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

Chen Zhongjin
2022-08-15T02:08:06Z

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);
>>