Re: [PATCH 2/3] bpf: Optimize get_modules_for_addrs()

["Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>]

On 2023/1/5 17:32, Petr Mladek wrote:
> On Wed 2023-01-04 17:25:08, Petr Mladek wrote:
>> On Fri 2022-12-30 19:27:28, Zhen Lei wrote:
>>> Function __module_address() can quickly return the pointer of the module
>>> to which an address belongs. We do not need to traverse the symbols of all
>>> modules to check whether each address in addrs[] is the start address of
>>> the corresponding symbol, because register_fprobe_ips() will do this check
>>> later.
>>>
>>> Assuming that there are m modules, each module has n symbols on average,
>>> and the number of addresses 'addrs_cnt' is abbreviated as K. Then the time
>>> complexity of the original method is O(K * log(K)) + O(m * n * log(K)),
>>> and the time complexity of current method is O(K * (log(m) + M)), M <= m.
>>> (m * n * log(K)) / (K * m) ==> n / log2(K). Even if n is 10 and K is 128,
>>> the ratio is still greater than 1. Therefore, the new method will
>>> generally have better performance.
>>>
>>> Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
>>> ---
>>>  kernel/trace/bpf_trace.c | 101 ++++++++++++++++-----------------------
>>>  1 file changed, 40 insertions(+), 61 deletions(-)
>>>
>>> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
>>> index 5f3be4bc16403a5..0ff9037098bd241 100644
>>> --- a/kernel/trace/bpf_trace.c
>>> +++ b/kernel/trace/bpf_trace.c
>>> @@ -2684,69 +2684,55 @@ static void symbols_swap_r(void *a, void *b, int size, const void *priv)
>>>  	}
>>>  }
>>>  
>>> -struct module_addr_args {
>>> -	unsigned long *addrs;
>>> -	u32 addrs_cnt;
>>> -	struct module **mods;
>>> -	int mods_cnt;
>>> -	int mods_cap;
>>> -};
>>> -
>>> -static int module_callback(void *data, const char *name,
>>> -			   struct module *mod, unsigned long addr)
>>> +static int get_modules_for_addrs(struct module ***out_mods, unsigned long *addrs, u32 addrs_cnt)
>>>  {
>>> -	struct module_addr_args *args = data;
>>> -	struct module **mods;
>>> -
>>> -	/* We iterate all modules symbols and for each we:
>>> -	 * - search for it in provided addresses array
>>> -	 * - if found we check if we already have the module pointer stored
>>> -	 *   (we iterate modules sequentially, so we can check just the last
>>> -	 *   module pointer)
>>> -	 * - take module reference and store it
>>> -	 */
>>> -	if (!bsearch(&addr, args->addrs, args->addrs_cnt, sizeof(addr),
>>> -		       bpf_kprobe_multi_addrs_cmp))
>>> -		return 0;
>>> +	int i, j, err;
>>> +	int mods_cnt = 0;
>>> +	int mods_cap = 0;
>>> +	struct module *mod;
>>> +	struct module **mods = NULL;
>>>  
>>> -	if (args->mods && args->mods[args->mods_cnt - 1] == mod)
>>> -		return 0;
>>> +	for (i = 0; i < addrs_cnt; i++) {
>>> +		mod = __module_address(addrs[i]);
>>
>> This must be called under module_mutex to make sure that the module
>> would not disappear.
>>
>>> +		if (!mod)
>>> +			continue;
>>>  
>>> -	if (args->mods_cnt == args->mods_cap) {
>>> -		args->mods_cap = max(16, args->mods_cap * 3 / 2);
>>> -		mods = krealloc_array(args->mods, args->mods_cap, sizeof(*mods), GFP_KERNEL);
>>> -		if (!mods)
>>> -			return -ENOMEM;
>>> -		args->mods = mods;
>>> -	}
>>> +		/* check if we already have the module pointer stored */
>>> +		for (j = 0; j < mods_cnt; j++) {
>>> +			if (mods[j] == mod)
>>> +				break;
>>> +		}
>>
>> This might get optimized like the original code.
>>
>> My understanding is that the addresses are sorted in "addrs" array.
>> So, the address is either part of the last found module or it belongs
>> to a completely new module.
> 
> I thought more about it and I think that I was wrong, see below.
> 
>> 	for (i = 0; i < addrs_cnt; i++) {
>> 		/*
>> 		 * The adresses are sorted. The adress either belongs
>> 		 * to the last found module or a new one.
>> 		 *
>> 		 * This is safe because we already have reference
>> 		 * on the found modules.
>> 		 */
>> 		 if (mods_cnt && within_module(addrs[i], mods[mods_cnt - 1]))
>> 			continue;
> 
> within_module() checks two sections (init and core). They are
> allocated separately, see module_alloc() called in move_module().
> 
> There might be a section from another modules between the init
> and core section of a module.
> 
> The optimization worked in the original code because
> module_kallsyms_on_each_symbol() always iterated over all
> symbols from a module.
> 
> That said, I am not sure if bpf trace might be added for
> symbols in the module init section. But it might be
> better to stay on the safe side.

Yes.

> 
> Best Regards,
> Petr
> .
> 

-- 
Regards,
  Zhen Lei