Re: patches to actually use markers?

["Frank Ch. Eigler" <fche at redhat dot com>]

Hi -

On Fri, Nov 16, 2007 at 05:03:14PM -0500, Mathieu Desnoyers wrote:
> [...]
> We currently have three distinct events for a system call :
> 
> syscall entry, with syscall id and instruction pointer
> the syscall specific instrumentation (opt)
> syscall exit

> [...]  Instrumentation within the syscall specific function helps
> knowing when/if the operation has really been done _within the
> kernel_. [...]

Not just that - but *what* the actual operation was.

> [...] Given these two opposite sets of constraints, I think having
> more than one instrumentation site per syscall makes sense.

Sure - what bothers me is the satisfaction with the inconsistency of
some system calls having no specific markers.

> Moreover, markers are really cheap... :)

I'm not the one who must buy what we're selling - it's the kernel
maintainers. :-)

> [...]  Yes, refcount would be the way to go. The code is currently
> in kernel/sched.c, since it touches the threads. I would have to add
> the refcount. It will be in the next LTTng prerelease.

But you see, if markers are not just really cheap but really really
cheap, then you don't need the task flag, nor the new API for
refcounting the flags' clients, nor the new machinery to propagate the
flag to new tasks.  You just put unconditional markers in there and
let the possible multiple marker handlers do their own filtering.


- FChE