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

[Luis Chamberlain]

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