Re: [PATCH v3 0/2] s390 vs. kprobes on ftrace

[Vojtech Pavlik <vojtech@suse.com>]

Hello Heiko,

I can confirm that kGraft works well on top of current mainline with
this patch added.

Another reason for a performance impact when kGraft is enabled is that
kGraft still adds two instructions to the syscall path on s390x, as
there is no space left for a kgraft TIF in the first eight bits of
thread info flags. Renumbering the thread info flags such that _TIF_WORK
occupies the first eight bits and TIF_TRACE the next eight would fix
that problem: Do you believe it is feasible?

Vojtech

On Tue, Oct 21, 2014 at 10:30:27AM +0200, Heiko Carstens wrote:
> v3:
> Changed patch 1/2 to incorporate feedback from Steven Rostedt and
> Masami Hiramatsu: rename helper function check_ftrace_location()
> to arch_check_ftrace_location() and convert it to a weak function,
> so architectures can override it without the need for new config
> option.
> 
> v2:
> Changed patch 1/2 to incorporate feedback from Masami Hiramatsu, and
> introduce a new helper function check_ftrace_location().
> The requested ftracetest has been sent as an own patch set, since it
> has no dependency to these patches.
> 
> v1:
> We would like to implement an architecture specific variant of "kprobes
> on ftrace" without using the current HAVE_KPROBES_ON_FTRACE infrastructure
> which is currently only used by x86.
> 
> The rationale for these two patches is:
> - we want to patch the first instruction of the mcount code block to
>   reduce the overhead of the function tracer
> - we'd like to keep the ftrace_caller function as simple as possible and
>   not require it to generate a 100% valid pt_regs structure as required
>   by the combination of DYNAMIC_FTRACE_WITH_REGS and HAVE_KPROBES_ON_FTRACE.
>   This allows us to not generate the psw mask field in the pt_regs
>   structure on each function tracer enabled function, which otherwise would
>   be very expensive. Besides that program check generated pt_regs contents
>   are "more" accurate than program generated ones and don't require any
>   maintenance.
>   And also we can keep the ftrace and kprobes backends quite separated.
> 
> In order to make this work a small common code change is necessary which
> removes a check if kprobe is being placed on an ftrace location (see
> first patch).
> 
> If possible, I'd like to have an ACK from at least one of the kprobes
> maintainers for the first patch and bring it upstream via the s390 tree.
> 
> Thanks,
> Heiko
> 
> Heiko Carstens (2):
>   kprobes: introduce weak arch_check_ftrace_location() helper function
>   s390/ftrace,kprobes: allow to patch first instruction
> 
>  arch/s390/include/asm/ftrace.h  |  52 ++++++++++++++--
>  arch/s390/include/asm/kprobes.h |   1 +
>  arch/s390/include/asm/lowcore.h |   4 +-
>  arch/s390/include/asm/pgtable.h |  12 ++++
>  arch/s390/kernel/asm-offsets.c  |   1 -
>  arch/s390/kernel/early.c        |   4 --
>  arch/s390/kernel/ftrace.c       | 132 +++++++++++++++++++++++++---------------
>  arch/s390/kernel/kprobes.c      |  92 ++++++++++++++++++++--------
>  arch/s390/kernel/mcount.S       |   1 +
>  arch/s390/kernel/setup.c        |   2 -
>  arch/s390/kernel/smp.c          |   1 -
>  include/linux/kprobes.h         |   1 +
>  kernel/kprobes.c                |  18 +++---
>  scripts/recordmcount.c          |   2 +-
>  scripts/recordmcount.pl         |   2 +-
>  15 files changed, 226 insertions(+), 99 deletions(-)
> 
> -- 
> 1.8.5.5