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

[Leizhen (ThunderTown)]

On 2023/1/5 0:25, 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@xxxxxxxxxx>
>> ---
>>  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'm in a hurry to get to the airport now. I will reply next week.
move_module() shows that a module has three layouts, and the memory
area is discontinuous. I originally wanted to implement what you
suggested below. I'll analyze it in depth next week. Maybe it'll work.


> 
> 	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;
> 
> 		mutex_lock(&module_mutex);
> 		mod = __module_address(addrs[i]);
> 		if (mod && !try_module_get(mod)) {
> 			mutex_unlock(&module_mutex);
> 			goto failed;
> 		}
> 		mutex_unlock(&module_mutex);
> 
> 		/*
> 		 * Nope when the address was not from a module.
> 		 *
> 		 * Is this correct? What if the module has gone in
> 		 * the meantime? Anyway, the original code
> 		 * worked this way.
> 		 *
> 		 * FIXME: I would personally make sure that it is part
> 		 * of vmlinux or so.
> 		 */
> 		if (!mod)
> 			continue;
> 
> 		/* store the module into mods array */
> 		...
> 
> 
> 
> 
>> +		if (j < mods_cnt)
>> +			continue;
>>  
>> -	if (!try_module_get(mod))
>> -		return -EINVAL;
>> +		if (mods_cnt == mods_cap) {
>> +			struct module **new_mods;
>>  
>> -	args->mods[args->mods_cnt] = mod;
>> -	args->mods_cnt++;
>> -	return 0;
>> -}
>> +			mods_cap = max(16, mods_cap * 3 / 2);
>> +			new_mods = krealloc_array(mods, mods_cap, sizeof(*mods), GFP_KERNEL);
>> +			if (!new_mods) {
>> +				err = -ENOMEM;
>> +				goto failed;
>> +			}
>> +			mods = new_mods;
>> +		}
>>  
>> -static int get_modules_for_addrs(struct module ***mods, unsigned long *addrs, u32 addrs_cnt)
>> -{
>> -	struct module_addr_args args = {
>> -		.addrs     = addrs,
>> -		.addrs_cnt = addrs_cnt,
>> -	};
>> -	int err;
>> +		if (!try_module_get(mod)) {
>> +			err = -EINVAL;
>> +			goto failed;
>> +		}
>>  
>> -	/* We return either err < 0 in case of error, ... */
>> -	err = module_kallsyms_on_each_symbol(NULL, module_callback, &args);
>> -	if (err) {
>> -		kprobe_multi_put_modules(args.mods, args.mods_cnt);
>> -		kfree(args.mods);
>> -		return err;
>> +		mods[mods_cnt] = mod;
>> +		mods_cnt++;
>>  	}
>>  
>> -	/* or number of modules found if everything is ok. */
>> -	*mods = args.mods;
>> -	return args.mods_cnt;
>> +	*out_mods = mods;
>> +	return mods_cnt;
>> +
>> +failed:
>> +	kprobe_multi_put_modules(mods, mods_cnt);
>> +	kfree(mods);
>> +	return err;
>>  }
>>  
>>  int bpf_kprobe_multi_link_attach(const union bpf_attr *attr, struct bpf_prog *prog)
> 
> Otherwise, it looks good. IMHO, the new code looks more straightforward
> than the original one.
> 
> Best Regards,
> Petr
> .
> 

-- 
Regards,
  Zhen Lei