Re: [PATCH 0/3] OPTPROBES for powerpc
From: Masami Hiramatsu
Date: Thu Sep 08 2016 - 12:11:57 EST
On Wed, 7 Sep 2016 15:03:09 +0530
Anju T Sudhakar <anju@xxxxxxxxxxxxxxxxxx> wrote:
> This is the patchset of the kprobes jump optimization
> (a.k.a OPTPROBES)for powerpc. Kprobe being an inevitable tool
> for kernel developers, enhancing the performance of kprobe has
> got much importance.
> Currently kprobes inserts a trap instruction to probe a running kernel.
> Jump optimization allows kprobes to replace the trap with a branch,
> reducing the probe overhead drastically.
Thank you for updating the series :)
I'll check that.
> In this series, conditional branch instructions are not considered for
> optimization as they have to be assessed carefully in SMP systems.
So, what kind of problem are there on PPC? (can conditional flag be
changed by other cpu?)
> An optimized kprobe in powerpc is 1.05 to 4.7 times faster than a kprobe.
> Placed a probe at an offset 0x50 in _do_fork().
> *Time Diff here is, difference in time before hitting the probe and
> after the probed instruction. mftb() is employed in kernel/fork.c for
> this purpose.
> # echo 0 > /proc/sys/debug/kprobes-optimization
> Kprobes globally unoptimized
> [ 233.607120] Time Diff = 0x1f0
> [ 233.608273] Time Diff = 0x1ee
> [ 233.609228] Time Diff = 0x203
> [ 233.610400] Time Diff = 0x1ec
> [ 233.611335] Time Diff = 0x200
> [ 233.612552] Time Diff = 0x1f0
> [ 233.613386] Time Diff = 0x1ee
> [ 233.614547] Time Diff = 0x212
> [ 233.615570] Time Diff = 0x206
> [ 233.616819] Time Diff = 0x1f3
> [ 233.617773] Time Diff = 0x1ec
> [ 233.618944] Time Diff = 0x1fb
> [ 233.619879] Time Diff = 0x1f0
> [ 233.621066] Time Diff = 0x1f9
> [ 233.621999] Time Diff = 0x283
> [ 233.623281] Time Diff = 0x24d
> [ 233.624172] Time Diff = 0x1ea
> [ 233.625381] Time Diff = 0x1f0
> [ 233.626358] Time Diff = 0x200
> [ 233.627572] Time Diff = 0x1ed
> # echo 1 > /proc/sys/debug/kprobes-optimization
> Kprobes globally optimized
> [ 70.797075] Time Diff = 0x103
> [ 70.799102] Time Diff = 0x181
> [ 70.801861] Time Diff = 0x15e
> [ 70.803466] Time Diff = 0xf0
> [ 70.804348] Time Diff = 0xd0
> [ 70.805653] Time Diff = 0xad
> [ 70.806477] Time Diff = 0xe0
> [ 70.807725] Time Diff = 0xbe
> [ 70.808541] Time Diff = 0xc3
> [ 70.810191] Time Diff = 0xc7
> [ 70.811007] Time Diff = 0xc0
> [ 70.812629] Time Diff = 0xc0
> [ 70.813640] Time Diff = 0xda
> [ 70.814915] Time Diff = 0xbb
> [ 70.815726] Time Diff = 0xc4
> [ 70.816955] Time Diff = 0xc0
> [ 70.817778] Time Diff = 0xcd
> [ 70.818999] Time Diff = 0xcd
> [ 70.820099] Time Diff = 0xcb
> [ 70.821333] Time Diff = 0xf0
> The trap instruction is replaced by a branch to a detour buffer. To address
> the limitation of branch instruction in power architecture detour buffer
> slot is allocated from a reserved area . This will ensure that the branch
> is within ± 32 MB range. Patch 2/3 furnishes this. The current kprobes
> insn caches allocate memory area for insn slots with module_alloc(). This
> will always be beyond ± 32MB range.
> The detour buffer contains a call to optimized_callback() which in turn
> call the pre_handler(). Once the pre-handler is run, the original
> instruction is emulated from the detour buffer itself. Also the detour
> buffer is equipped with a branch back to the normal work flow after the
> probed instruction is emulated. Before preparing optimization, Kprobes
> inserts original(breakpoint instruction)kprobe on the specified address.
> So, even if the kprobe is not possible to be optimized, it just uses a
> normal kprobe.
> - Number of probes which can be optimized is limited by the size of the
> area reserved.
> - Currently instructions which can be emulated are the only candidates for
> - Conditional branch instructions are not optimized.
> - Probes on kernel module region are not considered for optimization now.
> RFC patchset for optprobes: https://lkml.org/lkml/2016/5/31/375
> Changes from RFC-v3 :
> - Optimization for kporbe(in case of branch instructions) is limited to
> unconditional branch instructions only, since the conditional
> branches are to be assessed carefully in SMP systems.
> - create_return_branch() is omitted.
> - Comments by Masami are addressed.
> Anju T Sudhakar (3):
> arch/powerpc : Add detour buffer support for optprobes
> arch/powerpc : optprobes for powerpc core
> arch/powerpc : Enable optprobes support in powerpc
> .../features/debug/optprobes/arch-support.txt | 2 +-
> arch/powerpc/Kconfig | 1 +
> arch/powerpc/include/asm/kprobes.h | 24 ++
> arch/powerpc/include/asm/sstep.h | 1 +
> arch/powerpc/kernel/Makefile | 1 +
> arch/powerpc/kernel/optprobes.c | 329 +++++++++++++++++++++
> arch/powerpc/kernel/optprobes_head.S | 119 ++++++++
> arch/powerpc/lib/sstep.c | 21 ++
> 8 files changed, 497 insertions(+), 1 deletion(-)
> create mode 100644 arch/powerpc/kernel/optprobes.c
> create mode 100644 arch/powerpc/kernel/optprobes_head.S
Masami Hiramatsu <mhiramat@xxxxxxxxxx>