Re: [PATCH 14/14] scripts/sorttable: ftrace: Do not add weak functions to available_filter_functions

Peter Zijlstra
2025-01-02T20:32:16Z

On Thu, Jan 02, 2025 at 03:03:56PM -0500, Steven Rostedt wrote:

> Maybe I misunderstood you, if you are not talking about kallsyms, but for
> static calls or anything else that references weak functions.
>
> The reference is not a problem I'm trying to address. The problem with
> mcount_loc, is that it is used to create the ftrace_table that is exposed
> to user space, and I can't remove entries once they are added.
>
> To set filter functions you echo names into set_ftrace_filter. If you want
> to enabled 5000 filters, that can take over a minute complete. That's
> because echoing in names to set_ftrace_filter is an O(n^2) operation. It
> has to search every address, call kallsyms on the address then compare it
> to every function passed in. If you have 40,000 functions total, and pass
> in 5,000 functions, that's 40,000 * 5,000 compares!

I'm pretty sure kallsyms has an option to use tree lookups, which would
make it ~ 16*5000.

> Since tooling is what does add these large number of filters, a shortcut
> was added. If a number written into set_ftrace_filter, it doesn't do a
> kallsyms lookup, it will enable the nth function in
> available_filter_functions. This turns into a O(1) operation.
>
> libtracefs() will read the available_filter_functions, figure out what to
> enable from that, and then write the indexes of all the functions it wants
> to enable. This is a much faster operation then echoing the names one at a
> time.
>
> This is where the weak functions becomes an issue. If I just ignore them,
> and do not add a place holder in the mcount section. Then the index will be
> off, and will break.
>
> When the issue first came about, I simply ignored the weak functions, but
> then my libtracefs self tests started to fail.
>
> So yes, this is just fixing mcount_loc, but I believe it's the only one
> that has a user interface issue.

This is quite the insane interface -- but whatever. I still feel
strongly you should fix kallsyms so that we can all deal more sanely
with the weak crap.