[RFC PATCH 6/9] powerpc/ftrace: Update and move function profile instructions out-of-line

[Steven Rostedt]

On Thu, 21 Dec 2023 10:46:08 +0000
Christophe Leroy <christophe.leroy at csgroup.eu> wrote:

> > To enable ftrace, the nop at function entry is changed to an
> > unconditional branch to 'tramp'. The call to ftrace_caller() may be
> > updated to ftrace_regs_caller() depending on the registered ftrace ops.
> > On 64-bit powerpc, we additionally change the instruction at 'tramp' to
> > 'mflr r0' from an unconditional branch back to func+4. This is so that
> > functions entered through the GEP can skip the function profile sequence
> > unless ftrace is enabled.
> > 
> > With the context_switch microbenchmark on a P9 machine, there is a
> > performance improvement of ~6% with this patch applied, going from 650k
> > context switches to 690k context switches without ftrace enabled. With
> > ftrace enabled, the performance was similar at 86k context switches.  
> 
> Wondering how significant that context_switch micorbenchmark is.
> 
> I ran it on both mpc885 and mpc8321 and I'm a bit puzzled by some of the 
> results:
> # ./context_switch --no-fp
> Using threads with yield on cpus 0/0 touching FP:no altivec:no vector:no 
> vdso:no
> 
> On 885, I get the following results before and after your patch.
> 
> CONFIG_FTRACE not selected : 44,9k
> CONFIG_FTRACE selected, before : 32,8k
> CONFIG_FTRACE selected, after : 33,6k
> 
> All this is with CONFIG_INIT_STACK_ALL_ZERO which is the default. But 
> when I select CONFIG_INIT_STACK_NONE, the CONFIG_FTRACE not selected 
> result is only 34,4.
> 
> On 8321:
> 
> CONFIG_FTRACE not selected : 100,3k
> CONFIG_FTRACE selected, before : 72,5k
> CONFIG_FTRACE selected, after : 116k
> 
> So the results look odd to me.


BTW, CONFIG_FTRACE just enables the tracing system (I would like to change
that to CONFIG_TRACING, but not sure if I can without breaking .configs all
over the place).

The nops for ftrace is enabled with CONFIG_FUNCTION_TRACER.

-- Steve