Messages in this thread | | | Date | Mon, 15 Aug 2022 10:07:54 +0800 | Subject | Re: [RFC] ftrace: Add support to keep some functions out of ftrace | From | 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@gmail.com> 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); >>
| |