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

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

On 2023/1/5 17:05, Petr Mladek wrote:
> On Wed 2023-01-04 09:07:02, Song Liu wrote:
>> On Wed, Jan 4, 2023 at 8:25 AM Petr Mladek <pmladek@suse.com> 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.
>>
>> module_mutex is not available outside kernel/module/. The common
>> practice is to disable preempt before calling __module_address().
>> CONFIG_LOCKDEP should catch this.
> 
> I see. Sigh, it is always better to take mutex than disable
> preemption. But it might be acceptable in this case. We just need
> to be careful.
> 
> First, the preemption must stay disabled all the time until
> try_module_get(). Otherwise the returned struct module could
> disappear in the meantime.
> 
> Second, krealloc_array() has to be called with preemption
> enabled. It is perfectly fine to do it after try_module_get().

Okay, thanks for the heads-up.

> 
> Best Regards,
> Petr
> .
> 

-- 
Regards,
  Zhen Lei