linux-kernel - Re: [PATCH 2/2] convert to syscall tracepoints

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Date:	Mon, 8 Jun 2009 23:25:26 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Jason Baron <jbaron@...hat.com>, "H. Peter Anvin" <hpa@...or.com>
Cc:	linux-kernel@...r.kernel.org, fweisbec@...il.com,
	laijs@...fujitsu.com, rostedt@...dmis.org, peterz@...radead.org,
	mathieu.desnoyers@...ymtl.ca, jiayingz@...gle.com,
	mbligh@...gle.com, roland@...hat.com, fche@...hat.com
Subject: Re: [PATCH 2/2] convert to syscall tracepoints


* Jason Baron <jbaron@...hat.com> wrote:

> On Mon, Jun 08, 2009 at 10:40:56PM +0200, Ingo Molnar wrote:
> > * Jason Baron <jbaron@...hat.com> wrote:
> > 
> > > +#ifdef __NR_time
> > > +trace_event_syscall(1, time, time_t __user *, tloc);
> > > +#endif
> > > +
> > > +#ifdef __NR_stime
> > > +trace_event_syscall(1, stime, time_t __user *, tptr);
> > > +#endif
> > > +
> > > +#ifdef __NR_gettimeofday
> > > +trace_event_syscall(2, gettimeofday, struct timeval __user *, tv, struct timezone __user *, tz);
> > > +#endif
> > 
> > This could be reduced to a single line: just add a Kconfig entry 
> > (say TRACE_SYSCALL_TRACEPOINTS) wether an arch supports syscall 
> > tracepoints, enable it on a sane arch, make sure it has all the 
> > syscalls and list them ...
> > 
> > As more architectures turn on SYSCALL_TRACEPOINTS, they'll have to 
> > resolve any deviations in syscall entry points. Ideally we'd have 
> > one generic table that covers 95% of all syscalls, and the remaining 
> > 5% in some architecture specific #ifdef section.
> > 
> 
> true, but this implementation works for all arches now, why would 
> want to slowly add this over time? [...]

Because the current solution is butt-ugly ...

> [...] I think its unnecessary work that could be error prone.

This area needs cleanups - making it messier doesnt help. (I've 
Cc:-ed hpa - he has expressed interest in auto-generating all the 
syscall related details from another angle ...)

> > But, more generally, i'm not at all convinced that we need _any_ 
> > of this enumeration. Look how much the above lines duplicate 
> > DEFINE_SYSCALL macros. Why arent those macros re-used?
> 
> The DEFINE_SYSCALL() are located all over the code in various .c files.

yes, and that's good.

> Thus, if we define the tracpoints via the DEFINE_SYSCALL() macros 
> we are going to have 'static inline functions' (which is how 
> tracepoints are implemented) defined in all these .c files. Now, I 
> need to call all these 'static inline functions' from ptrace.c. 
> How do I do that? [...]

And that's bad.

We dont want a per syscall tracepoint call site. AT ALL.

We want to collect the record information, we want to construct 
/debug/tracing/events/syscalls/ directories with all the proper 
tracepoint-lookalike entries, and then we want to use the 
_existing_, _zero overhead_ method implemented by Frederic to get 
per syscall functionality.

Have you looked at how the syscall attributes information is 
constructed by using .section tricks? See: 
kernel/trace/trace_syscalls.c.

	Ingo
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