On 15/12/16 03:18, Anju T Sudhakar wrote: > This is the V2 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. > > In this series, conditional branch instructions are not considered for > optimization as they have to be assessed carefully in SMP systems. > > The kprobe placed on the kretprobe_trampoline during boot time, is also > optimized in this series. Patch 4/4 furnishes this. > > The first two patches can go independently of the series. The helper > functions in these patches are invoked in patch 3/4. > > 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. The current kprobes insn caches allocate memory > area for insn slots with module_alloc(). This will always be beyond > ± 32MB range. >
The paragraph is a little confusing. We need the detour buffer to be within +-32 MB, but then you say we always get memory from module_alloc() beyond 32MB. > 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. Does the branch itself use registers that need to be saved? I presume we are going to rely on the +-32MB, what are the guarantees of success of such a mechanism? Balbir Singh.
