lkml.org 
[lkml]   [2010]   [Oct]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] tracing: Cleanup the convoluted softirq tracepoints
On Tue, Oct 19, 2010 at 02:16:54PM -0700, David Daney wrote:
> On 10/19/2010 12:49 PM, Thomas Gleixner wrote:
> [...]
> >So that saves _TWO_ bytes of text and replaces:
> >
> >- 1e: 83 3d 00 00 00 00 00 cmpl $0x0,0x0(%rip) # 25<test+0x25>
> >- 25: 74 4d je 74<test+0x74>
> >+ 1e: e9 00 00 00 00 jmpq 23<test+0x23>
> >+ 23: eb 4d jmp 72<test+0x72>
> >
> >So it trades a conditional vs. two jumps ? WTF ??
> >
> >I thought that jumplabel magic was supposed to get rid of the jump
> >over the tracing code ? In fact it adds another jump. Whatfor ?
>
> The 'asm goto' construct in GCC-4.5 is deficient in this area.
>
> GCC assumes that all exit paths from an 'asm goto' are equally
> likely, so the tracing (or dynamic printk etc.) code is assumed to
> be hot and is emitted inline. Since they are inline like this,
> there are all these jumps around them and they pollute the I-Cache.
>
> I was looking at fixing it, but I think a true general purpose fix
> would require enhancing GCC's grammar to allow specifying of the
> 'likelyness' of each exit path from 'asm goto'.
>
> David Daney
>

right, the next step is adding support for hot/cold labels, so the
tracing code will be annotaed with a 'cold' label. Thus, not adding the
'jmp' above on line '23', and in fact moving the tracing code
out-of-line. Maybe I haven't been clear on this.

thanks,

-Jason


\
 
 \ /
  Last update: 2010-10-19 23:35    [from the cache]
©2003-2011 Jasper Spaans