linux-kernel - Re: [PATCH v7 00/11] kallsyms: Optimizes the performance of lookup symbols

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Message-ID: <ad9e51c6-f77d-d9e9-9c13-42fcbbde7147@huawei.com>
Date:   Wed, 19 Oct 2022 22:11:58 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     Luis Chamberlain <mcgrof@...nel.org>
CC:     Josh Poimboeuf <jpoimboe@...nel.org>,
        Jiri Kosina <jikos@...nel.org>,
        Miroslav Benes <mbenes@...e.cz>,
        Petr Mladek <pmladek@...e.com>,
        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>,
        <linux-modules@...r.kernel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        "Ingo Molnar" <mingo@...hat.com>
Subject: Re: [PATCH v7 00/11] kallsyms: Optimizes the performance of lookup
 symbols



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.

> 
>   Luis
> .
> 

-- 
Regards,
  Zhen Lei