() with 3.16-rc

[Nick Krause <xerofoify@gmail.com>]

On Thu, Jul 24, 2014 at 10:33 PM, Linus Torvalds
<torvalds@linux-foundation.org> wrote:
> On Thu, Jul 24, 2014 at 6:25 PM, Michel Dänzer <michel@daenzer.net> wrote:
>>
>> Attached is fair.s from Debian gcc 4.8.3-5. Does that look better? I'm
>> going to try reproducing the problem with a kernel built by that now.
>
> This looks better. For roughly that same code sequence it does
> (ignoring the debug line and cfi information):
>
>         subq    $184, %rsp      #,
>         movq    (%r12), %rax    # sd_22(D)->parent, sd_parent
>         movl    %edi, -156(%rbp)        # this_cpu, %sfp
>         movl    %ecx, -160(%rbp)        # idle, %sfp
>         movq    %r8, -184(%rbp) # continue_balancing, %sfp
>         movq    %rax, -176(%rbp)        # sd_parent, %sfp
>         movq    $load_balance_mask, %rax        #, tcp_ptr__
> #APP
>         add %gs:this_cpu_off, %rax      # this_cpu_off, tcp_ptr__
> #NO_APP
>
> so it updates the stack pointer before any spills, and it also doesn't
> spill that constant value.
>
> I still have no idea why it does the 4-byte rep stosl/movsl thing, but
> that's a whole separate guessing game and might have something to do
> with the fact that you do CONFIG_CC_OPTIMIZE_FOR_SIZE and the 4-byte
> form is one byte smaller.
>
> I'm a big believer in not blowing up the I$ footprint, and I have to
> admit to pushing that myself a few years ago, but gcc does some rather
> bad things with '-Os', so it's not actually suggested for the kernel
> any more. I wish there was some middle ground model that cared about
> size, but not to exclusion of everything else. The string instructions
> are not good for performance when it's a compile-time known small
> size.
>
>                  Linus
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Seems better after looking at it too, seems I don't need to test this and this
bug is in gcc 4.9 related versions.
Cheers Nick