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

[Jiri Olsa]

On Wed, Jan 11, 2023 at 04:41:21PM +0800, Leizhen (ThunderTown) wrote:
> 
> 
> 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].

ok, will do

jirka