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Message-ID: <dbb556d0-cede-63ac-4afa-2c52baef7960@huawei.com>
Date: Mon, 9 Jan 2023 12:10:58 +0800
From: "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To: Petr Mladek <pmladek@...e.com>
CC: Josh Poimboeuf <jpoimboe@...nel.org>,
Jiri Kosina <jikos@...nel.org>,
Miroslav Benes <mbenes@...e.cz>,
Joe Lawrence <joe.lawrence@...hat.com>,
Alexei Starovoitov <ast@...nel.org>,
Daniel Borkmann <daniel@...earbox.net>,
Andrii Nakryiko <andrii@...nel.org>,
Martin KaFai Lau <martin.lau@...ux.dev>,
Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>,
John Fastabend <john.fastabend@...il.com>,
KP Singh <kpsingh@...nel.org>,
Stanislav Fomichev <sdf@...gle.com>,
Hao Luo <haoluo@...gle.com>, Jiri Olsa <jolsa@...nel.org>,
Steven Rostedt <rostedt@...dmis.org>,
Masami Hiramatsu <mhiramat@...nel.org>,
Mark Rutland <mark.rutland@....com>, <bpf@...r.kernel.org>,
<linux-trace-kernel@...r.kernel.org>,
<live-patching@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
Luis Chamberlain <mcgrof@...nel.org>,
<linux-modules@...r.kernel.org>
Subject: Re: [PATCH 2/3] bpf: Optimize get_modules_for_addrs()
On 2023/1/5 17:32, Petr Mladek wrote:
> On Wed 2023-01-04 17:25:08, 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.
>>>
>>> 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@...wei.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.
>>
>>> + if (!mod)
>>> + continue;
>>>
>>> - if (args->mods_cnt == args->mods_cap) {
>>> - args->mods_cap = max(16, args->mods_cap * 3 / 2);
>>> - mods = krealloc_array(args->mods, args->mods_cap, sizeof(*mods), GFP_KERNEL);
>>> - if (!mods)
>>> - return -ENOMEM;
>>> - args->mods = mods;
>>> - }
>>> + /* check if we already have the module pointer stored */
>>> + for (j = 0; j < mods_cnt; j++) {
>>> + if (mods[j] == mod)
>>> + break;
>>> + }
>>
>> This might get optimized like the original code.
>>
>> My understanding is that the addresses are sorted in "addrs" array.
>> So, the address is either part of the last found module or it belongs
>> to a completely new module.
>
> I thought more about it and I think that I was wrong, see below.
>
>> for (i = 0; i < addrs_cnt; i++) {
>> /*
>> * The adresses are sorted. The adress either belongs
>> * to the last found module or a new one.
>> *
>> * This is safe because we already have reference
>> * on the found modules.
>> */
>> if (mods_cnt && within_module(addrs[i], mods[mods_cnt - 1]))
>> continue;
>
> within_module() checks two sections (init and core). They are
> allocated separately, see module_alloc() called in move_module().
>
> There might be a section from another modules between the init
> and core section of a module.
>
> The optimization worked in the original code because
> module_kallsyms_on_each_symbol() always iterated over all
> symbols from a module.
>
> That said, I am not sure if bpf trace might be added for
> symbols in the module init section. But it might be
> better to stay on the safe side.
Yes.
>
> Best Regards,
> Petr
> .
>
--
Regards,
Zhen Lei
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