Re: [PATCH] riscv/ftrace: Add basic support

[Steven Rostedt]

On Mon, 4 Dec 2017 13:52:30 +0800
Alan Kao <nonerkao@xxxxxxxxx> wrote:

> > > Note that the functions in both ftrace.c and setup.c should not be
> > > hooked with the compiler's -pg option: to prevent infinite self-
> > > referencing for the former, and to ignore early setup stuff for the latter.  
> > 
> > I'm curious to what is in setup.c that ftrace uses.  
> 
> In the scenario for some embedded systems, the __init prefix does not give 
> us the notrace feature without the MODULE config.  Therefore, all functions 
> would have been hooked with the _mcount trampoline if the -pg flag was not 
> specifically disabled.

But is there functions you may want to trace. There's an effort going
on to allow function tracing to start in early boot up.

> 
> And a terrible result would have happened after function setup_vm called
> _mcount.  As _mcount compared the value of ftrace_trace_function and 
> the position of ftrace_stub, it crashed the kernel because one of them 
> was a physical address while the other was a virtual address but
> actually they pointed to the same.
> 
> Adding notrace to setup_vm can solve the described issue, but it might be 
> redundant once the MODULE config becomes stable and default on most 
> platforms. To be honest, nobody really needs those init procedures to be
> ftrace-able.

Um no, because MODULE init code can now be traced. It use to be that we
didn't trace any __init, but I worked on having both inits be traced.
The module code was a little bit trickier because it can be loaded
multiple times and we needed to figure out the best way to handle init
functions in the buffer that went stale and is replaced by other module
init functions.

 
> > > +config TRACE_IRQFLAGS_SUPPORT
> > > +	def_bool y
> > > +
> > > +config LOCKDEP_SUPPORT
> > > +	def_bool y  
> > 
> > Hmm, not sure the above is needed for function tracing.
> >  
> 
> FTRACE depends on TRACING_SUPPORT, and TRACING_SUPPORT depends on
> TRACE_IRQFLAGS_SUPPORT. But LOCKDEP_SUPPORT is not actually needed
> for any of the ftrace features implemented in this patch.

Hmm, I think that's stale. Thanks for bringing that to my attention,
and don't believe that dependency still exists.

> 
> The LOCKDEP_SUPPORT will be removed in the next version.
>

I should have also asked, is lockdep really supported on this arch, and
is IRQSFLAGS really supported too? I vaguely remember making ftrace
depend on IRQFLAGS because we wanted archs to support TRACE_IRQFLAGS
before they supported ftrace. Maybe I'll keep that dependency.

> > > +ENTRY(_mcount)
> > > +	la	t4, ftrace_stub
> > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> > > +	la	t0, ftrace_graph_return
> > > +	ld	t1, 0(t0)
> > > +	bne	t1, t4, do_ftrace_graph_caller  
> > 
> > If function graph is enabled, you jump straight to the graph tracer,
> > but never return back to here?
> >  
> 
> Because prepare_ftrace_return function can return to the caller of
> _mcount directly without messing up the stack.

Yes, is that required?

> 
> > > +
> > > +	la	t3, ftrace_graph_entry
> > > +	ld	t2, 0(t3)
> > > +	la	t6, ftrace_graph_entry_stub
> > > +	bne	t2, t6, do_ftrace_graph_caller
> > > +#endif
> > > +	la	t3, ftrace_trace_function
> > > +	ld	t5, 0(t3)
> > > +	bne	t5, t4, do_trace
> > > +	ret
> > > +
> > > +#ifdef CONFIG_FUNCTION_GRAPH_TRACER
> > > +/*
> > > + * A pseudo representation for the function graph tracer:
> > > + * prepare_to_return(&ra_to_caller_of_caller, ra_to_caller)
> > > + */
> > > +do_ftrace_graph_caller:
> > > +	addi	a0, s0, -8
> > > +	mv	a1, ra
> > > +#ifdef HAVE_FUNCTION_GRAPH_FP_TEST
> > > +	ld	a2, -16(s0)
> > > +#endif
> > > +	SAVE_ABI_STATE
> > > +	la	t0, prepare_ftrace_return
> > > +	jalr	t0
> > > +	STORE_ABI_STATE  
> > 
> > I'm guessing you don't support function tracer and function graph
> > tracer running at the same time?
> > 
> > -- Steve
> >   
> 
> This code section implements similar logic as those for arm, arm64,
> blackfin, and others.  Also, according to Documentation/trace/ftrace.txt,
> the current_tracer is introduced as singular.
> 
> Is it necessary to support simultaneous tracers?

Well, you can do things like have multiple buffers today (different
tracers recording in different buffers). We can have function tracing
happening at the same time as the graph tracer.

Is this a requirement? No. Just letting you know.

While you only support static ftrace, and not dynamic (code modifying)
ftrace, this isn't yet an issue. I'm just trying to let you know of
some of the current features that are supported in other archs, in case
you extend this.

-- Steve