linux-kernel - Re: [PATCH v1] ftrace: Make ftrace_graph_is

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Message-ID: <eb94fd9c-39f4-cdfb-c04d-abe9ad26a15e@csgroup.eu>
Date:   Fri, 25 Mar 2022 07:06:45 +0000
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     Ingo Molnar <mingo@...hat.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>
Subject: Re: [PATCH v1] ftrace: Make ftrace_graph_is_dead() static inline



Le 24/03/2022 à 23:33, Steven Rostedt a écrit :
> On Thu, 24 Mar 2022 11:01:45 +0100
> Christophe Leroy <christophe.leroy@...roup.eu> wrote:
> 
>> @@ -1006,7 +1006,20 @@ unsigned long ftrace_graph_ret_addr(struct task_struct *task, int *idx,
>>   extern int register_ftrace_graph(struct fgraph_ops *ops);
>>   extern void unregister_ftrace_graph(struct fgraph_ops *ops);
>>   
>> -extern bool ftrace_graph_is_dead(void);
>> +/**
>> + * ftrace_graph_is_dead - returns true if ftrace_graph_stop() was called
>> + *
>> + * ftrace_graph_stop() is called when a severe error is detected in
>> + * the function graph tracing. This function is called by the critical
>> + * paths of function graph to keep those paths from doing any more harm.
>> + */
>> +extern bool kill_ftrace_graph;
>> +
>> +static inline bool ftrace_graph_is_dead(void)
>> +{
>> +	return kill_ftrace_graph;
>> +}
>> +
>>   extern void ftrace_graph_stop(void);
> 
> The reason I did not expose that variable, is because I didn't want it to
> be touched outside of the kernel/trace directory. Or the ftrace.c file for
> that matter (although, I could put it in fgraph.c :-/)
> 
> What would be better, is to make it a static branch.
> 
> extern struct static_key fgraph_dead;
> 
> static inline bool ftrace_graph_is_dead(void)
> {
> 	if (static_key_false(&fgraph_dead))
> 		return true;
> 	return false;
> }
> 
> That way we even get rid of the conditional branch.
> 
> Yeah, the fgraph_dead is still exposed for anyone to touch, but it still
> requires a function to modify it, so I'm not as worried it will be touched
> as easily.
> 

If your concern is about kill_ftrace_graph being visible in ftrace.h, we 
could have hidden it with:

static inline bool ftrace_graph_is_dead(void)
{
	extern bool kill_ftrace_graph;

	return kill_ftrace_graph;
}


But using a static branch is even better, I'll do that.

By the way, I was wondering if that call is still necessary.

In 2014, in commit 545d47b8f359 ("ftrace-graph: Remove usage of 
ftrace_stop() in ftrace_graph_stop()") you wrote: "All archs now use 
ftrace_graph_is_dead() to stop function graph tracing".

But when I grep it nowadays, it seems not all architecture are using it:

$ git grep "prepare_ftrace_return(" arch/*ftrace.c

arch/arm/kernel/ftrace.c:void prepare_ftrace_return(unsigned long 
*parent, unsigned long self_addr,
arch/arm64/kernel/ftrace.c:void prepare_ftrace_return(unsigned long 
self_addr, unsigned long *parent,
arch/csky/kernel/ftrace.c:void prepare_ftrace_return(unsigned long 
*parent, unsigned long self_addr,
arch/microblaze/kernel/ftrace.c:void prepare_ftrace_return(unsigned long 
*parent, unsigned long self_addr)
arch/mips/kernel/ftrace.c:void prepare_ftrace_return(unsigned long 
*parent_ra_addr, unsigned long self_ra,
arch/nds32/kernel/ftrace.c:void prepare_ftrace_return(unsigned long 
*parent, unsigned long self_addr,
arch/nds32/kernel/ftrace.c:	prepare_ftrace_return(parent_ip, selfpc, 
frame_pointer);
arch/parisc/kernel/ftrace.c:static void __hot 
prepare_ftrace_return(unsigned long *parent,
arch/parisc/kernel/ftrace.c:		prepare_ftrace_return(parent_rp, self_addr);
arch/powerpc/kernel/trace/ftrace.c:unsigned long 
prepare_ftrace_return(unsigned long parent, unsigned long ip,
arch/powerpc/kernel/trace/ftrace.c:	fregs->regs.link = 
prepare_ftrace_return(parent_ip, ip, fregs->regs.gpr[1]);
arch/riscv/kernel/ftrace.c:void prepare_ftrace_return(unsigned long 
*parent, unsigned long self_addr,
arch/s390/kernel/ftrace.c:unsigned long prepare_ftrace_return(unsigned 
long ra, unsigned long sp,
arch/sh/kernel/ftrace.c:void prepare_ftrace_return(unsigned long 
*parent, unsigned long self_addr)
arch/sparc/kernel/ftrace.c:unsigned long prepare_ftrace_return(unsigned 
long parent,
arch/x86/kernel/ftrace.c:void prepare_ftrace_return(unsigned long ip, 
unsigned long *parent,
arch/x86/kernel/ftrace.c:void prepare_ftrace_return(unsigned long ip, 
unsigned long *parent,
arch/x86/kernel/ftrace.c:	prepare_ftrace_return(ip, (unsigned long 
*)stack, 0);

$ git grep "ftrace_graph_is_dead()" arch/*ftrace.c

arch/microblaze/kernel/ftrace.c:	if (unlikely(ftrace_graph_is_dead()))
arch/mips/kernel/ftrace.c:	if (unlikely(ftrace_graph_is_dead()))
arch/parisc/kernel/ftrace.c:	if (unlikely(ftrace_graph_is_dead()))
arch/powerpc/kernel/trace/ftrace.c:	if (unlikely(ftrace_graph_is_dead()))
arch/s390/kernel/ftrace.c:	if (unlikely(ftrace_graph_is_dead()))
arch/sh/kernel/ftrace.c:	if (unlikely(ftrace_graph_is_dead()))
arch/x86/kernel/ftrace.c:	if (unlikely(ftrace_graph_is_dead()))

But back in commit 545d47b8f359, there was already architectures 
implementing prepare_ftrace_return() without calling 
ftrace_graph_is_dead(), for instance s390. So I must be missing 
something I guess ?

Thanks
Christophe