Re: [RFC] ftrace: Add support to keep some functions out of ftrace

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Aug 23, 2022 at 10:23:24AM -0700, Alexei Starovoitov wrote:
> On Fri, Aug 19, 2022 at 4:45 AM Jiri Olsa <olsajiri@gmail.com> wrote:
> >
> > On Thu, Aug 18, 2022 at 11:32:55PM +0200, Jiri Olsa wrote:
> > > On Thu, Aug 18, 2022 at 02:00:21PM -0700, Alexei Starovoitov wrote:
> > > > On Thu, Aug 18, 2022 at 1:50 PM Steven Rostedt <rostedt@goodmis.org> wrote:
> > > > >
> > > > > On Thu, 18 Aug 2022 22:27:07 +0200
> > > > > Jiri Olsa <olsajiri@gmail.com> wrote:
> > > > >
> > > > > > ok, so the problem with __attribute__((patchable_function_entry(5))) is that
> > > > > > it puts function address into __patchable_function_entries section, which is
> > > > > > one of ftrace locations source:
> > > > > >
> > > > > >   #define MCOUNT_REC()    . = ALIGN(8);     \
> > > > > >     __start_mcount_loc = .;                 \
> > > > > >     KEEP(*(__mcount_loc))                   \
> > > > > >     KEEP(*(__patchable_function_entries))   \
> > > > > >     __stop_mcount_loc = .;                  \
> > > > > >    ...
> > > > > >
> > > > > >
> > > > > > it looks like __patchable_function_entries is used for other than x86 archs,
> > > > > > so we perhaps we could have x86 specific MCOUNT_REC macro just with
> > > > > > __mcount_loc section?
> > > > >
> > > > > So something like this:
> > > > >
> > > > > #ifdef CONFIG_X86
> > > > > # define NON_MCOUNT_PATCHABLE KEEP(*(__patchable_function_entries))
> > > > > # define MCOUNT_PATCHABLE
> > > > > #else
> > > > > # define NON_MCOUNT_PATCHABLE
> > > > > # define MCOUNT_PATCHABLE  KEEP(*(__patchable_function_entries))
> > > > > #endif
> > > > >
> > > > >   #define MCOUNT_REC()    . = ALIGN(8);     \
> > > > >     __start_mcount_loc = .;                 \
> > > > >     KEEP(*(__mcount_loc))                   \
> > > > >     MCOUNT_PATCHABLE                        \
> > > > >     __stop_mcount_loc = .;                  \
> > > > >     NON_MCOUNT_PATCHABLE                    \
> > > > >    ...
> > > > >
> > > > > ??
> > > >
> > > > That's what more or less Peter's patch is doing:
> > > > Here it is again for reference:
> > > > https://git.kernel.org/pub/scm/linux/kernel/git/peterz/queue.git/commit/?id=8d075bdf11193f1d276bf19fa56b4b8dfe24df9e
> > >
> > > ah nice, and discards the __patchable_function_entries section, great
> > >
> >
> > tested change below with Peter's change above and it seems to work,
> > once it get merged I'll send full patch
> 
> Peter,
> what is the ETA to land your changes?
> That particular commit is detached in your git tree.
> Did you move it to a different branch?
> 
> Just trying to figure out the logistics to land Jiri's fix below.
> We can take it into bpf-next, since it's harmless as-is,
> but it won't have an effect until your change lands.
> Sounds like they both will get in during the next merge window?

I discussed with Peter and I'll send his change together with my fix

jirka