(2011/09/21 5:53), Stefan Hajnoczi wrote:

On Tue, Sep 20, 2011 at 02:12:25PM -0400, Christoph Hellwig wrote:

quoted

On Tue, Sep 20, 2011 at 06:13:10PM +0100, Stefan Hajnoczi wrote:

quoted

You've probably thought of this but it would be nice to skip XOL for
nops.  This would be a common case with static probes (e.g. sdt.h) where
the probe template includes a nop where we can easily plant int $0x3.

Do we now have sdt.h support for uprobes?  That's one of the killer
features that always seemed to get postponed.

Not yet but it's a question of doing roughly what SystemTap does to
parse the appropriate ELF sections and then putting those probes into
uprobes.

Masami looked at this and found that SystemTap sdt.h currently requires
an extra userspace memory store in order to activate probes.  Each probe
has a "semaphore" 16-bit counter which applications may test before
hitting the probe itself.  This is used to avoid overhead in
applications that do expensive argument processing (e.g. creating
strings) for probes.

Indeed, originally, those semaphores designed for such use cases.
However, some applications *always* use it (e.g. qemu-kvm).

But this should be solvable so it would be possible to use perf-probe(1)
on a std.h-enabled binary.  Some distros already ship such binaries!

I'm not sure that we should stick on the current implementation
of the sdt.h. I think we'd better modify the sdt.h to replace
such semaphores with checking whether the tracepoint is changed from nop.

Or, we can introduce an add-hoc ptrace code to perftools for modifying
those semaphores. However, this means that user always has to use
perf to trace applications, and it's hard to trace multiple applications
at a time (can we attach all of them?)...

Thank you,

-- 
Masami HIRAMATSU
Software Platform Research Dept. Linux Technology Center
Hitachi, Ltd., Yokohama Research Laboratory
E-mail: masami.hiramatsu.pt@hitachi.com

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