linux-kernel - [BUG] ftrace: support for SOFT_DISABLE to syscall events caused huge slowdown of ftrace startup test

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Date:	Tue, 03 Dec 2013 18:16:09 +0100
From:	Petr Mladek <pmladek@...e.cz>
To:	Tom Zanussi <tom.zanussi@...ux.intel.com>,
	Steven Rostedt <rostedt@...dmis.org>
Cc:	Jiri Kosina <jkosina@...e.cz>, linux-kernel@...r.kernel.org
Subject: [BUG] ftrace: support for SOFT_DISABLE to syscall events caused
 huge slowdown of ftrace startup test

Hi,

I tested my ftrace-related patches and enabled also the startup test:

  CONFIG_FTRACE_SELFTEST=y
  CONFIG_FTRACE_STARTUP_TEST=y

On today's kernel/git/tip/tip.git, the boot "freezed" for 1 minute when
doing "Testing event system syscalls".

A bisect pointed finger to the commit d562aff93bfb530b099 (tracing: Add
support for SOFT_DISABLE to syscall events).

It seems that the culprit are the "synchronize_sched" calls added to
"unreg_event_syscall_enter" and "unreg_event_syscall_exit" functions.
They are called 2312 times during the boot here and are responsible for
the delay.

I wonder if it might cause slowdown also at runtime.

I am going to look deeper into the code and eventually come up with a
solution. But it might take some time. I am still newbie in the kernel
development. Any hints or tips are welcome ;-)

Best Regards,
Petr

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