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

[Chen Zhongjin]

On 2022/8/14 23:22, Jiri Olsa wrote:
> On Sat, Aug 13, 2022 at 03:02:52PM -0400, Steven Rostedt wrote:
> > On Fri, 12 Aug 2022 23:18:15 +0200
> > Jiri Olsa <olsajiri@xxxxxxxxx> wrote:
> >
> > > the patch below moves the bpf function into sepatate object and switches
> > > off the -mrecord-mcount for it.. so the function gets profile call
> > > generated but it's not visible to ftrace
> > >
> > > this seems to work, but it depends on -mrecord-mcount support in gcc and
> > > it's x86 specific... other archs seems to use -fpatchable-function-entry,
> > > which does not seem to have option to omit symbol from being collected
> > > to the section
> > As I stated. the __mcount_loc section was created by ftrace. It has
> > nothing to do with -fpatchable-function-entry. It's just that the archs
> > that use that, do not have a gcc that creates the __mcount_loc section.
> >
> > > disabling specific ftrace symbol with FTRACE_FL_DISABLED flag seems to
> > > be easir and generic solution.. I'll send RFC for that
> > It's not easier.
> >
> > Here, I have a POC for you and some more history.
> >
> > The recordmcount.pl Perl script was the first to create the
> > __mcount_loc section in all objects that ftrace needed to hook to. It
> > did an objdump, found the locations of the calls to mcount, created
> > another file that had a section __mcount_loc that referenced all those
> > locations. Compiled and relinked that back into the original object.
> >
> > This was performed on all object files during the build, and had an
> > impact on build times. But this is when I also created and introduced
> > "make localmodconfig", which shrunk the build times for many people, so
> > nobody noticed the build time increase! :-)
> >
> > Then John Reiser sent me a patch that created recordmcount.c that did
> > the same work but directly modified the ELF object files without having
> > to run scripts. This got rid of that horrible overhead from the scripts.
> >
> > Then, the gcc folks decided to be helpful here as well and created the
> > --mrecord-mcount option that would create the __mcount_loc section for
> > us, where we no longer needed the recordmcount scripts/C program. But
> > is not available across the board.
> >
> > Today, objtool has also got involved, and added an "--mcount" option
> > that will create the section too.
> I overlooked that objtool is involved as well,
> will check on that

objtool --mcount option only involves mcount_loc generation (see 
annotate_call_site) and other validation check call destination directly 
(see is_fentry_call).

Some simply removing --mcount option dose work for this.


Another question, it seems we can export and use DEFINE_BPF_DISPATCHER 
out of kernel, does that means we should add NO_MCOUNT_FILES for these 
single uages as well?

I dont think it can be made automatically. If ignored, this can be a 
trouble.


