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

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Date:   Thu, 27 Oct 2022 11:26:07 +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 2022/10/27 3:03, Luis Chamberlain wrote:
> On Wed, Oct 26, 2022 at 02:44:36PM +0800, Leizhen (ThunderTown) wrote:
>> On 2022/10/26 1:53, Luis Chamberlain wrote:
>>> 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.
> 
> For instance, you can look at evaluating if alloc_large_system_hash() would help.

OK, I found the right hash function. In this way, the tool does not need to consider
the byte order.

include/linux/stringhash.h

/*
 * Version 1: one byte at a time.  Example of use:
 *
 * unsigned long hash = init_name_hash;
 * while (*p)
 *      hash = partial_name_hash(tolower(*p++), hash);
 * hash = end_name_hash(hash);


> 
>   Luis
> .
> 

-- 
Regards,
  Zhen Lei