Re: [PATCH bpf-next v9] selftests/bpf: trace_helpers.c: optimize kallsyms cache

[Jiri Olsa <olsajiri@gmail.com>]

On Tue, Aug 29, 2023 at 10:14:01PM +0800, Rong Tao wrote:
> From: Rong Tao <rongtao@cestc.cn>
> 
> Static ksyms often have problems because the number of symbols exceeds the
> MAX_SYMS limit. Like changing the MAX_SYMS from 300000 to 400000 in
> commit e76a014334a6("selftests/bpf: Bump and validate MAX_SYMS") solves
> the problem somewhat, but it's not the perfect way.
> 
> This commit uses dynamic memory allocation, which completely solves the
> problem caused by the limitation of the number of kallsyms.
> 
> Acked-by: Stanislav Fomichev <sdf@google.com>
> Signed-off-by: Rong Tao <rongtao@cestc.cn>
> ---
> v9: Add load_kallsyms_local,ksym_search_local,ksym_get_addr_local functions.
> v8: https://lore.kernel.org/lkml/tencent_6D23FE187408D965E95DFAA858BC7E8C760A@qq.com/
>     Resolves inter-thread contention for ksyms global variables.
> v7: https://lore.kernel.org/lkml/tencent_BD6E19C00BF565CD5C36A9A0BD828CFA210A@qq.com/
>     Fix __must_check macro.
> v6: https://lore.kernel.org/lkml/tencent_4A09A36F883A06EA428A593497642AF8AF08@qq.com/
>     Apply libbpf_ensure_mem()
> v5: https://lore.kernel.org/lkml/tencent_0E9E1A1C0981678D5E7EA9E4BDBA8EE2200A@qq.com/
>     Release the allocated memory once the load_kallsyms_refresh() upon error
>     given it's dynamically allocated.
> v4: https://lore.kernel.org/lkml/tencent_59C74613113F0C728524B2A82FE5540A5E09@qq.com/
>     Make sure most cases we don't need the realloc() path to begin with,
>     and check strdup() return value.
> v3: https://lore.kernel.org/lkml/tencent_50B4B2622FE7546A5FF9464310650C008509@qq.com/
>     Do not use structs and judge ksyms__add_symbol function return value.
> v2: https://lore.kernel.org/lkml/tencent_B655EE5E5D463110D70CD2846AB3262EED09@qq.com/
>     Do the usual len/capacity scheme here to amortize the cost of realloc, and
>     don't free symbols.
> v1: https://lore.kernel.org/lkml/tencent_AB461510B10CD484E0B2F62E3754165F2909@qq.com/
> ---
>  samples/bpf/Makefile                          |   4 +
>  .../selftests/bpf/prog_tests/bpf_cookie.c     |   9 +-
>  .../selftests/bpf/prog_tests/fill_link_info.c |   9 +-
>  .../bpf/prog_tests/get_stack_raw_tp.c         |  10 +-
>  .../bpf/prog_tests/kprobe_multi_test.c        |  15 +-
>  .../prog_tests/kprobe_multi_testmod_test.c    |  19 ++-
>  tools/testing/selftests/bpf/trace_helpers.c   | 142 +++++++++++++-----
>  tools/testing/selftests/bpf/trace_helpers.h   |   8 +
>  8 files changed, 159 insertions(+), 57 deletions(-)
> 
> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
> index 4ccf4236031c..6c707ebcebb9 100644
> --- a/samples/bpf/Makefile
> +++ b/samples/bpf/Makefile
> @@ -175,6 +175,7 @@ TPROGS_CFLAGS += -I$(srctree)/tools/testing/selftests/bpf/
>  TPROGS_CFLAGS += -I$(LIBBPF_INCLUDE)
>  TPROGS_CFLAGS += -I$(srctree)/tools/include
>  TPROGS_CFLAGS += -I$(srctree)/tools/perf
> +TPROGS_CFLAGS += -I$(srctree)/tools/lib
>  TPROGS_CFLAGS += -DHAVE_ATTR_TEST=0
>  
>  ifdef SYSROOT
> @@ -314,6 +315,9 @@ XDP_SAMPLE_CFLAGS += -Wall -O2 \
>  
>  $(obj)/$(XDP_SAMPLE): TPROGS_CFLAGS = $(XDP_SAMPLE_CFLAGS)
>  $(obj)/$(XDP_SAMPLE): $(src)/xdp_sample_user.h $(src)/xdp_sample_shared.h
> +# Override includes for trace_helpers.o because __must_check won't be defined
> +# in our include path.
> +$(obj)/$(TRACE_HELPERS): TPROGS_CFLAGS := $(TPROGS_CFLAGS) -D__must_check=
>  
>  -include $(BPF_SAMPLES_PATH)/Makefile.target
>  
> diff --git a/tools/testing/selftests/bpf/prog_tests/bpf_cookie.c b/tools/testing/selftests/bpf/prog_tests/bpf_cookie.c
> index 1454cebc262b..4ed47dc75669 100644
> --- a/tools/testing/selftests/bpf/prog_tests/bpf_cookie.c
> +++ b/tools/testing/selftests/bpf/prog_tests/bpf_cookie.c
> @@ -104,8 +104,10 @@ static void kprobe_multi_link_api_subtest(void)
>  	LIBBPF_OPTS(bpf_link_create_opts, opts);
>  	unsigned long long addrs[8];
>  	__u64 cookies[8];
> +	struct ksyms *ksyms;
>  
> -	if (!ASSERT_OK(load_kallsyms(), "load_kallsyms"))
> +	ksyms = load_kallsyms_local();
> +	if (!ASSERT_OK(ksyms != NULL, "load_kallsyms_local"))
>  		goto cleanup;

hum, this wont work right? I think you should use ASSERT_OK_PTR in here
and ther similar places below

