Re: [RFC] convert ftrace syscall tracer to TRACE_EVENT()

Mathieu Desnoyers
2009-05-09T16:32:45Z

* Frank Ch. Eigler (fche@redhat.com) wrote:
> Hi -
>
> On Sat, May 09, 2009 at 10:37:37AM +0200, Ingo Molnar wrote:
> > [...]
> > Firstly, it adds two new tracepoints to every system call. That is
> > unnecessary - we already have the TIF flag based callbacks, and we
> > can use the existing syscall attributes table to get to tracepoints
> > - without slow down (or impacting) the fast path in any way.
> > [...]
>
> However, as the TIF_SYSCALL_FTRACE and/or TIF_SYSCALL_TRACE flags are
> tied to a single consumer, it would limit the usefulness of the
> naturally multiple-client tracepoints, if they were made conditional
> on them. Would you be interested in a proper reference-counting API?
>

Hrm, actually, looking at

void start_ftrace_syscalls(void)
{
unsigned long flags;
struct task_struct *g, *t;

mutex_lock(&syscall_trace_lock);

/* Don't enable the flag on the tasks twice */
if (++refcount != 1)
goto unlock;

shows me that there is in fact some refcounting done there. But I'll
send the TIF_KERNEL_TRACE patchset anyway, given it supports much more
architectures and has a more generic thread flag name.

Mathieu

> - FChE

--
Mathieu Desnoyers
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