Re: Warning from ring buffer code (Was: Re: linux-next: tip tree build warning)

Re: Warning from ring buffer code (Was: Re: linux-next: tip tree build warning)

[Steven Rostedt]

On Sat, 2009-09-12 at 16:42 +0530, Jaswinder Singh Rajput wrote:

> 
> Here are some more trace related warnings in current linus (as well as
> -tip) tree :
> 
>   CHECK   arch/x86/kernel/ptrace.c
> include/trace/events/syscalls.h:18:1: warning: symbol 'ftrace_raw_output_sys_enter' was not declared. Should it be static?
> include/trace/events/syscalls.h:42:1: warning: symbol 'ftrace_raw_output_sys_exit' was not declared. Should it be static?
> include/trace/events/syscalls.h:18:1: warning: symbol 'ftrace_define_fields_sys_enter' was not declared. Should it be static?
> include/trace/events/syscalls.h:42:1: warning: symbol 'ftrace_define_fields_sys_exit' was not declared. Should it be static?

I just wrote a patch to fix the above.

> include/trace/events/syscalls.h:18:1: error: bad constant expression
> include/trace/events/syscalls.h:42:1: error: bad constant expression

Not sure why sparse is failing on this. Looking at the sched.c code, I
ran "make kernel/sched.i" and then removed the CPP expressions and then
expanded the macros and here's where it is failing:

static void ftrace_profile_sched_kthread_stop(struct task_struct *t)
{
	struct ftrace_data_offsets_sched_kthread_stop __attribute__((unused)) __data_offsets;
	struct ftrace_event_call *event_call = &event_sched_kthread_stop;
	extern void perf_tpcounter_event(int, u64, u64, void *, int);
	struct ftrace_raw_sched_kthread_stop *entry;
	u64 __addr = 0, __count = 1;
	unsigned long irq_flags;
	
	int __entry_size;
	int __data_size;
	int pc;

	do { ({ unsigned long __dummy;
				typeof(irq_flags) __dummy2;
				(void)(&__dummy == &__dummy2);
				1;
			});
		do { (irq_flags) = __raw_local_save_flags();
		} while (0);
	} while (0);
	pc = (current_thread_info()->preempt_count);
	__data_size = ftrace_get_offsets_sched_kthread_stop(&__data_offsets, t);
	__entry_size = (((__data_size + sizeof(*entry) + sizeof(u32))+((typeof(__data_size + sizeof(*entry) + sizeof(u32)))(sizeof(u64))-1))&~((typeof(__data_size + sizeof(*entry) + sizeof(u32)))(sizeof(u64))-1));
	__entry_size -= sizeof(u32);
	do {
		char raw_data[__entry_size];   <<<<----------- FAILURE HERE
		struct trace_entry *ent;
		*(u64 *)(&raw_data[__entry_size - sizeof(u64)]) = 0ULL;
		entry = (struct ftrace_raw_sched_kthread_stop *)raw_data;
		ent = &entry->ent;
		tracing_generic_entry_update(ent, irq_flags, pc);
		ent->type = event_call->id;
		{ memcpy(entry->comm, t->comm, 16);
			entry->pid = t->pid;
			;
		} perf_tpcounter_event(event_call->id, __addr, __count, entry, __entry_size);
	} while (0);
};

Sure enough, sparse does not like the __entry_size. I replaced it with
"10" and sparse was happy with it. That is a perfectly legal entry, so
this looks more like a bug with sparse.

I just tested this too:

static void func(int size_me) {
	char array[size_me];

	memcpy(array, "hello", size);
};

and sparse failed on it as well. Note, you need to have something call
func, or sparse will ignore it.


-- Steve

Re: Warning from ring buffer code (Was: Re: linux-next: tip tree build warning)

[Christopher Li]

On Mon, Sep 14, 2009 at 8:16 AM, Steven Rostedt <srostedt@redhat.com> wrote:
> static void func(int size_me) {
>        char array[size_me];
>
>        memcpy(array, "hello", size);
> };
>
> and sparse failed on it as well. Note, you need to have something call
> func, or sparse will ignore it.

Gcc allows variable size. Sparse expects the size of an array is constant.
For the kernel using variable array size is consider bad. Because the kernel
has very limited stack size. (8K if I remember correctly). Using dynamic array
is very easy to overflow the stack without realizing it.