>  
>  	skel = kprobe_multi__open_and_load();
> @@ -116,8 +118,8 @@ static void kprobe_multi_link_api_subtest(void)
>  	skel->bss->test_cookie = true;
>  
>  #define GET_ADDR(__sym, __addr) ({				\
> -	__addr = ksym_get_addr(__sym);				\
> -	if (!ASSERT_NEQ(__addr, 0, "ksym_get_addr " #__sym))	\
> +	__addr = ksym_get_addr_local(ksyms, __sym);		\
> +	if (!ASSERT_NEQ(__addr, 0, "ksym_get_addr_local " #__sym))	\
>  		goto cleanup;					\
>  })
>  
> @@ -171,6 +173,7 @@ static void kprobe_multi_link_api_subtest(void)
>  cleanup:
>  	close(link1_fd);
>  	close(link2_fd);
> +	free_kallsyms_local(ksyms);

I think we don't need to change any test that's calling load_kallsyms,
it should use load_kallsyms and global ksyms data

the load_kallsyms_local would be used in tests that use load_kallsyms_refresh
and need updated ksyms data for bpf_testmod symbols

we just need to make sure that global ksyms initialization won't race, like with:

	+static pthread_mutex_t ksyms_mutex = PTHREAD_MUTEX_INITIALIZER;
	+
	 int load_kallsyms(void)
	 {
	-       /*
	-        * This is called/used from multiplace places,
	-        * load symbols just once.
	-        */
	-       if (sym_cnt)
	-               return 0;
	-       return load_kallsyms_refresh();
	+       pthread_mutex_lock(&ksyms_mutex);
	+       if (!ksyms)
	+               ksyms = load_kallsyms_local();
	+       pthread_mutex_unlock(&ksyms_mutex);
	+       return ksyms ? 0 : 1;
	 }

it could be in separate patch perhaps, because currently there's the same race

>  	kprobe_multi__destroy(skel);
>  }
>  
> diff --git a/tools/testing/selftests/bpf/prog_tests/fill_link_info.c b/tools/testing/selftests/bpf/prog_tests/fill_link_info.c
> index 9d768e083714..154aaa08761f 100644
> --- a/tools/testing/selftests/bpf/prog_tests/fill_link_info.c
> +++ b/tools/testing/selftests/bpf/prog_tests/fill_link_info.c
> @@ -302,16 +302,18 @@ void test_fill_link_info(void)
>  {
>  	struct test_fill_link_info *skel;
>  	int i;
> +	struct ksyms *ksyms;
>  
>  	skel = test_fill_link_info__open_and_load();
>  	if (!ASSERT_OK_PTR(skel, "skel_open"))
>  		return;
>  
>  	/* load kallsyms to compare the addr */
> -	if (!ASSERT_OK(load_kallsyms_refresh(), "load_kallsyms_refresh"))
> +	ksyms = load_kallsyms_refresh_local(NULL);
> +	if (!ASSERT_OK(ksyms != NULL, "load_kallsyms_refresh_local"))

ASSERT_OK_PTR

SNIP

> -int load_kallsyms_refresh(void)
> +struct ksyms *load_kallsyms_refresh_local(struct ksyms *ksyms)
>  {
>  	FILE *f;
>  	char func[256], buf[256];
>  	char symbol;
>  	void *addr;
> -	int i = 0;
> +	int ret;
>  
> -	sym_cnt = 0;
> +	/* flush kallsyms, free the previously allocated dynamic memory */
> +	free_kallsyms_local(ksyms);
>  
>  	f = fopen("/proc/kallsyms", "r");
>  	if (!f)
> -		return -ENOENT;
> +		return NULL;
> +
> +	ksyms = calloc(1, sizeof(struct ksyms));
> +	if (!ksyms)
> +		return NULL;
>  
>  	while (fgets(buf, sizeof(buf), f)) {
>  		if (sscanf(buf, "%p %c %s", &addr, &symbol, func) != 3)
>  			break;
>  		if (!addr)
>  			continue;
> -		if (i >= MAX_SYMS)
> -			return -EFBIG;
>  
> -		syms[i].addr = (long) addr;
> -		syms[i].name = strdup(func);
> -		i++;
> +		ret = libbpf_ensure_mem((void **) &ksyms->syms, &ksyms->sym_cap,
> +					sizeof(struct ksym), ksyms->sym_cnt + 1);
> +		if (ret)
> +			goto error;
> +		ret = ksyms__add_symbol(ksyms, func, (unsigned long)addr);
> +		if (ret)
> +			goto error;
>  	}
>  	fclose(f);
> -	sym_cnt = i;
> -	qsort(syms, sym_cnt, sizeof(struct ksym), ksym_cmp);
> -	return 0;
> +	qsort(ksyms->syms, ksyms->sym_cnt, sizeof(struct ksym), ksym_cmp);
> +	return ksyms;
> +
> +error:
> +	free_kallsyms_local(ksyms);
> +	return NULL;
> +}
> +
> +int load_kallsyms_refresh(void)
> +{
> +	ksyms = load_kallsyms_refresh_local(NULL);
> +	return ksyms ? 0 : 1;
> +}
> +
> +struct ksyms *load_kallsyms_local(void)
> +{
> +	return load_kallsyms_refresh_local(NULL);
>  }

do we need to have load_kallsyms_refresh_local?

we could have ksyms arg passed directly to load_kallsyms_local

  struct ksyms *load_kallsyms_local(struct ksyms *old);

it would return fresh ksyms and release old ksyms if it's passed as an argument

basically I mean just to drop load_kallsyms_local above and rename
load_kallsyms_refresh_local to load_kallsyms_local


jirka