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

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

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.

> 
> thanks,
> jirka
> .
> 

-- 
Regards,
  Zhen Lei