On Thu, Mar 09, 2017 at 09:14:47AM -0500, Steven Rostedt wrote: > On Wed, 8 Mar 2017 15:29:59 -0600 > Josh Poimboeuf <jpoim...@redhat.com> wrote: > > > [adding Steven Rostedt to CC as an FYI] > > > > On Wed, Mar 08, 2017 at 10:25:01AM -0800, Linus Torvalds wrote: > > > On Wed, Mar 8, 2017 at 9:37 AM, Josh Poimboeuf <jpoim...@redhat.com> > > > wrote: > > > > - CONFIG_FUNCTION_GRAPH_TRACER sets it on x86-32 because of a gcc bug > > > > where the stack gets aligned before the mcount call. This issue > > > > should be mostly obsolete as most modern compilers now have -mfentry. > > > > We could make it dependent on CC_USING_FENTRY. > > > > > > Yeah. At some point we might even upgrade the compiler requirements to > > > no longer accept the mcount model. > > > > The plot slightly thickens... > > > > So I was mistaken about this problem not existing with newer versions of > > gcc, because the x86-32 ftrace code doesn't use -mfentry. It still > > relies on mcount. So CONFIG_FUNCTION_GRAPH_TRACER will still need > > -maccumulate-outgoing-args for *all* versions of gcc on x86-32. > > OK, I admit, I was lazy here. I thought, who cares about x86-32 > anymore ;-)
As we just saw in another thread where somebody ran into this problem with -Os, apparently some people still do care... > > (Of course, that situation would improve if ftrace on x86-32 were ported > > to use -mfentry.) > > That can easily be done. You weren't on CC earlier, so just to summarize the benefits of doing fentry on x86-32, thus removing the need for -maccumulate-outgoing-args: - graph tracer compatibility with -Os - text size decrease of ~3% - possible performance improvement - more uniformity (-maccumulate-outgoing-args disabled everywhere for modern gccs) But either way I'll still work up a patch to make the changes I suggested. -- Josh
