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

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Date:   Tue, 25 Oct 2022 10:53:53 -0700
From:   Luis Chamberlain <mcgrof@...nel.org>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
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 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?

  Luis