linux-kernel - Re: [PATCH 14/14] scripts/sorttable: ftrace: Do not add weak functions to available_filter

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Message-ID: <20250102203200.GE7274@noisy.programming.kicks-ass.net>
Date: Thu, 2 Jan 2025 21:32:00 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: linux-kernel@...r.kernel.org, linux-trace-kernel@...r.kernel.org,
	linux-kbuild@...r.kernel.org, bpf <bpf@...r.kernel.org>,
	Masami Hiramatsu <mhiramat@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Masahiro Yamada <masahiroy@...nel.org>,
	Nathan Chancellor <nathan@...nel.org>,
	Nicolas Schier <nicolas@...sle.eu>,
	Zheng Yejian <zhengyejian1@...wei.com>,
	Martin Kelly <martin.kelly@...wdstrike.com>,
	Christophe Leroy <christophe.leroy@...roup.eu>,
	Josh Poimboeuf <jpoimboe@...hat.com>
Subject: Re: [PATCH 14/14] scripts/sorttable: ftrace: Do not add weak
 functions to available_filter_functions

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.