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

Petr Mladek
2022-09-22T07:02:18Z

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.

Best Regards,
Petr