It deserves a warning. I agree the warning message can use a better description
though.

Chris

Re: Warning from ring buffer code (Was: Re: linux-next: tip tree build warning)

[Steven Rostedt]

On Mon, 2009-09-14 at 10:09 -0700, Christopher Li wrote:
> On Mon, Sep 14, 2009 at 8:16 AM, Steven Rostedt <srostedt@redhat.com> wrote:
> > static void func(int size_me) {
> >        char array[size_me];
> >
> >        memcpy(array, "hello", size);
> > };
> >
> > and sparse failed on it as well. Note, you need to have something call
> > func, or sparse will ignore it.
> 
> Gcc allows variable size. Sparse expects the size of an array is constant.
> For the kernel using variable array size is consider bad. Because the kernel
> has very limited stack size. (8K if I remember correctly). Using dynamic array
> is very easy to overflow the stack without realizing it.
> 
> It deserves a warning. I agree the warning message can use a better description
> though.

Good point!

I've added Frederic to the Cc list, since he wrote the code.

Frederic, how big can one of those events get. The ring buffer (and
TRACE_EVENT) allow up to almost a page size, which is very hefty for the
stack. This code needs to either be rewritten or we need to set a limit
to the size of a profile entry.

We could add:

	if (__entry_size > 256)
		return;

Thoughts?

-- Steve

Re: Warning from ring buffer code (Was: Re: linux-next: tip tree build warning)

[Peter Zijlstra]

On Mon, 2009-09-14 at 14:17 -0400, Steven Rostedt wrote:
> Frederic, how big can one of those events get. The ring buffer (and
> TRACE_EVENT) allow up to almost a page size, which is very hefty for the
> stack. This code needs to either be rewritten or we need to set a limit
> to the size of a profile entry.

Yeah, that needs to get a re-write.. I've complained about this when it
went in.

Re: Warning from ring buffer code (Was: Re: linux-next: tip tree build warning)

[Steven Rostedt]

On Mon, 2009-09-14 at 20:23 +0200, Peter Zijlstra wrote:
> On Mon, 2009-09-14 at 14:17 -0400, Steven Rostedt wrote:
> > Frederic, how big can one of those events get. The ring buffer (and
> > TRACE_EVENT) allow up to almost a page size, which is very hefty for the
> > stack. This code needs to either be rewritten or we need to set a limit
> > to the size of a profile entry.
> 
> Yeah, that needs to get a re-write.. I've complained about this when it
> went in.

One answer is to create a per cpu buffer that is big enough to hold the
data needed. Then you can disable interrupts an use it without worry.

If you need to also handle NMIs, then create a per_cpu NMI buffer too,
and use that if "in_nmi()" is true.

-- Steve

Re: Warning from ring buffer code (Was: Re: linux-next: tip tree build warning)

[Frederic Weisbecker]

On Mon, Sep 14, 2009 at 02:31:16PM -0400, Steven Rostedt wrote:
> On Mon, 2009-09-14 at 20:23 +0200, Peter Zijlstra wrote:
> > On Mon, 2009-09-14 at 14:17 -0400, Steven Rostedt wrote:
> > > Frederic, how big can one of those events get. The ring buffer (and
> > > TRACE_EVENT) allow up to almost a page size, which is very hefty for the
> > > stack. This code needs to either be rewritten or we need to set a limit
> > > to the size of a profile entry.
> > 
> > Yeah, that needs to get a re-write.. I've complained about this when it
> > went in.
> 
> One answer is to create a per cpu buffer that is big enough to hold the
> data needed. Then you can disable interrupts an use it without worry.
> 
> If you need to also handle NMIs, then create a per_cpu NMI buffer too,
> and use that if "in_nmi()" is true.
> 
> -- Steve


Looks like a nice idea.

Peter, does that sound acceptable to you to disable interrupts during a
profiled tracepoint event?

