Re: [PATCH bpf-next v2] selftests/bpf: trace_helpers.c: optimize kallsyms cache

[Stanislav Fomichev <sdf@google.com>]

On 08/11, 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.
> 
> Signed-off-by: Rong Tao <rongtao@cestc.cn>
> ---
> v2: 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/
> ---
>  tools/testing/selftests/bpf/trace_helpers.c | 73 ++++++++++++++-------
>  1 file changed, 48 insertions(+), 25 deletions(-)
> 
> diff --git a/tools/testing/selftests/bpf/trace_helpers.c b/tools/testing/selftests/bpf/trace_helpers.c
> index f83d9f65c65b..cda5a2328450 100644
> --- a/tools/testing/selftests/bpf/trace_helpers.c
> +++ b/tools/testing/selftests/bpf/trace_helpers.c
> @@ -18,9 +18,37 @@
>  #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 int sym_cnt;
> +static struct {
> +	struct ksym *syms;
> +	unsigned int sym_cap;
> +	unsigned int sym_cnt;
> +} ksyms = {
> +	.syms = NULL,
> +	.sym_cap = 1024,
> +	.sym_cnt = 0,
> +};

Not sure what the struct buys you here (besides grouping everything
nicely), maybe do the following?
static struct ksym *syms;
static int sym_cnt;
static int sym_cap = 1024;

Will reduce the churn elsewhere..

> +static int ksyms__add_symbol(const char *name, unsigned long addr)
> +{
> +	void *tmp;
> +	unsigned int new_cap;
> +
> +	if (ksyms.sym_cnt + 1 > ksyms.sym_cap) {
> +		new_cap = ksyms.sym_cap * 4 / 3;
> +		tmp = realloc(ksyms.syms, sizeof(struct ksym) * new_cap);
> +		if (!tmp)
> +			return -ENOMEM;
> +		ksyms.syms = tmp;
> +		ksyms.sym_cap = new_cap;
> +	}
> +
> +	ksyms.syms[ksyms.sym_cnt].addr = addr;
> +	ksyms.syms[ksyms.sym_cnt].name = strdup(name);
> +
> +	ksyms.sym_cnt++;
> +
> +	return 0;
> +}
>  
>  static int ksym_cmp(const void *p1, const void *p2)
>  {
> @@ -33,9 +61,10 @@ int load_kallsyms_refresh(void)
>  	char func[256], buf[256];
>  	char symbol;
>  	void *addr;
> -	int i = 0;
>  
> -	sym_cnt = 0;
> +	ksyms.syms = malloc(sizeof(struct ksym) * ksyms.sym_cap);
> +	if (!ksyms.syms)
> +		return -ENOMEM;
>  
>  	f = fopen("/proc/kallsyms", "r");
>  	if (!f)
> @@ -46,16 +75,10 @@ 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++;
> +		ksyms__add_symbol(func, (unsigned long)addr);

Need to check the return of ksyms__add_symbol here?