Re: [PATCH V2 0/4] OPTPROBES for powerpc

[Anju T Sudhakar <anju@linux.vnet.ibm.com>]

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Hi Balbir,



On Friday 16 December 2016 08:16 PM, Balbir Singh wrote:
>
> 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 last two lines in the paragraph talks about the*current 
*method**which the regular kprobe uses
for allocating instruction slot. So in our case, we can't use 
module_alloc() since there is no guarantee that the slot allocated will 
be within +/- 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.
> 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?

For branching back to the next instruction, after the execution of the 
kprobe's pre-handler,
we place the branch instruction in the detour buffer itself. Hence we 
don't have to clobber any registers
after restoring them.
Before optimizing the kprobe we make sure that , 'branch to detour 
buffer' and 'branch back from detour buffer' is within +/- 32MB range. 
This ensures the working of optimized kprobe.


Thanks ,
Anju

>
> Balbir Singh.
>


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