Re: [PATCH v4 4/8] kallsyms: Improve the performance of kallsyms_lookup_name()

Leizhen (ThunderTown)
2022-09-30T11:44:22Z

On 2022/9/28 9:35, Leizhen (ThunderTown) wrote:
>
>
> 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@huawei.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
>>
>>>>> @@ -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.
>
> These days sleep better. When I got up this morning, my subconscious told me that
> compiled LLVM could also be optimized. In fact, the method is simple, that is,
> check whether the next token starts with '.' or '$' after being expanded.
>
> I will post v7 before the holidays.

Sorry, I'm going to break my promise. A lot of code needs to be modified on
the tool side, to make sure that the first '.' or '$' will not be in the middle
or end of the expanded substring. The lab is powered off, so I can only post v7
after the holidays (one week).

>
>>
>> Best Regards,
>> Petr
>> .
>>
>

--
Regards,
Zhen Lei