linux-kernel - Re: [RFC/PATCH] ftrace: Allow to change size of function graph filters

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Message-ID: <20170113111618.53709bf8@gandalf.local.home>
Date:   Fri, 13 Jan 2017 11:16:18 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Namhyung Kim <namhyung@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [RFC/PATCH] ftrace: Allow to change size of function graph
 filters

On Fri, 13 Jan 2017 13:22:43 +0900
Namhyung Kim <namhyung@...nel.org> wrote:

> It's currently fixed to 32 and it ignores when user gives a pattern
> which match to functions more than the size.  So filtering like all
> system calls or many functions with common prefix cannot be set all.
> Not sure this is right though.

Yes it's small, and there's a reason for it. So I'm giving a
conditional nack to the patch.
> 
> This patch adds 'graph_filter_size' file in the tracefs to adjust the
> size.  It can be changed only if the current tracer is not set.
> 
> Signed-off-by: Namhyung Kim <namhyung@...nel.org>

The condition is, we need to fix ftrace_graph_addr() first.

	for (i = 0; i < ftrace_graph_count; i++) {

That gets called at every function being traced. See where I'm heading
with that? ;-)

We need to create a hash table or binary search first and make that
function handle a large ftrace_graph_count before implementing your
patch. Remove the linear search, replace it with either a binary search
or a hash. But an O(n) algorithm at every function call is out of the
question.

Thanks!

-- Steve