Re: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression

Andrew Morton
2006-10-04T22:06:41Z

On Wed, 4 Oct 2006 12:47:00 -0400
Andrew James Wade <andrew.j.wade@gmail.com> wrote:

> On Tuesday 03 October 2006 23:32, Andrew Morton wrote:
>
> > It might help, but we still don't know what's going on (I think).
> >
> > I mean, if cache misses against __warn_once were sufficiently high for it
> > to affect performance, then __warn_once would be, err, in cache?
>
> Yes, of course. I'm embarrassed.
>
> I took a look at the generated code, and GCC is having difficulty
> optimizing WARN_ON_ONCE. Here is the start of __local_bh_enable:
>
> 00000130 <__local_bh_enable>:
> 130: 83 ec 10 sub $0x10,%esp
> 133: 8b 15 04 00 00 00 mov 0x4,%edx <-+
> 139: 89 e0 mov %esp,%eax |
> 13b: 25 00 e0 ff ff and $0xffffe000,%eax | !!!
> 140: 8b 40 14 mov 0x14(%eax),%eax |
> 143: 25 00 00 ff 0f and $0xfff0000,%eax |

This is the evaluation of in_irq(): calculate `current', grab
current->thread_info->preempt_count.

Normally gcc does manage to CSE the value of current.

> 148: 85 d2 test %edx,%edx <-+
> 14a: 74 04 je 150 <__local_bh_enable+0x20>
> 14c: 85 c0 test %eax,%eax
> 14e: 75 35 jne 185 <__local_bh_enable+0x55>
> 150: 89 e0 mov %esp,%eax
> 152: 8b 0d 00 00 00 00 mov 0x0,%ecx <-+
> 158: 25 00 e0 ff ff and $0xffffe000,%eax |

but this time it went and reevaluated it.

> 15d: 8b 40 14 mov 0x14(%eax),%eax | !!!
> 160: 25 00 ff 00 00 and $0xff00,%eax |
> 165: 3d 00 01 00 00 cmp $0x100,%eax |
> 16a: 0f 94 c0 sete %al |
> 16d: 85 c9 test %ecx,%ecx <-+
> 16f: 0f b6 c0 movzbl %al,%eax
> 172: 74 04 je 178 <__local_bh_enable+0x48>
> 174: 85 c0 test %eax,%eax
> 176: 75 42 jne 1ba <__local_bh_enable+0x8a>
> 178: b8 00 01 00 00 mov $0x100,%eax
> 17d: 83 c4 10 add $0x10,%esp
> 180: e9 fc ff ff ff jmp 181 <__local_bh_enable+0x51>
> 185: c7 44 24 0c 3e 00 00 movl $0x3e,0xc(%esp)
> ...
>
> WARN_ON is better, but still has problems:
>
> 000011a0 <do_exit>:
> 11a0: 55 push %ebp
> 11a1: 57 push %edi
> 11a2: 56 push %esi
> 11a3: 53 push %ebx
> 11a4: 83 ec 30 sub $0x30,%esp
> 11a7: 89 44 24 18 mov %eax,0x18(%esp)
> 11ab: 89 e0 mov %esp,%eax
> 11ad: 25 00 e0 ff ff and $0xffffe000,%eax
> 11b2: 8b 30 mov (%eax),%esi
> 11b4: 8b 86 58 0a 00 00 mov 0xa58(%esi),%eax
> 11ba: 89 44 24 2c mov %eax,0x2c(%esp) <-+
> 11be: 8b 44 24 2c mov 0x2c(%esp),%eax <-+
> 11c2: 85 c0 test %eax,%eax | !!!
> 11c4: 0f 85 65 07 00 00 jne 192f <do_exit+0x78f> |
> 11ca: 8b 44 24 2c mov 0x2c(%esp),%eax <-+
> 11ce: 89 e0 mov %esp,%eax
> ...

No, that's pretty much the same. In the __local_bh_enable case we have the
evaluation of in_irq() as well as softirq_count(). do_exit() just has a
single WARN_ON().

> This is gcc (GCC) 4.0.3 (Ubuntu 4.0.3-1ubuntu5), -O2.
>
> I don't know why this would show up as cache misses, but it does look
> like a compiler bug is being tickled.

The code could be better, but nope, there are no additional cache misses
there.

Still weird.