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powerpc/kprobes: Prefer ftrace when probing function entry

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KPROBES_ON_FTRACE avoids much of the overhead of regular kprobes as it
eliminates the need for a trap, as well as the need to emulate or single-step
instructions.

Though OPTPROBES provides us with similar performance, we have limited
optprobes trampoline slots. As such, when asked to probe at a function
entry, default to using the ftrace infrastructure.

With:
  # cd /sys/kernel/debug/tracing
  # echo 'p _do_fork' > kprobe_events

before patch:
  # cat ../kprobes/list
  c0000000000daf08  k  _do_fork+0x8    [DISABLED]
  c000000000044fc0  k  kretprobe_trampoline+0x0    [OPTIMIZED]

and after patch:
  # cat ../kprobes/list
  c0000000000d074c  k  _do_fork+0xc    [DISABLED][FTRACE]
  c0000000000412b0  k  kretprobe_trampoline+0x0    [OPTIMIZED]

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
This commit is contained in:
Naveen N. Rao 2017-04-19 18:22:28 +05:30 committed by Michael Ellerman
parent 1b32cd1715
commit 24bd909e94
1 changed files with 15 additions and 2 deletions
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17
arch/powerpc/kernel/kprobes.c
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@ -49,8 +49,21 @@ kprobe_opcode_t *kprobe_lookup_name(const char *name, unsigned int offset)
#ifdef PPC64_ELF_ABI_v2
/* PPC64 ABIv2 needs local entry point */
addr = (kprobe_opcode_t *)kallsyms_lookup_name(name);
if (addr && !offset)
addr = (kprobe_opcode_t *)ppc_function_entry(addr);
if (addr && !offset) {
#ifdef CONFIG_KPROBES_ON_FTRACE
unsigned long faddr;
/*
* Per livepatch.h, ftrace location is always within the first
* 16 bytes of a function on powerpc with -mprofile-kernel.
*/
faddr = ftrace_location_range((unsigned long)addr,
(unsigned long)addr + 16);
if (faddr)
addr = (kprobe_opcode_t *)faddr;
else
#endif
addr = (kprobe_opcode_t *)ppc_function_entry(addr);
}
#elif defined(PPC64_ELF_ABI_v1)
/*
* 64bit powerpc ABIv1 uses function descriptors: