linux-kernel - Re: [PATCH v4 4/8] kallsyms: Improve the performance of kallsyms_lookup

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Message-ID: <ba8979fb-df77-c387-3c7b-cf5c46050c43@huawei.com>
Date:   Thu, 22 Sep 2022 15:21:57 +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>,
        <live-patching@...r.kernel.org>, <linux-kernel@...r.kernel.org>,
        "Masahiro Yamada" <masahiroy@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Luis Chamberlain <mcgrof@...nel.org>,
        <linux-modules@...r.kernel.org>
Subject: Re: [PATCH v4 4/8] kallsyms: Improve the performance of
 kallsyms_lookup_name()



On 2022/9/22 15:02, Petr Mladek wrote:
> On Thu 2022-09-22 10:15:22, Leizhen (ThunderTown) wrote:
>>
>>
>> On 2022/9/21 23:25, Petr Mladek wrote:
>>> On Tue 2022-09-20 15:13:13, Zhen Lei wrote:
>>>> Currently, to search for a symbol, we need to expand the symbols in
>>>> 'kallsyms_names' one by one, and then use the expanded string for
>>>> comparison. This process can be optimized.
>>>>
>>>> And now scripts/kallsyms no longer compresses the symbol types, each
>>>> symbol type always occupies one byte. So we can first compress the
>>>> searched symbol and then make a quick comparison based on the compressed
>>>> length and content. In this way, for entries with mismatched lengths,
>>>> there is no need to expand and compare strings. And for those matching
>>>> lengths, there's no need to expand the symbol. This saves a lot of time.
>>>> According to my test results, the average performance of
>>>> kallsyms_lookup_name() can be improved by 20 to 30 times.
>>>>
>>>> The pseudo code of the test case is as follows:
>>>> static int stat_find_name(...)
>>>> {
>>>> 	start = sched_clock();
>>>> 	(void)kallsyms_lookup_name(name);
>>>> 	end = sched_clock();
>>>> 	//Update min, max, cnt, sum
>>>> }
>>>>
>>>> /*
>>>>  * Traverse all symbols in sequence and collect statistics on the time
>>>>  * taken by kallsyms_lookup_name() to lookup each symbol.
>>>>  */
>>>> kallsyms_on_each_symbol(stat_find_name, NULL);
>>>>
>>>> The test results are as follows (twice):
>>>> After : min=5250, max=  726560, avg= 302132
>>>> After : min=5320, max=  726850, avg= 301978
>>>> Before: min=170,  max=15949190, avg=7553906
>>>> Before: min=160,  max=15877280, avg=7517784
>>>>
>>>> The average time consumed is only 4.01% and the maximum time consumed is
>>>> only 4.57% of the time consumed before optimization.
>>>>
>>>> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
>>>> ---
>>>>  kernel/kallsyms.c | 79 +++++++++++++++++++++++++++++++++++++++++++++--
>>>>  1 file changed, 76 insertions(+), 3 deletions(-)
>>>>
>>>> diff --git a/kernel/kallsyms.c b/kernel/kallsyms.c
>>>> index 3e7e2c2ad2f75ef..2d76196cfe89f34 100644
>>>> --- a/kernel/kallsyms.c
>>>> +++ b/kernel/kallsyms.c
>>>> @@ -87,6 +87,71 @@ static unsigned int kallsyms_expand_symbol(unsigned int off,
>>>> +{
>>>> +	int i, j, k, n;
>>>> +	int len, token_len;
>>>> +	const char *token;
>>>> +	unsigned char token_idx[KSYM_NAME_LEN];
>>>> +	unsigned char token_bak[KSYM_NAME_LEN];
>>>
>>> Why do we need two buffers? It should be possible to compress the name
>>> in the same buffer as it is done in compress_symbols() in scripts/callsyms.c.
>>
>> Because the performance would be a little better. Now this function takes
>> just over a microsecond. Currently, it takes about 250 microseconds on
>> average to lookup a symbol, so adding a little more time to this function
>> doesn't affect the overall picture. I'll modify and test it as you suggest
>> below.
> 
> We need to be careful about a stack overflow. I have seen that
> KSYM_NAME_LEN might need to be increased to 512 because of
> Rust support, see
> https://lore.kernel.org/r/20220805154231.31257-6-ojeda@kernel.org

OK. Thanks for your information. I decided to add kallsyms_best_token_table[],
kallsyms_best_token_table_len, so that we only need one namebuf[], like
kallsyms_expand_symbol().

> 
>>>> @@ -192,20 +257,28 @@ unsigned long kallsyms_lookup_name(const char *name)
>>>>  	for (i = 0, off = 0; i < kallsyms_num_syms; i++) {
>>>>  		off = kallsyms_expand_symbol(off, namebuf, ARRAY_SIZE(namebuf));
>>>>  
>>>> -		if (strcmp(namebuf, name) == 0)
>>>> -			return kallsyms_sym_address(i);
>>>> -
>>>>  		if (cleanup_symbol_name(namebuf) && strcmp(namebuf, name) == 0)
>>>>  			return kallsyms_sym_address(i);
>>>
>>> Hmm, it means that the speedup is not usable when kernel is compiled LLVM?
>>> It might actually slow down the search because we would need to use
>>> both fast and slow search?
>>
>> Theoretically, I don't think so. A string comparison was removed from the
>> slow search. "if (name_len != len)" is faster than
>> "if (strcmp(namebuf, name) == 0)". Even if they're equal,
>> kallsyms_compress_symbol_name() only takes 1-2us, it doesn't affect the
>> overall picture. The average lookup time before optimization is
>> millisecond-level.
>>
>> Before: min=170,  max=15949190, avg=7553906
> 
> Good point! I agree that the potential extra overhead is negligible
> when using the old code as a fallback.
> 
> Best Regards,
> Petr
> .
> 

-- 
Regards,
  Zhen Lei