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

[Leizhen (ThunderTown)]

On 2023/1/9 23:11, Leizhen (ThunderTown) wrote:
> 
> 
> On 2023/1/9 21:48, Jiri Olsa wrote:
>> On Mon, Jan 09, 2023 at 04:51:37PM +0800, Leizhen (ThunderTown) wrote:
>>>
>>>
>>> On 2023/1/6 17:45, Jiri Olsa wrote:
>>>> On Thu, Jan 05, 2023 at 10:31:12PM +0100, Jiri Olsa wrote:
>>>>> On Wed, Jan 04, 2023 at 05:25:08PM +0100, 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.
>>>>>
>>>>> hum, for some reason I can see only replies to this patch and
>>>>> not the actual patch.. I'll dig it out of the lore I guess
>>>>>
>>>>>>>
>>>>>>> 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.
>>>>>
>>>>> could you try to benchmark that? I tried something similar but was not
>>>>> able to get better performance
>>>>
>>>> hm looks like I tried the smilar thing (below) like you did,
>>>
>>> Yes. I just found out you're working on this improvement, too.
>>>
>>>> but wasn't able to get better performace
>>>
>>> Your implementation below is already the limit that can be optimized.
>>> If the performance is not improved, it indicates that this place is
>>> not the bottleneck.
>>>
>>>>
>>>> I guess your goal is to get rid of the module arg in
>>>> module_kallsyms_on_each_symbol callback that we use?
>>>
>>> It's not a bad thing to keep argument 'mod' for function
>>> module_kallsyms_on_each_symbol(), but for kallsyms_on_each_symbol(),
>>> it's completely redundant. Now these two functions often use the
>>> same hook function. So I carefully analyzed get_modules_for_addrs(),
>>> which is the only place that involves the use of parameter 'mod'.
>>> Looks like there's a possibility of eliminating parameter 'mod'.
>>>
>>>> I'm ok with the change if the performace is not worse
>>>
>>> OK, thanks.
>>>
>>>>
>>>> jirka
>>>>
>>>>
>>>> ---
>>>> diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
>>>> index 5b9008bc597b..3280c22009f1 100644
>>>> --- a/kernel/trace/bpf_trace.c
>>>> +++ b/kernel/trace/bpf_trace.c
>>>> @@ -2692,23 +2692,16 @@ struct module_addr_args {
>>>>  	int mods_cap;
>>>>  };
>>>>  
>>>> -static int module_callback(void *data, const char *name,
>>>> -			   struct module *mod, unsigned long addr)
>>>> +static int add_module(struct module_addr_args *args, struct module *mod)
>>>>  {
>>>> -	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)
>>>> +	/* We iterate sorted addresses and for each within module we:
>>>> +	 * - check if we already have the module pointer stored for it
>>>> +	 *   (we iterate sorted addresses 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;
>>>> -
>>>>  	if (args->mods && args->mods[args->mods_cnt - 1] == mod)
>>>>  		return 0;
>>>
>>> There'll be problems Petr mentioned.
>>>
>>> https://lkml.org/lkml/2023/1/5/191
>>
>> ok, makes sense.. I guess we could just search args->mods in here?
>> are you going to send new version, or should I update my patch with that?
> 
> It's better for you to update! I'm not familiar with the bpf module.

Hi Jiri:
  Can you attach patch 1/3 when you send the new patch? There's a little
dependency. Petr has already replied OK to patch 1/3, see [1].
  Patch 3/3 is just a cleanup, I'll delay updating it after v6.3-rc1, it
also has a dependency on another patch [2].

[1] https://lkml.org/lkml/2023/1/4/627
[2] https://lkml.org/lkml/2023/1/10/534



> 
>>
>> thanks,
>> jirka
>> .
>>
> 

-- 
Regards,
  Zhen Lei