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

From: H. Peter Anvin
Date: Mon Aug 05 2013 - 14:29:08 EST

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.


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