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

Leizhen (ThunderTown)
2022-09-22T02:15:45Z

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,
>> return off;
>> }
>>
>> +static int kallsyms_name_to_tokens(const char *name, char *buf)
>
> This is not safe API. It is always needed to pass the size of the
> buffer.

OK

>
> Also it should be called "compress". "token" is just an implementation
> detail.
>
> I would do:
>
> static int kallsyms_compress_symbol_name(const char *name,
> char *buf, size_t size)

This's a wonderful name. Thanks.

>
>
>> +{
>> + 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.

>
>> +
>> + /*
>> + * n, number of tokens in the string name.
>> + * token_idx[i], the start index of the ith token.
>> + * token_idx[n] is used to calculate the length of the last token.
>> + */
>> + n = strlen(name);
>> + if (n >= KSYM_NAME_LEN) {
>> + buf[0] = 0;
>> + return 0;
>> + }
>> + for (i = 0; i <= n; i++)
>> + token_idx[i] = (unsigned char)i;
>> +
>> + /*
>> + * For tokens whose token_len >= 2, a larger index value indicates
>> + * a higher occurrence frequency. See scripts/kallsyms.c
>> + */
>> + for (i = 255; i >= 0; i--) {
>> + token = &kallsyms_token_table[kallsyms_token_index[i]];
>> + token_len = strlen(token);
>> + if (token_len <= 1)
>> + continue;
>> +
>> + /*
>> + * Find and merge two tokens into one.
>> + *
>> + * |<-- new_token -->|
>> + * | token1 | token2 |
>> + * token_idx[]: j j+1 j+2
>> + */
>> + for (j = 0; j < n - 1; j++) {
>> + len = token_idx[j + 2] - token_idx[j];
>> + if (len == token_len &&
>> + !strncmp(name + token_idx[j], token, len)) {
>> + token_bak[token_idx[j]] = (unsigned char)i;
>> + for (k = j + 1; k < n; k++)
>> + token_idx[k] = token_idx[k + 1];
>> + n--;
>> + }
>> + }
>> + }
>> +
>> + for (j = 0; j < n; j++) {
>> + len = token_idx[j + 1] - token_idx[j];
>> + if (len <= 1) {
>> + buf[j] = name[token_idx[j]];
>> + continue;
>> + }
>> +
>> + buf[j] = token_bak[token_idx[j]];
>
> Maybe, I do not understand the compression format correctly but
> this code looks too complicated. Honestly, I even did not try to
> understand it.
>
> My understanding is the we just need to find all tokens and
> replace them with index.
>
> It should be even easier than compress_symbols() in scripts/callsyms.c.
> The token_table already exists and we do not need to handle the token_profit...
>
> The following looks more strigtforward (not even compile tested):

OK, I will try this one. Or refer to compress_symbols() in scripts/callsyms.c.

>
> ssize_t len, size;
>
> len = strscpy(buf, symname, size);
> if (WARN_ON_ONCE(len < 0))
> return -EINVAL;
>
> /* the tokens with higher index are used first */
> for (idx = 255; idx >= 0; idx--) {
> token = &kallsyms_token_table[kallsyms_token_index[i]];
> token_len = strlen(token);
>
> p1 = buf;
> /* length of the remaining symname including the trailing '\0' */
> remaining = len + 1;
>
> /* find the token in the symbol name */
> p2 = strstr(token, p1);
>
> while (p2) {
> /* replace token with index */
> *p2 = idx;
> remaining -= ((p2 - p1) + token_len);
> memmove(p2 + 1, p2 + token_len, remaining);
> len -= (token_len - 1);
> p1 = p2;
>
> /* find the token in the rest of the symbol name */
> p2 = strstr(token, p1);
> }
> }
>
> return len;
>
>> + }
>> + buf[n] = 0;
>> +
>> + return n;
>> +}
>> +
>> /*
>> * Get symbol type information. This is encoded as a single char at the
>> * beginning of the symbol name.
>> @@ -192,20 +257,28 @@ unsigned long kallsyms_lookup_name(const char *name)
>> char namebuf[KSYM_NAME_LEN];
>> unsigned long i;
>> unsigned int off;
>> + int len;
>>
>> /* Skip the search for empty string. */
>> if (!*name)
>> return 0;
>>
>> + len = kallsyms_name_to_tokens(name, namebuf);
>> + for (i = 0, off = 0; len && i < kallsyms_num_syms; i++) {
>> + if (kallsyms_names[off] == len + 1 &&
>> + !memcmp(&kallsyms_names[off + 2], namebuf, len))
>> + return kallsyms_sym_address(i);
>> +
>> + off += kallsyms_names[off] + 1;
>
> These complicated checks are hard to review. The following looks much
> more readable to me:

Yes, it looks well.

>
> for (i = 0, off = 0; len && i < kallsyms_num_syms; i++) {
> /* the stored symbol name is prefixed by symbol type */
> name_len = kallsyms_names[off] - 1;
> name = &kallsyms_names[off + 2];
> off += name_len + 2;
>
> if (name_len != len)
> continue;
>
> if (!memcmp(name, namebuf, len))
> return kallsyms_sym_address(i);
> }
>
>
>> + }
>> +
>> 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. To allay your concerns, I can run a test.

Before: min=170, max=15949190, avg=7553906

>
> Best Regards,
> Petr
> .
>

--
Regards,
Zhen Lei