On Sat, 2011-01-15 at 21:17 -0800, David Miller wrote:
> From: Richard Mortimer <ri...@oldelvet.org.uk>
> Date: Fri, 14 Jan 2011 16:08:30 +0000
>
> [ Frederic, Steven, Ingo, the short version of the story is that we
> need to make it such that the _ftrace_events section is aligned
> properly for 64-bit systems, and in particular that GCC can see this
> too. Otherwise GCC thinks 64-bit words will be unaligned and we get
> all kinds of funky relocations being used in modules on sparc64 that
> really there is never any reason to see. ]
>
> > They seem to come from scsi.o at
> >
> > .LLC51:
> > .asciz "scsi_eh_wakeup"
> > .section _ftrace_events,"aw",@progbits
> > .align 4
> > .type event_scsi_eh_wakeup, #object
> > .size event_scsi_eh_wakeup, 136
> > event_scsi_eh_wakeup:
> > .skip 16
> > .uaxword event_class_scsi_eh_wakeup
> > .uaxword .LLC51
> > .skip 8
> > .skip 40
> > .uaxword ftrace_event_type_funcs_scsi_eh_wakeup
> > .uaxword print_fmt_scsi_eh_wakeup
> > .skip 40
>
> These ".uaxword" emissions are a bug, and if we let them stand then
> ftrace events are going to load every long word object using a
> sequence of byte-sized accesses. This will absolutely kill
> performance.
>
> This is exactly why I wanted to find out why this started happening
> out of nowhere, instead of blindly adding support for new relocation
> types to the sparc module loader.
>
> The set of relocations supported by the sparc module loading code is
> not meant to be "all relocations which are legal" but rather it's
> meant to support the most minimum set of relocations that the kernel
> actually _needs_ and should be making use of.
>
> I think the problem we have here is that the _ftrace_events section is
> not aligned sufficiently. That ".align 4" mnemonic is a good indication
> of this. It should at least "8" on sparc64.
>
> So we need to figure out why that is happening, and fix that instead.
>
> Thanks.
I'm wondering if gcc is just getting better at honouring the source
code. The DEFINE_EVENT macros in include/trace/ftrace.h have a
__aligned__(4) attribute in them. Maybe that should be 8 on sparc64
systems.
The aligned 4 seems to be unchanged since include/trace/ftrace.h was
created in f42c85e74faa422cf0bc747ed808681145448f88 in April 2009.
example
#undef DEFINE_EVENT
#define DEFINE_EVENT(template, call, proto, args) \
\
static struct ftrace_event_call __used \
__attribute__((__aligned__(4))) \
__attribute__((section("_ftrace_events"))) event_##call = { \
.name = #call, \
.class = &event_class_##template, \
.event.funcs = &ftrace_event_type_funcs_##template, \
.print_fmt = print_fmt_##template, \
};
I haven't tried making any changes/recompiling yet because that change
will pretty much force a full recompile and that takes upwards of 12
hours on the old sparc64 box that I have. I will wait for feedback
before I try anything.
Regards
Richard
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