Re: [RFC 1/2] ftrace porting of ia64

[Steven Rostedt <srostedt@redhat.com>]

On Thu, 2008-12-18 at 15:28 +0100, Ingo Molnar wrote:
> * Steven Rostedt <srostedt@redhat.com> wrote:
> 
> > 
> > On Thu, 2008-12-18 at 11:25 +0100, Ingo Molnar wrote:
> > > * Shaohua Li <shaohua.li@intel.com> wrote:
> > > 
> > > > ftrace porting of IA64.
> > > > TBD:
> > > > 1. I don't how to add unwind info to the assemble code, so please
> > > > advise.
> > > > 2. The generic ftrace ring buffer code doesn't handle alignment well.
> > > > With the patch, kernel will report a lot of unalignment. This is still
> > > > under investigation.
> > > 
> > > hm, that's weird - i recently profiled 64-bit x86 for unaligned accesses 
> > > and there didnt seem to be many. In any case, feel free to fix any 
> > > unaligned structure fields by reordering them.
> > 
> > Will x86_64 break if something is aligned by 32 bits?
> 
> No, but there are tools to detect misalignment as they happen. (there are 
> CPU hw counters for access misalignment and kerneltop can profile based on 
> that info :-)
> 
> > The records in the buffer use to be 64 bit aligned. They are now 32 bit 
> > aligned. I wonder if we should make that alignment arch specific.
> 
> No, we should just align them to 64 bits and be done with it.

I originally had it 64 bit aligned, but a lot of people complained about
the wasted space. The minimum record was 8 bytes, but you could have a
12 byte record. This allows for more compact recording.

Hmm, the header is 32 bits. Maybe that's the problem. The data part
starts at the 32bit mark. Maybe it is not the alignment of the record,
but the alignment of where the data starts. If the size is small enough,
it might align to the 32bit boundary.

I'll have to look more into that.

-- Steve