linux-kernel - Re: [PATCH 3/3] tracing/function-return-tracer: add the overrun field

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Date:	Tue, 18 Nov 2008 16:02:49 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] tracing/function-return-tracer: add the overrun
	field


* Steven Rostedt <rostedt@...dmis.org> wrote:

> 
> On Tue, 18 Nov 2008, Ingo Molnar wrote:
> > * Steven Rostedt <rostedt@...dmis.org> wrote:
> > > 
> > > I was just looking at the stack tracer, and it pretty much gives us 
> > > the answer ;-) I'm hitting on max traces around 55, but some of 
> > > those are asm calls. We could do 50 or 60?  We probably want to make 
> > > sure that the two do not come close to hitting. That is, the bottom 
> > > of the stack to overwrite the saved return addresses.
> > 
> > does the stack tracer properly nest across IRQ entry boundaries 
> > already on x86? We used to have problems in that area.
> 
> Actually, because the stack tracer is in generic code, we punt on IRQ 
> stacks:
> 
> 	/* we do not handle interrupt stacks yet */
> 	if (!object_is_on_stack(&this_size))
> 		return;
> 
> I check if the local variable "this_size" is on the current->stack 
> and if it is not then this means that we are using some other stack, 
> and we do not record it.
> 
> What would be needed is to make a per-arch stack call. Perhaps have 
> a:
> 
>    arch_check_stack(&this_size, &max_stack_trace, &max_stack_size);
> 
> Where a weak function can be defined to return nothing. But the arch 
> can check which stack the "this_size" variable is on and run the 
> stack tracer against that stack.
> 
> Maybe we should have two stack traces, a stack_trace file and a 
> stack_trace_irq ?
> 
> Because, some archs, like x86_64 have different size stacks. The 
> thread stack is 8K where as the IRQ stack is 4K.  We may want to see 
> which IRQ stack call is the worst, and not compare it to the thread 
> stack call.

... and on 64-bit x86 the IRQ stacks are 16K, and some of the IST 
exception stacks have different sizes as well.

	Ingo
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