netdev - Re: [PATCH v2 bpf-next 1/2] bpftool: introduce "prog profile" command

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Date:   Sun, 1 Mar 2020 22:37:06 +0000
From:   Song Liu <songliubraving@...com>
To:     Yonghong Song <yhs@...com>
CC:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        "Kernel Team" <Kernel-team@...com>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "arnaldo.melo@...il.com" <arnaldo.melo@...il.com>,
        "jolsa@...nel.org" <jolsa@...nel.org>
Subject: Re: [PATCH v2 bpf-next 1/2] bpftool: introduce "prog profile" command



> On Feb 29, 2020, at 7:52 PM, Yonghong Song <yhs@...com> wrote:
> 
> 
> 
> On 2/28/20 3:40 PM, Song Liu wrote:
>> With fentry/fexit programs, it is possible to profile BPF program with
>> hardware counters. Introduce bpftool "prog profile", which measures key
>> metrics of a BPF program.
>> bpftool prog profile command creates per-cpu perf events. Then it attaches
>> fentry/fexit programs to the target BPF program. The fentry program saves
>> perf event value to a map. The fexit program reads the perf event again,
>> and calculates the difference, which is the instructions/cycles used by
>> the target program.
>> Example input and output:
>>   ./bpftool prog profile 3 id 337 cycles instructions llc_misses
>>         4228 run_cnt
>>      3403698 cycles                                              (84.08%)
>>      3525294 instructions   #  1.04 insn per cycle               (84.05%)
>>           13 llc_misses     #  3.69 LLC misses per million isns  (83.50%)
>> This command measures cycles and instructions for BPF program with id
>> 337 for 3 seconds. The program has triggered 4228 times. The rest of the
>> output is similar to perf-stat. In this example, the counters were only
>> counting ~84% of the time because of time multiplexing of perf counters.
>> Note that, this approach measures cycles and instructions in very small
>> increments. So the fentry/fexit programs introduce noticeable errors to
>> the measurement results.
>> The fentry/fexit programs are generated with BPF skeletons. Therefore, we
>> build bpftool twice. The first time _bpftool is built without skeletons.
>> Then, _bpftool is used to generate the skeletons. The second time, bpftool
>> is built with skeletons.
>> Signed-off-by: Song Liu <songliubraving@...com>
>> ---
>>  tools/bpf/bpftool/Makefile                |  18 +
>>  tools/bpf/bpftool/prog.c                  | 428 +++++++++++++++++++++-
>>  tools/bpf/bpftool/skeleton/profiler.bpf.c | 171 +++++++++
>>  tools/bpf/bpftool/skeleton/profiler.h     |  47 +++
>>  tools/scripts/Makefile.include            |   1 +
>>  5 files changed, 664 insertions(+), 1 deletion(-)
>>  create mode 100644 tools/bpf/bpftool/skeleton/profiler.bpf.c
>>  create mode 100644 tools/bpf/bpftool/skeleton/profiler.h
>> diff --git a/tools/bpf/bpftool/Makefile b/tools/bpf/bpftool/Makefile
>> index c4e810335810..c035fc107027 100644
>> --- a/tools/bpf/bpftool/Makefile
>> +++ b/tools/bpf/bpftool/Makefile
>> @@ -59,6 +59,7 @@ LIBS = $(LIBBPF) -lelf -lz
>>    INSTALL ?= install
>>  RM ?= rm -f
>> +CLANG ?= clang
>>    FEATURE_USER = .bpftool
>>  FEATURE_TESTS = libbfd disassembler-four-args reallocarray zlib
>> @@ -110,6 +111,22 @@ SRCS += $(BFD_SRCS)
>>  endif
>>    OBJS = $(patsubst %.c,$(OUTPUT)%.o,$(SRCS)) $(OUTPUT)disasm.o
>> +_OBJS = $(filter-out $(OUTPUT)prog.o,$(OBJS)) $(OUTPUT)_prog.o
>> +
>> +$(OUTPUT)_prog.o: prog.c
>> +	$(QUIET_CC)$(COMPILE.c) -MMD -DBPFTOOL_WITHOUT_SKELETONS -o $@ $<
>> +
>> +$(OUTPUT)_bpftool: $(_OBJS) $(LIBBPF)
>> +	$(QUIET_LINK)$(CC) $(CFLAGS) $(LDFLAGS) -o $@ $(_OBJS) $(LIBS)
>> +
>> +skeleton/profiler.bpf.o: skeleton/profiler.bpf.c
>> +	$(QUIET_CLANG)$(CLANG) -g -O2 -target bpf -c $< -o $@
> 
> With a fresh checkout, applying this patch and just selftests like
>  make -C tools/testing/selftests/bpf
> 
> I got the following build error:
> 
> make[2]: Leaving directory `/data/users/yhs/work/net-next/tools/lib/bpf'
> clang -g -O2 -target bpf -c skeleton/profiler.bpf.c -o skeleton/profiler.bpf.o
> skeleton/profiler.bpf.c:5:10: fatal error: 'bpf/bpf_helpers.h' file not found
> #include <bpf/bpf_helpers.h>
>         ^~~~~~~~~~~~~~~~~~~
> 1 error generated.
> make[1]: *** [skeleton/profiler.bpf.o] Error 1
> 
> I think Makefile should be tweaked to avoid selftest failure.

Hmm... I am not seeing this error. The build succeeded in the test. 

Thanks,
Song