LKML Archive on
help / color / mirror / Atom feed
From: Ingo Molnar <>
To: Frederic Weisbecker <>
Cc: Steven Rostedt <>,
Subject: Re: [RFC][PATCH] Make ftrace able to trace function return
Date: Thu, 30 Oct 2008 19:20:56 +0100	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

* Frederic Weisbecker <> wrote:

> Hi.
> First of all, I want to say this patch is absolutely not ready for 
> inclusion. Some parts are really ugly and the thing is only 
> partially functionning.
> It's just an idea or a kind of proof of concept. I just wanted to 
> make ftrace able to measure the time of execution of a function. For 
> that I had to hook both the function call and its return.
> By using mcount, we can hook the function on enter and we can 
> override its return address. So we can catch the time at those two 
> points. The problem comes when a function run concurrently through 
> preemption or smp. We can measure the return time but how to be sure 
> which time capture we had on call since this time could have been 
> captured multiple times. And for the same reason, how to make sure 
> of the return address.
> So the idea is to allocate a general set of slots on which we can 
> save our original return address and the call time. After that we 
> change the return address of the hooked function to jump on a 
> trampoline which will push the offset for us to retrieve the slot on 
> the set for this function call. Then the trampoline will call a 
> return handler that will trace the return time and send all of these 
> informations to a specific tracer. And then the return handler will 
> return to the original return address.
> To determine quickly which slot is free, I use a bitmap of 32 bits. 
> Perhaps it is a bad assumption but I could enlarge it and there is 
> an overrun counter. This is the only point which needs to be 
> protected against concurrent access.
> I made a tracer for this but the problem is that the capture by 
> ftrace will hang the system if we can use several slots. When I 
> dedicate only one free slot, wherever on the set, there is no 
> problem but I miss a lot of calls. So by default on this patch, 
> there is only one slot dedicated on the bitmap.
> Don't hesitate to comment this patch made of trashes...

hm, are you aware of the -finstrument-functions feature of GCC?

that feature generates such entry points at build time:

                   void __cyg_profile_func_enter (void *this_fn,
                                                  void *call_site);
                   void __cyg_profile_func_exit  (void *this_fn,
                                                  void *call_site);

this might be faster/cleaner than using a trampoline approach IMO.

OTOH, entry+exit profiling has about double the cost of just entry 
profiling - so maybe there should be some runtime flexibility there. 
Plus the same recordmcount trick should be used to patch up these 
entry points to NOP by default.


  reply	other threads:[~2008-10-30 18:21 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2008-10-30 19:20 Frederic Weisbecker
2008-10-30 18:20 ` Ingo Molnar [this message]
2008-10-30 18:30   ` Frédéric Weisbecker
2008-10-30 18:37     ` Ingo Molnar
2008-10-30 19:17       ` Steven Rostedt
2008-10-30 19:26         ` Ingo Molnar
2008-11-03 11:16           ` Ananth N Mavinakayanahalli
2008-11-03 13:32             ` Frédéric Weisbecker
2008-10-31  1:35       ` Frédéric Weisbecker
2008-10-30 20:30 ` Steven Rostedt
2008-10-31  0:05   ` Frédéric Weisbecker
2008-10-31  2:43     ` Steven Rostedt
2008-10-31 20:18       ` Frederic Weisbecker
2008-11-03 10:06         ` Ingo Molnar
2008-11-03 13:11           ` Frédéric Weisbecker

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \ \ \ \ \ \
    --subject='Re: [RFC][PATCH] Make ftrace able to trace function return' \

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).