Hi Anju, On Wed, 7 Sep 2016 15:03:09 +0530 Anju T Sudhakar <[email protected]> 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?) Thanks, > > > Performance: > ============= > An optimized kprobe in powerpc is 1.05 to 4.7 times faster than a kprobe. > > Example: > > 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 > > Implementation: > =================== > > 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. > > Limitations: > ============== > - 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 > optimization. > - 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 > https://lkml.org/lkml/2016/5/31/376 > https://lkml.org/lkml/2016/5/31/377 > https://lkml.org/lkml/2016/5/31/378 > > 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 > > -- > 2.7.4 > -- Masami Hiramatsu <[email protected]>
