Re: [PATCH] Fix WARN_ON / WARN_ON_ONCE regression

Andrew James Wade
2006-10-05T08:17:55Z

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.

Andrew Wade