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

[Ingo Molnar <mingo@elte.hu>]

* Shaohua Li <shaohua.li@intel.com> wrote:

> On Thu, 2008-12-18 at 23:31 +0800, Steven Rostedt wrote:
> > 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.
> I port my patch to latest -tip, the unalignment issue seems disappear.

thanks for checking!

	Ingo