Re: [PATCH 5/7] powerpc/ftrace: Update ftrace_location() for powerpc -mprofile-kernel

[Naveen N. Rao]

Steven Rostedt wrote:
> On Tue, 18 Jun 2019 23:53:11 +0530
> "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx> wrote:
>
> > Naveen N. Rao wrote:
> > > Steven Rostedt wrote:  
> > >> On Tue, 18 Jun 2019 20:17:04 +0530
> > >> "Naveen N. Rao" <naveen.n.rao@xxxxxxxxxxxxxxxxxx> wrote:
> > >>   
> > >>> @@ -1551,7 +1551,7 @@ unsigned long ftrace_location_range(unsigned long start, unsigned long end)
> > >>>  	key.flags = end;	/* overload flags, as it is unsigned long */
> > >>>  
> > >>>  	for (pg = ftrace_pages_start; pg; pg = pg->next) {
> > >>> -		if (end < pg->records[0].ip ||
> > >>> +		if (end <= pg->records[0].ip ||  
> > >> 
> > >> This breaks the algorithm. "end" is inclusive. That is, if you look for
> > >> a single byte, where "start" and "end" are the same, and it happens to
> > >> be the first ip on the pg page, it will be skipped, and not found.  
> > > 
> > > Thanks. It looks like I should be over-riding ftrace_location() instead.  
> > > I will update this patch.  
> >
> > I think I will have ftrace own the two instruction range, regardless of 
> > whether the preceding instruction is a 'mflr r0' or not. This simplifies 
> > things and I don't see an issue with it as of now. I will do more 
> > testing to confirm.
> >
> > - Naveen
> >
> >
> > --- a/arch/powerpc/kernel/trace/ftrace.c
> > +++ b/arch/powerpc/kernel/trace/ftrace.c
> > @@ -951,6 +951,16 @@ void arch_ftrace_update_code(int command)
> >  }
> >  
> >  #ifdef CONFIG_MPROFILE_KERNEL
> > +/*
> > + * We consider two instructions -- 'mflr r0', 'bl _mcount' -- to be part
> > + * of ftrace. When checking for the first instruction, we want to include
> > + * the next instruction in the range check.
> > + */
> > +unsigned long ftrace_location(unsigned long ip)
> > +{
> > +	return ftrace_location_range(ip, ip + MCOUNT_INSN_SIZE);
> > +}
> > +
> >  /* Returns 1 if we patched in the mflr */
> >  static int __ftrace_make_call_prep(struct dyn_ftrace *rec)
> >  {
> > diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> > index 21d8e201ee80..122e2bb4a739 100644
> > --- a/kernel/trace/ftrace.c
> > +++ b/kernel/trace/ftrace.c
> > @@ -1573,7 +1573,7 @@ unsigned long ftrace_location_range(unsigned long start, unsigned long end)
> >   * the function tracer. It checks the ftrace internal tables to
> >   * determine if the address belongs or not.
> >   */
> > -unsigned long ftrace_location(unsigned long ip)
> > +unsigned long __weak ftrace_location(unsigned long ip)
> >  {
> >  	return ftrace_location_range(ip, ip);
> >  }
>
> Actually, instead of making this a weak function, let's do this:
>
>
> In include/ftrace.h:
>
> #ifndef FTRACE_IP_EXTENSION
> # define FTRACE_IP_EXTENSION	0
> #endif
>
>
> In arch/powerpc/include/asm/ftrace.h
>
> #define FTRACE_IP_EXTENSION	MCOUNT_INSN_SIZE
>
>
> Then we can just have:
>
> unsigned long ftrace_location(unsigned long ip)
> {
> 	return ftrace_location_range(ip, ip + FTRACE_IP_EXTENSION);
> }

Thanks, that's indeed nice. I hope you don't mind me adding your SOB for 
that.

- Naveen