linux-kernel - Re: [RFC] convert ftrace syscall tracer to TRACE

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Date:	Sat, 9 May 2009 12:32:16 -0400
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	"Frank Ch. Eigler" <fche@...hat.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Jason Baron <jbaron@...hat.com>,
	Tom Zanussi <tzanussi@...il.com>, linux-kernel@...r.kernel.org,
	fweisbec@...il.com, laijs@...fujitsu.com, rostedt@...dmis.org,
	peterz@...radead.org, jiayingz@...gle.com, mbligh@...gle.com,
	roland@...hat.com
Subject: Re: [RFC] convert ftrace syscall tracer to TRACE_EVENT()

* Frank Ch. Eigler (fche@...hat.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
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68
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