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

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

On 2023/1/5 1:07, 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.

Yes, mod needs to be protected, thanks.

> 
> module_mutex is not available outside kernel/module/. The common
> practice is to disable preempt before calling __module_address().

Yes, I've looked elsewhere, and all calling preempt_disable() for
RCU read protection. I will fix it.

> CONFIG_LOCKDEP should catch this.
> 
> Thanks,
> Song
> 
> [...]
> .
> 

-- 
Regards,
  Zhen Lei