Re: [RFC PATCH] ftrace: speed up module enable by binary searching module symbols

[Steven Rostedt <rostedt@goodmis.org>]

On Wed, 16 Sep 2026 16:27:15 +0800
Yaxiong Tian <tianyaxiong@kylinos.cn> wrote:

Hi Yaxiong,

I've been traveling since September 14th to October 10th and my backlog is
very long. Unfortunately, I will not be able to look at this patch until
after the merge window. But it is not forgotten, just postponed.

-- Steve



> Loading a module with many functions takes seconds, almost all of it in
> ftrace_module_enable().  amdgpu modules stall for about ten seconds, and
> disabling CONFIG_DYNAMIC_FTRACE makes the stall disappear.
> 
> Every record is validated by test_for_valid_rec(), which resolves its ip
> through kallsyms_lookup(); for a module address that ends up in
> find_kallsyms_symbol(), which walks the whole module symbol table from
> index 1.  The core kernel instead binary searches its sorted kallsyms
> table in get_symbol_pos().  So only modules pay, and the cost is
> quadratic: both the record count and the symbol count grow with the
> module.  An amdgpu.ko from this tree has 17282 records and 55332 symbol
> table entries.
> 
> Fix this by collecting the addresses of a module's symbols once, sorting
> them, and binary searching that array instead of walking the table once
> per record; a loaded module's symbol table cannot change, so the array
> stays valid for the whole pass.  What comes back is what the linear scan
> returned: the same entries are skipped - SHN_UNDEF, unnamed and mapping
> symbols - and the closest address not below the base of the record's
> memory region is taken.  Whenever the array cannot answer, because it
> could not be built or because no such symbol exists, the record is handed
> to test_for_valid_rec(), so the rest of kallsyms_lookup() still has its
> say and no verdict can change.
> 
> On the machine that reported the stall, modprobe of that amdgpu.ko takes
> 12.242 s before and 2.870 s after, and the time spent inside ftrace_lock
> drops from 9.368 s to 1.585 ms - 15.195 ms if the price of building the
> array is included, that build being 13.6 ms, 0.15% of the 9.37 s saved.
> The array costs O(symbols * log symbols), it does not grow with the
> record count.  Across the 74 modules loaded at boot, total lock time
> drops from 491 ms to 2.33 ms.
> 
> Signed-off-by: Yaxiong Tian <tianyaxiong@kylinos.cn>