RE: [RFC][PATCH 0/4] ftrace: Use -mfentry when supported (this isfor x86_64 right now)

[Dominique Toupin]

My understanding is stop_machine will stop all processors for many ms.
Even if most of our systems are not hard real-time they are soft real-time and stopping all cores for a few ms is not allowed.
We can stop a few threads while we are jump patching but all processors is too much for us.

I can send other real use cases if you are interested.


> -----Original Message-----
> From: Mathieu Desnoyers [mailto:mathieu.desnoyers@xxxxxxxxxxxx] 
> Sent: 18-Feb-11 10:20
> To: Steven Rostedt; Dominique Toupin
> Cc: Masami Hiramatsu; linux-kernel@xxxxxxxxxxxxxxx; Ingo 
> Molnar; Andrew Morton; Thomas Gleixner; Frederic Weisbecker; 
> H. Peter Anvin; Andi Kleen; 2nddept-manager@xxxxxxxxxxxxxxxxx
> Subject: Re: [RFC][PATCH 0/4] ftrace: Use -mfentry when 
> supported (this is for x86_64 right now)
> 
> [Adding Dominique Toupin, from Ericsson, to CC list]
> 
> * Steven Rostedt (rostedt@xxxxxxxxxxx) wrote:
> > On Fri, 2011-02-18 at 20:45 +0900, Masami Hiramatsu wrote:
> > > (2011/02/18 5:11), Steven Rostedt wrote:
> > > > On Fri, 2011-02-18 at 01:07 +0900, Masami Hiramatsu wrote:
> > > > 
> > > >> I just thought that frequent stop-machine is not so 
> good from the 
> > > >> user's POV. I agree that disabled probe ignoring the 
> call is enough.
> > > >> Maybe, it could be done with the similar mechanism of 
> jump optimization.
> > > > 
> > > > I thought jump optimization still calls stop_machine too?
> > > 
> > > Yes, but now it does batch optimization.
> > > Even if hundreds kprobes are registered separately, jump 
> > > optimization has been done in background with a 
> stop_machine per every 256 probes.
> > > (Until optimizing, kprobes can use breakpoints instead)
> > 
> > But a single optimized kprobe still must use stopmachine.
> > 
> > But it is true that the function tracer does it as one big 
> shot. That 
> > is, it will do all functions in a single stop machine that 
> needs to be 
> > changed. It too is batched, but there is not a limit to that batch.
> > 
> > I would be interested in hearing from users and real use cases that 
> > someone would like to trace functions but stopmachine is 
> too big of a 
> > hammer.
> 
> Hi Steven,
> 
> Telecom end users are one of such cases where the latency 
> induced by stop machine while the system is running is a 
> problem. Dominique Toupin could certainly tell us more about 
> Ericsson's use-cases.
> 
> Thanks,
> 
> Mathieu
> 
> --
> Mathieu Desnoyers
> Operating System Efficiency R&D Consultant EfficiOS Inc.
> http://www.efficios.com
> --
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