Re: [PATCH] Linux Kernel Markers

[Martin Bligh <mbligh@google.com>]

Karim Yaghmour wrote:
> Martin Bligh wrote:
> 
>>It's looking to me like it might still need djprobes to implement, in
>>order to get the atomic and safe switchover from the original function
>>into the traced one. All rather sad, but seems to be true from all the
>>CPU errata, etc. If anyone can see a way round that, I'd love to hear
>>it.
> 
> 
> But we don't need to fight the errata, there are fortunately solutions
> that take care of it where it does exist (x86: djprobes/kprobes.)
> What's more interesting, though, is that the method as it is proposed
> at this stage *seems* to be easily portable to other archs. And where
> such binary trickery is difficult to pull off, nothing precludes
> having a universally "portable" mechanism including something akin to
> switching between instrumented vs. normal function at function entry.
> Even such conditional ifs can be optimized by the CPU nowadays.
> 
> The picture is, nevertheless, very bright at the moment (I think).
> Just have a 5byte filler at function entry such as Hiramatsu-san
> suggested, and use djprobes to fork to instrumented function. The
> unconditional jump in the filler will most likely be utterly
> unmeasurable, and benchmarks should confirm this.
> 
> So:
> On x86: use 5byte filler and djprobes.
> On "sane" archs: use filler and override as explained earlier.
> Elsewhere: use standard "if" or function pointer at function entry.

Do we even need the filler padding? I thought we could insert kprobes
at the beginning of any function without that ... it was only a
requirement for mid-function (sometimes). If we copy the whole function,
we don't even need that any more ...

if kprobes can do it, I don't see why djprobes can't ... after all, it
just seems to use kprobes to insert a jump, AFAICS.

M.