Messages in this thread |  | | | Subject | Re: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression | | Date | Wed, 4 Oct 2006 12:47:00 -0400 | | From | Andrew James Wade <> |
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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 | 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 | 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 ... 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.
Andrew Wade - To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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