Re: [Xen-devel] ftrace_enabled set to 1 on bootup, slow downs with CONFIG_FUNCTION_TRACER in virt environments?

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Tue, Feb 14, 2012 at 01:22:02PM -0500, Steven Rostedt wrote:
> On Tue, 2012-02-14 at 10:29 -0500, Konrad Rzeszutek Wilk wrote: 
> > Hey,
> > 
> > I was running some benchmarks (netserver/netperf) where the init script 
> > just launched
> > the netserver and nothing else and was concerned to see the performance not 
> > up to par.
> > This was an HVM guest running with PV drivers.
> > 
> > If I compile the kernel without CONFIG_FUNCTION_TRACER it is much better
> 
> There is a known performance degrade of 1 or 2% with function tracing
> enabled, on some work loads. Anything more that needs to be
> investigated.
> 
> Did you also keep FRAME_POINTERS enabled? FUNCTION_TRACER selects frame
> pointers which can also slow down the system.

Not yet. Doing the compile now.
> 
> > - but it was 
> > my understanding that the tracing code does not impact the machine unless 
> > it is enabled.
> > And when I inserted a bunch of print_dump_bytes I do see instructions such 
> > as
> > e8 6a 90 60 e1 get replaced with 66 66 66 90 so I see the the instructions 
> > getting
> > patched over.
> 
> Right on boot up (and module load) the calls do get changed to nops. Now
> note that there's some calls that do not get changed at boot up, but the
> most recent scripts/recordmcount.c should change them to nops at compile
> time.
> > 
> > To get a better feel for this I tried this on baremetal, and (this is going
> > to sound a bit round-about way, but please bear with me), I was working on 
> > making
> > the pte_flags be paravirt (so it is a function instead of being a macro) 
> > and noticed
> > that on on an AMD A8-3850, with a CONFIG_PARAVIRT and 
> > CONFIG_FUNCTION_TRACER and
> > running kernelbench it would run slower than without CONFIG_FUNCTION_TRACER.
> 
> Have you tried what the difference is between !CONFIG_PARAVIRT and with
> and without CONFIG_FUNCTION_TRACER?

Hadn't tried that, but let do that.

> 
> > 
> > I am not really sure what the problem is, but based on those experiments
> > four things come to my mind:
> >  - Lots of nops and we choke the CPU instruction decoder with 20-30 bytes
> >    of 'nop', so the CPU is stalling waiting for some real instructions.
> 
> But the nop is only placed at the beginning of functions.

Right, and I was thinking that with paravirt enabled that some of the operations
end up having nops as well. So you kind of get:
66 66 66 90 66 66 66 90 or more

Thought let me double check which instructions I was thinking of that
get patched over to NOPs when running with pvops under baremetal.

> 
> > - The compiler has choosen to compile most of the paravirt instructions as
> >    functions making the call to mcount (which gets patched over), but the
> >    end result is that we have an extra 'call' in the chain.
> 
> You mean that we get a lot more functions because the compiler made them
> functions? Maybe we should add "notrace" to all paravirt functions? Then
> they wont have the calls or nops.

<nods> Do you remember the rational of why some have notrace but not all?

> 
> > - Somehow the low-level para-virt (like the assembler ones) calls don't get
> >    patched over and still end up calling mcount? (but I really doubt that 
> > is the
> >    case - but you never know).
> 
> We only live patch code in a white list of sections. But with the latest
> scripts/recordmcount.c, as I stated above, the ones that don't get
> patched at boot up, should be patched at compile time. But that still
> keeps the nops there.

So the ideal_nop in the looks to be different from what the trace code
decides to patch during execution. Is that OK? I am not that familiar with the
variants of nops to know if some are just not ok on certain architectures?

> 
> > - Something else?
> > 
> > My thought was to crash the kernel as it is up and running and look at the
> > diassembled core to see what the instructions end up looking to get a 
> > further feel
> > for this.  But before I go with this are there some other ideas of what I 
> > should look
> > for?
> 
> You can just look at the objdump of vmlinux, as the recordmcount.c would
> have already patched the code that is not whitelisted, and you can also
> see if things are function calls.

OK. Let me start doing that.

Thank for your email with lots of hints/pointers to what to try out!

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