linux-kernel - Re: [RFC] gcc feature request: Moving blocks into sections

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Date:	Mon, 05 Aug 2013 11:29:00 -0700
From:	"H. Peter Anvin" <hpa@...ux.intel.com>
To:	Steven Rostedt <rostedt@...dmis.org>
CC:	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>, gcc <gcc@....gnu.org>,
	Ingo Molnar <mingo@...nel.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	David Daney <ddaney.cavm@...il.com>,
	Behan Webster <behanw@...verseincode.com>,
	Peter Zijlstra <peterz@...radead.org>,
	Herbert Xu <herbert@...dor.apana.org.au>
Subject: Re: [RFC] gcc feature request: Moving blocks into sections

On 08/05/2013 11:23 AM, Steven Rostedt wrote:
> On Mon, 2013-08-05 at 11:17 -0700, H. Peter Anvin wrote:
>> On 08/05/2013 10:55 AM, Steven Rostedt wrote:
>>>
>>> Well, as tracepoints are being added quite a bit in Linux, my concern is
>>> with the inlined functions that they bring. With jump labels they are
>>> disabled in a very unlikely way (the static_key_false() is a nop to skip
>>> the code, and is dynamically enabled to a jump).
>>>
>>
>> Have you considered using traps for tracepoints?  A trapping instruction
>> can be as small as a single byte.  The downside, of course, is that it
>> is extremely suppressed -- the trap is always expensive -- and you then
>> have to do a lookup to find the target based on the originating IP.
> 
> No, never considered it, nor would I. Those that use tracepoints, do use
> them extensively, and adding traps like this would probably cause
> heissenbugs and make tracepoints useless.
> 
> Not to mention, how would we add a tracepoint to a trap handler?
> 

Traps nest, that's why there is a stack.  (OK, so you don't want to take
the same trap inside the trap handler, but that code should be very
limited.)  The trap instruction just becomes very short, but rather
slow, call-return.

However, when you consider the cost you have to consider that the
tracepoint is doing other work, so it may very well amortize out.

	-hpa


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