Best,

Chen

> > Below is a patch that extends yours by adding a NO_MCOUNT_FILES list,
> > that you add the object file to and it will prevent the other methods
> > from adding an mcount_loc location.
> thanks,
> jirka
>
> > I'm adding the objtool folks to make sure this is fine with them.
> > Again, this is a proof of concept, but works. It may need to be cleaned
> > a bit before it is final.
> >
> > -- Steve
> >
> > Index: linux-trace.git/scripts/Makefile.build
> > ===================================================================
> > --- linux-trace.git.orig/scripts/Makefile.build
> > +++ linux-trace.git/scripts/Makefile.build
> > @@ -206,8 +206,10 @@ sub_cmd_record_mcount = perl $(srctree)/
> >   	"$(if $(part-of-module),1,0)" "$(@)";
> >   recordmcount_source := $(srctree)/scripts/recordmcount.pl
> >   endif # BUILD_C_RECORDMCOUNT
> > -cmd_record_mcount = $(if $(findstring $(strip $(CC_FLAGS_FTRACE)),$(_c_flags)),	\
> > +chk_sub_cmd_record_mcount = $(if $(filter $(shell basename $@),$(NO_MCOUNT_FILES)),, \
> >   	$(sub_cmd_record_mcount))
> > +cmd_record_mcount = $(if $(findstring $(strip $(CC_FLAGS_FTRACE)),$(_c_flags)),	\
> > +	$(chk_sub_cmd_record_mcount))
> >   endif # CONFIG_FTRACE_MCOUNT_USE_RECORDMCOUNT
> >   
> >   # 'OBJECT_FILES_NON_STANDARD := y': skip objtool checking for a directory
> > Index: linux-trace.git/scripts/Makefile.lib
> > ===================================================================
> > --- linux-trace.git.orig/scripts/Makefile.lib
> > +++ linux-trace.git/scripts/Makefile.lib
> > @@ -233,7 +233,8 @@ objtool_args =								\
> >   	$(if $(CONFIG_HAVE_JUMP_LABEL_HACK), --hacks=jump_label)	\
> >   	$(if $(CONFIG_HAVE_NOINSTR_HACK), --hacks=noinstr)		\
> >   	$(if $(CONFIG_X86_KERNEL_IBT), --ibt)				\
> > -	$(if $(CONFIG_FTRACE_MCOUNT_USE_OBJTOOL), --mcount)		\
> > +	$(if $(filter $(shell basename $@),$(NO_MCOUNT_FILES)),,	\
> > +		$(if $(CONFIG_FTRACE_MCOUNT_USE_OBJTOOL), --mcount))	\
> >   	$(if $(CONFIG_UNWINDER_ORC), --orc)				\
> >   	$(if $(CONFIG_RETPOLINE), --retpoline)				\
> >   	$(if $(CONFIG_SLS), --sls)					\
> > Index: linux-trace.git/net/core/Makefile
> > ===================================================================
> > --- linux-trace.git.orig/net/core/Makefile
> > +++ linux-trace.git/net/core/Makefile
> > @@ -11,10 +11,15 @@ obj-$(CONFIG_SYSCTL) += sysctl_net_core.
> >   obj-y		     += dev.o dev_addr_lists.o dst.o netevent.o \
> >   			neighbour.o rtnetlink.o utils.o link_watch.o filter.o \
> >   			sock_diag.o dev_ioctl.o tso.o sock_reuseport.o \
> > -			fib_notifier.o xdp.o flow_offload.o gro.o
> > +			fib_notifier.o xdp.o flow_offload.o gro.o \
> > +			dispatcher.o
> >   
> >   obj-$(CONFIG_NETDEV_ADDR_LIST_TEST) += dev_addr_lists_test.o
> >   
> > +# remove dispatcher function from ftrace sight
> > +CFLAGS_REMOVE_dispatcher.o = -mrecord-mcount
> > +NO_MCOUNT_FILES += dispatcher.o
> > +
> >   obj-y += net-sysfs.o
> >   obj-$(CONFIG_PAGE_POOL) += page_pool.o
> >   obj-$(CONFIG_PROC_FS) += net-procfs.o
> > Index: linux-trace.git/net/core/dispatcher.c
> > ===================================================================
> > --- /dev/null
> > +++ linux-trace.git/net/core/dispatcher.c
> > @@ -0,0 +1,3 @@
> > +#include <linux/bpf.h>
> > +
> > +DEFINE_BPF_DISPATCHER(xdp)
> > Index: linux-trace.git/net/core/filter.c
> > ===================================================================
> > --- linux-trace.git.orig/net/core/filter.c
> > +++ linux-trace.git/net/core/filter.c
> > @@ -11162,8 +11162,6 @@ const struct bpf_verifier_ops sk_lookup_
> >   
> >   #endif /* CONFIG_INET */
> >   
> > -DEFINE_BPF_DISPATCHER(xdp)
> > -
> >   void bpf_prog_change_xdp(struct bpf_prog *prev_prog, struct bpf_prog *prog)
> >   {
> >   	bpf_dispatcher_change_prog(BPF_DISPATCHER_PTR(xdp), prev_prog, prog);