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Subject: Re: [PATCH v7 00/11] kallsyms: Optimizes the performance of lookup
 symbols
To:     Luis Chamberlain <mcgrof@kernel.org>
CC:     Josh Poimboeuf <jpoimboe@kernel.org>,
        Jiri Kosina <jikos@kernel.org>,
        Miroslav Benes <mbenes@suse.cz>,
        Petr Mladek <pmladek@suse.com>,
        Joe Lawrence <joe.lawrence@redhat.com>,
        <live-patching@vger.kernel.org>, <linux-kernel@vger.kernel.org>,
        Masahiro Yamada <masahiroy@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Jiri Olsa <jolsa@kernel.org>,
        Kees Cook <keescook@chromium.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        <linux-modules@vger.kernel.org>,
        Steven Rostedt <rostedt@goodmis.org>,
        "Ingo Molnar" <mingo@redhat.com>
References: <20221017064950.2038-1-thunder.leizhen@huawei.com>
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 <Y1gisUFzgt1D1Jle@bombadil.infradead.org>
From:   "Leizhen (ThunderTown)" <thunder.leizhen@huawei.com>
Message-ID: <77f1c8f0-5e67-0e57-9285-15ba613044fb@huawei.com>
Date:   Wed, 26 Oct 2022 14:44:36 +0800
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On 2022/10/26 1:53, Luis Chamberlain wrote:
> On Wed, Oct 19, 2022 at 10:11:58PM +0800, Leizhen (ThunderTown) wrote:
>>
>>
>> On 2022/10/19 20:01, Luis Chamberlain wrote:
>>> On Mon, Oct 17, 2022 at 02:49:39PM +0800, 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 is very slow.
>>>>
>>>> In fact, we can first compress the name being looked up and then use
>>>> it for comparison when traversing 'kallsyms_names'.
>>>>
>>>> This patch series optimizes the performance of function kallsyms_lookup_name(),
>>>> and function klp_find_object_symbol() in the livepatch module. Based on the
>>>> test results, the performance overhead is reduced to 5%. That is, the
>>>> performance of these functions is improved by 20 times.
>>>
>>> Stupid question, is a hash table in order?
>>
>> No hash table.
>>
>> All symbols are arranged in ascending order of address. For example: cat /proc/kallsyms
>>
>> The addresses of all symbols are stored in kallsyms_addresses[], and names of all symbols
>> are stored in kallsyms_names[]. The elements in these two arrays are in a one-to-one
>> relationship. For any symbol, it has the same index in both arrays.
>>
>> Therefore, when we look up a symbolic name based on an address, we use a binary lookup.
>> However, when we look up an address based on a symbol name, we can only traverse array
>> kallsyms_names[] in sequence. I think the reason why hash is not used is to save memory.
> 
> This answers how we don't use a hash table, the question was *should* we
> use one?

I'm not the original author, and I can only answer now based on my understanding. Maybe
the original author didn't think of the hash method, or he has weighed it out.

Hash is a good solution if only performance is required and memory overhead is not
considered. Using hash will increase the memory size by up to "4 * kallsyms_num_syms +
4 * ARRAY_SIZE(hashtable)" bytes, kallsyms_num_syms is about 1-2 million.

Because I don't know what hash algorithm will be used, the cost of generating the
hash value corresponding to the symbol name is unknown now. But I think it's gonna
be small. But it definitely needs a simpler algorithm, the tool needs to implement
the same hash algorithm.

If the hash is not very uniform or ARRAY_SIZE(hashtable) is small, then my current
approach still makes sense. So maybe hash can be deferred to the next phase of
improvement.

> 
>   Luis
> .
> 

-- 
Regards,
  Zhen Lei