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

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Message-ID: <20081118151326.GH30358@elte.hu>
Date:	Tue, 18 Nov 2008 16:13:26 +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:
> > 
> > > 
> > > On Tue, 18 Nov 2008, Ingo Molnar wrote:
> > > > > 
> > > > > Ok I will try with 50. If there are still a lot and often missing 
> > > > > traces with this depth, perhaps should we consider a hybrid solution 
> > > > > between ret stack and trampolines? We could use the normal ret stack 
> > > > > on struct info for most common cases and the trampoline when we are 
> > > > > exceeding the depth....
> > > > 
> > > > dunno, trampolines make me feel uneasy.
> > > > 
> > > > Could you set it to some really large value (200) and add a "max 
> > > > depth seen" variable perhaps, and see the maximum depth?
> > > 
> > > Don't run that on a box you care about ;-) But hopefully the stacks 
> > > will not collide. This should also depend on IRQSTACKS.
> > 
> > that reminds me: ti->ret_stack[] should be moved to task->ret_stack[]. 
> > That way we decouple its size from any kernel stack size limits. 
> > (thread-info resides at one end of the kernel stack, on x86)
> 
> Yeah, I recommended that to Frederic to save space. But that can be 
> dangerous. Using task instead would be safer with the downside of 
> making the task struct even bigger.

We almost never put new stuff into thread_info - we have the lockdep 
lock stack in the task structure too, for similar reasons.

	Ingo
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