linux-kernel - Re: [PATCH 4/4] s390/ftrace: add -mfentry and -mnop-mcount support

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Date:   Wed, 8 Aug 2018 13:10:13 +0200
From:   Vasily Gorbik <gor@...ux.ibm.com>
To:     Heiko Carstens <heiko.carstens@...ibm.com>
Cc:     Masahiro Yamada <yamada.masahiro@...ionext.com>,
        Michal Marek <michal.lkml@...kovi.net>,
        Martin Schwidefsky <schwidefsky@...ibm.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ingo Molnar <mingo@...hat.com>, linux-kbuild@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-s390@...r.kernel.org,
        Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH 4/4] s390/ftrace: add -mfentry and -mnop-mcount support

On Tue, Aug 07, 2018 at 07:30:17AM +0200, Heiko Carstens wrote:
> On Mon, Aug 06, 2018 at 03:17:47PM +0200, Vasily Gorbik wrote:
> > Utilize -mfentry and -mnop-mcount gcc options together with
> > -mrecord-mcount to get compiler generated calls to the profiling functions
> > as nops which are compatible with current -mhotpatch=0,3 approach.  At the
> > same time -mrecord-mcount enables __mcount_loc section generation by
> > the compiler which allows to avoid using scripts/recordmcount.pl script.
> > 
> > Signed-off-by: Vasily Gorbik <gor@...ux.ibm.com>
> > ---
> >  arch/s390/Kconfig              |  2 ++
> >  arch/s390/Makefile             | 16 +++++++++-------
> >  arch/s390/include/asm/ftrace.h |  6 +++---
> >  arch/s390/kernel/ftrace.c      |  2 +-
> >  arch/s390/kernel/mcount.S      |  2 +-
> >  5 files changed, 16 insertions(+), 12 deletions(-)
> 
> Do you have numbers which tell how much this reduces the compile time of
> the kernel on s390? I assume this change makes quite some difference.
> 

Very roughly, best time across several runs. With -j24 on 24 core lpar
with -mfentry -mrecord-mcount etc
real    0m54.748s
user    12m32.041s
sys     1m17.778s

vs -mhotpatch=0,3 + scripts/recordmcount.pl:
real    0m56.752s
user    12m52.386s
sys     1m35.873s