Re: Warning from ring buffer code (Was: Re: linux-next: tip tree build warning)

[Peter Zijlstra]

On Mon, 2009-09-14 at 20:41 +0200, Frederic Weisbecker wrote:
> On Mon, Sep 14, 2009 at 02:31:16PM -0400, Steven Rostedt wrote:
> > On Mon, 2009-09-14 at 20:23 +0200, Peter Zijlstra wrote:
> > > On Mon, 2009-09-14 at 14:17 -0400, Steven Rostedt wrote:
> > > > Frederic, how big can one of those events get. The ring buffer (and
> > > > TRACE_EVENT) allow up to almost a page size, which is very hefty for the
> > > > stack. This code needs to either be rewritten or we need to set a limit
> > > > to the size of a profile entry.
> > > 
> > > Yeah, that needs to get a re-write.. I've complained about this when it
> > > went in.
> > 
> > One answer is to create a per cpu buffer that is big enough to hold the
> > data needed. Then you can disable interrupts an use it without worry.
> > 
> > If you need to also handle NMIs, then create a per_cpu NMI buffer too,
> > and use that if "in_nmi()" is true.
> > 
> > -- Steve
> 
> 
> Looks like a nice idea.
> 
> Peter, does that sound acceptable to you to disable interrupts during a
> profiled tracepoint event?

It does anyway, a little further down the line, so sure.

Re: Warning from ring buffer code (Was: Re: linux-next: tip tree build warning)

[Frédéric Weisbecker]

2009/9/15, Peter Zijlstra <peterz@infradead.org>:
> On Mon, 2009-09-14 at 20:41 +0200, Frederic Weisbecker wrote:
>> On Mon, Sep 14, 2009 at 02:31:16PM -0400, Steven Rostedt wrote:
>> > On Mon, 2009-09-14 at 20:23 +0200, Peter Zijlstra wrote:
>> > > On Mon, 2009-09-14 at 14:17 -0400, Steven Rostedt wrote:
>> > > > Frederic, how big can one of those events get. The ring buffer (and
>> > > > TRACE_EVENT) allow up to almost a page size, which is very hefty for
>> > > > the
>> > > > stack. This code needs to either be rewritten or we need to set a
>> > > > limit
>> > > > to the size of a profile entry.
>> > >
>> > > Yeah, that needs to get a re-write.. I've complained about this when
>> > > it
>> > > went in.
>> >
>> > One answer is to create a per cpu buffer that is big enough to hold the
>> > data needed. Then you can disable interrupts an use it without worry.
>> >
>> > If you need to also handle NMIs, then create a per_cpu NMI buffer too,
>> > and use that if "in_nmi()" is true.
>> >
>> > -- Steve
>>
>>
>> Looks like a nice idea.
>>
>> Peter, does that sound acceptable to you to disable interrupts during a
>> profiled tracepoint event?
>
> It does anyway, a little further down the line, so sure.


Ok I'll fix it soon using Steve's idea then.

Thanks.

Re: Warning from ring buffer code (Was: Re: linux-next: tip tree build warning)

[Frederic Weisbecker]

On Mon, Sep 14, 2009 at 02:17:18PM -0400, Steven Rostedt wrote:
> On Mon, 2009-09-14 at 10:09 -0700, Christopher Li wrote:
> > On Mon, Sep 14, 2009 at 8:16 AM, Steven Rostedt <srostedt@redhat.com> wrote:
> > > static void func(int size_me) {
> > >        char array[size_me];
> > >
> > >        memcpy(array, "hello", size);
> > > };
> > >
> > > and sparse failed on it as well. Note, you need to have something call
> > > func, or sparse will ignore it.
> > 
> > Gcc allows variable size. Sparse expects the size of an array is constant.
> > For the kernel using variable array size is consider bad. Because the kernel
> > has very limited stack size. (8K if I remember correctly). Using dynamic array
> > is very easy to overflow the stack without realizing it.
> > 
> > It deserves a warning. I agree the warning message can use a better description
> > though.
> 
> Good point!
> 
> I've added Frederic to the Cc list, since he wrote the code.
> 
> Frederic, how big can one of those events get. The ring buffer (and
> TRACE_EVENT) allow up to almost a page size, which is very hefty for the
> stack. This code needs to either be rewritten or we need to set a limit
> to the size of a profile entry.
> 
> We could add:
> 
> 	if (__entry_size > 256)
> 		return;
> 
> Thoughts?
> 


Well it can be big, especially once we play with array fields or
__string().

I can manage the __string() that said, by only copying their
pointer and later delay the copy.

Well actually I would like to rewrite all that entirely to avoid
any stack allocation, especially for arrays and string.

Lemme think about a CPP magic way to directly interact with perf
buffer. I think it's possible.