linux-kernel - Re: [RFC][PATCH 0/3] ftrace: Add dynamically allocated trampolines

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Date:	Wed, 23 Jul 2014 13:20:20 -0400
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Oleg Nesterov <oleg@...hat.com>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Namhyung Kim <namhyung@...nel.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Josh Poimboeuf <jpoimboe@...hat.com>,
	Jiri Kosina <jkosina@...e.cz>,
	Seth Jennings <sjenning@...hat.com>,
	Jiri Slaby <jslaby@...e.cz>
Subject: Re: [RFC][PATCH 0/3] ftrace: Add dynamically allocated trampolines

On Wed, 23 Jul 2014 19:05:52 +0200
Oleg Nesterov <oleg@...hat.com> wrote:


> > kprobe ftrace_ops are allocated which sets the FTRACE_OPS_FL_DYNAMIC
> > flag. You'll see that flag checked in update_ftrace_function(), and if
> > it is set, it forces the ftrace_ops_list_func() to be used.
> 
> No? __register_ftrace_function() sets if !core_kernel_data(ops), and
> kprobe_ftrace_ops is not dynamic?

Oh, you're right. I thought it was allocated.

What it is missing is the FTRACE_OPS_RECURSION_SAFE flag. Although, I'm
working on a patch that makes a non loop func that does the recursion
checks for just a single ftrace_ops->func if only one is registered.

> 
> > Why?
> >
> > [...snip..]
> 
> Yes, thanks, I understand why, at least to some degree.
> 
> > 	foo()
> > 	  [mcount called --> ftrace_caller trampoline]
> >            ftrace_caller
> >              load ftrace_ops into parameter
> >              <interrupt>
> >              preempt_schedule()
> >        [new task]
> >        kfree(kprobe ftrace_ops);
> 
> see above.
> 
> And to be sure, I compiled your rfc/trampoline kernel which I pulled
> yesterday with the same patch and did the same test. __ftrace_ops_list_func()
> prints nothing.

Note, I'm still working on fixes to that branch ;-)

> 
> So I also added WARN_ON(1) into kprobe_ftrace_handler() to ensure that
> it is actually called, and yes, dmesg reports
> 
> 	WARNING: ... kprobe_ftrace_handler+0x38/0x140()
> 	...
> 	Call Trace:
> 	 [<ffffffff8136a3eb>] dump_stack+0x5b/0xa8
> 	 [<ffffffff810423ec>] warn_slowpath_common+0x8c/0xc0
> 	 [<ffffffff8105772c>] ? SyS_prctl+0x1c/0x730
> 	 [<ffffffff8104243a>] warn_slowpath_null+0x1a/0x20
> 	 [<ffffffff810325c8>] kprobe_ftrace_handler+0x38/0x140
> 	 [<ffffffff8137148a>] ? retint_swapgs+0xe/0x13
> 	 [<ffffffff81057731>] ? SyS_prctl+0x21/0x730
> 	 [<ffffffff81057731>] ? SyS_prctl+0x21/0x730
> 	 [<ffffffff8122424e>] ? trace_hardirqs_on_thunk+0x3a/0x3f
> 	 [<ffffffff81370912>] ? system_call_fastpath+0x16/0x1b
> 

BTW, you may want to look at

/sys/kernel/debug/tracing/enabled_functions

as that has a lot of debug info for the trampolines in that branch.

> after "perl -e 'syscall 157,-1'".
> 
> and, as expected, if I do "echo SyS_prctl >| set_ftrace_filter" and
> "echo function >| current_tracer", then the command above also triggers
> 2 printk's in __ftrace_ops_list_func() :
> 
> 	LIST_FUNC -> function_trace_call()
> 	LIST_FUNC -> kprobe_ftrace_handler()
> 
> so it seems that your patches can potentially buy more than you think ;)
> 
> Oleg.


I'll play with this some more to understand everything you are stating.

Thanks,

-- Steve
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