Re: [PATCH bpf-next v6] selftests/bpf: trace_helpers.c: optimize kallsyms cache

[Jiri Olsa]

On Mon, Aug 21, 2023 at 02:45:20PM +0800, Rong Tao wrote:
> From: Rong Tao <rongtao@xxxxxxxx>
> 
> 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@xxxxxxxxxx>
> Signed-off-by: Rong Tao <rongtao@xxxxxxxx>
> ---
> v6: Apply libbpf_ensure_mem()
> v5: https://lore.kernel.org/lkml/tencent_0E9E1A1C0981678D5E7EA9E4BDBA8EE2200A@xxxxxx/
>     Release the allocated memory once the load_kallsyms_refresh() upon error
>     given it's dynamically allocated.
> v4: https://lore.kernel.org/lkml/tencent_59C74613113F0C728524B2A82FE5540A5E09@xxxxxx/
>     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@xxxxxx/
>     Do not use structs and judge ksyms__add_symbol function return value.
> v2: https://lore.kernel.org/lkml/tencent_B655EE5E5D463110D70CD2846AB3262EED09@xxxxxx/
>     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@xxxxxx/
> ---
>  samples/bpf/Makefile                        |  1 +
>  tools/testing/selftests/bpf/trace_helpers.c | 59 +++++++++++++++++----
>  2 files changed, 49 insertions(+), 11 deletions(-)
> 
> diff --git a/samples/bpf/Makefile b/samples/bpf/Makefile
> index 595b98d825ce..a49d0f759f5a 100644
> --- a/samples/bpf/Makefile
> +++ b/samples/bpf/Makefile
> @@ -199,6 +199,7 @@ 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
> diff --git a/tools/testing/selftests/bpf/trace_helpers.c b/tools/testing/selftests/bpf/trace_helpers.c
> index f83d9f65c65b..316a7874a12b 100644
> --- a/tools/testing/selftests/bpf/trace_helpers.c
> +++ b/tools/testing/selftests/bpf/trace_helpers.c
> @@ -14,14 +14,48 @@
>  #include <linux/limits.h>
>  #include <libelf.h>
>  #include <gelf.h>
> +#ifndef __must_check
> +#define __must_check
> +#endif

I think you need to fix this on samples/bpf side

I tried to play with the samples/bpf/ includes, but couldn't find a way to
make this work.. selftests base includes on tools/include, while samples
have $(objtree)/usr/include as first include and AFAICS the __must_check is
defined under __KERNEL__ ifdef

I guess the reason samples use $(objtree)/usr/include is to get some struct
definitions which are not in tools/include, but looks like some samples objects
already use vmlinux.h include, so that could be the way forward to fix that

or get rid of the trace_helpers.c or bpf/selftests dependency ;-)

> +#include "bpf/libbpf_internal.h"
>  
>  #define TRACEFS_PIPE	"/sys/kernel/tracing/trace_pipe"
>  #define DEBUGFS_PIPE	"/sys/kernel/debug/tracing/trace_pipe"
>  
> -#define MAX_SYMS 400000
> -static struct ksym syms[MAX_SYMS];
> +static struct ksym *syms;
> +static size_t sym_cap;
>  static int sym_cnt;

nit, sym_cnt should be size_t

jirka

>  
> +static int ksyms__add_symbol(const char *name, unsigned long addr)
> +{
> +	void *tmp;
> +
> +	tmp = strdup(name);
> +	if (!tmp)
> +		return -ENOMEM;
> +	syms[sym_cnt].addr = addr;
> +	syms[sym_cnt].name = tmp;
> +
> +	sym_cnt++;
> +
> +	return 0;
> +}
> +
> +static void ksyms__free(void)
> +{
> +	unsigned int i;
> +
> +	if (!syms)
> +		return;
> +
> +	for (i = 0; i < sym_cnt; i++)
> +		free(syms[i].name);
> +	free(syms);
> +	syms = NULL;
> +	sym_cnt = 0;
> +	sym_cap = 0;
> +}
> +
>  static int ksym_cmp(const void *p1, const void *p2)
>  {
>  	return ((struct ksym *)p1)->addr - ((struct ksym *)p2)->addr;
> @@ -33,9 +67,7 @@ int load_kallsyms_refresh(void)
>  	char func[256], buf[256];
>  	char symbol;
>  	void *addr;
> -	int i = 0;
> -
> -	sym_cnt = 0;
> +	int ret;
>  
>  	f = fopen("/proc/kallsyms", "r");
>  	if (!f)
> @@ -46,17 +78,22 @@ int load_kallsyms_refresh(void)
>  			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 **) &syms, &sym_cap,
> +					sizeof(struct ksym), sym_cnt + 1);
> +		if (ret)
> +			goto error;
> +		ret = ksyms__add_symbol(func, (unsigned long)addr);
> +		if (ret)
> +			goto error;
>  	}
>  	fclose(f);
> -	sym_cnt = i;
>  	qsort(syms, sym_cnt, sizeof(struct ksym), ksym_cmp);
>  	return 0;
> +
> +error:
> +	ksyms__free();
> +	return ret;
>  }
>  
>  int load_kallsyms(void)
> -- 
> 2.39.3
> 
>