Messages in this thread | | | Subject | Re: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression | Date | Thu, 5 Oct 2006 04:13:07 -0400 | From | Andrew James Wade <> |
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On Wednesday 04 October 2006 18:06, Andrew Morton wrote: > 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.
Actually I was confusing "mov 0x4,%edx" with "mov $0x4,%edx". That code's fine (albeit unlinked). There are stupid inefficiencies in some of the other code, but nothing really stood out at me in __local_bh_enable, _local_bh_enable, or local_bh_Enable.
(from earlier) > Perhaps the `static int __warn_once' is getting put in the same cacheline > as some frequently-modified thing.
hmm:
00000460 l O .data 00000044 task_exit_notifier 000004c0 l O .data 0000002c task_free_notifier 000004ec l O .data 00000004 warnlimit.15904 000004f0 l O .data 00000004 firsttime.15774 000004f4 l O .data 00000004 __warn_once.15180 000004f8 l O .data 00000004 __warn_once.15174 000004fc l O .data 00000004 __warn_once.15213 00000500 l O .data 00000004 __warn_once.15207 00000504 l O .data 00000004 __warn_once.15145 00000508 l O .data 00000004 __warn_once.15309 0000050c l O .data 00000004 __warn_once.15256 00000510 l O .data 00000004 __warn_once.15250 000005a0 l O .data 0000006c proc_iomem_operations (extracted from objdump -t kernel/built-in.o)
warnlimit and firsttime are fine, and proc_iomem_operations is presumably fine as well. But I'm not so sure task_free_notifier is infrequently modified. But that's just my .config and I'm out of my depth.
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