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    [W:0.027 / U:29.824 seconds]
    ©2003-2016 Jasper Spaans. hosted at Digital OceanAdvertise on this site