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SubjectRe: [PATCH v9 0/5] lib,kprobes: kretprobe scalability improvement
On 2023/9/23 16:57, Masami Hiramatsu (Google) wrote:
> Hi Wuqiang,
>
> I dug my mail box and found this. Sorry for replying late.
>
> On Tue, 5 Sep 2023 09:52:50 +0800
> "wuqiang.matt" <wuqiang.matt@bytedance.com> wrote:
>
>> This patch series introduces a scalable and lockless ring-array based
>> object pool and replaces the original freelist (a LIFO queue based on
>> singly linked list) to improve scalability of kretprobed routines.
>>
>> v9:
>> 1) objpool: raw_local_irq_save/restore added to prevent interruption
>>
>> To avoid possible ABA issues, we must ensure objpool_try_add_slot
>> and objpool_try_add_slot are uninterruptible. If these operations
>> are blocked or interrupted in the middle, other cores could overrun
>> the same slot's ages[] of uint32, then after resuming back, the
>> interrupted pop() or push() could see same value of ages[], which
>> is a typical ABA problem though the possibility is small.
>>
>> The pair of pop()/push() costs about 8.53 cpu cycles, measured
>> by IACA (Intel Architecture Code Analyzer). That is, on a 4Ghz
>> core dedicated for pop() & push(), theoretically it would only
>> need 8.53 seconds to overflow a 32bit value. Testings upon Intel
>> i7-10700 (2.90GHz) cost 71.88 seconds to overrun a 32bit integer.
>
> What does this mean? This sounds like "There is a timing issue if it's enough fast".

Yes, that's why local irq must be disabled. If push()/pop() is interrupted or
preempted long enough (> 10 seconds for the extreme cases), other nodes could
overrun the same ages[] of 32-bit, then after resuming to execution the push()
or pop() would see the same value without notifying the overrun, which is a
typical ABA.

Changing ages[] to 64-bit could be a solution, but it's inappropriate for
32-bit OS and looks too heavy. With local irg disabled, push() or pop() is
uninterrupted,thus the ABA is avoided.

push() or pop() consumes only ~4 cycles to complete (most of the use cases),
so raw_local_irq_save/restore are used instead of local_irq_save/restore to
minimize the overhead.

> Let me reivew the patch itself.
>
> Thanks,
>
>>
>> 2) codes improvements: thanks to Masami for the thorough inspection
>>
>> v8:
>> 1) objpool: refcount added for objpool lifecycle management
>>
>> wuqiang.matt (5):
>> lib: objpool added: ring-array based lockless MPMC
>> lib: objpool test module added
>> kprobes: kretprobe scalability improvement with objpool
>> kprobes: freelist.h removed
>> MAINTAINERS: objpool added
>>
>> MAINTAINERS | 7 +
>> include/linux/freelist.h | 129 --------
>> include/linux/kprobes.h | 11 +-
>> include/linux/objpool.h | 174 ++++++++++
>> include/linux/rethook.h | 16 +-
>> kernel/kprobes.c | 93 +++---
>> kernel/trace/fprobe.c | 32 +-
>> kernel/trace/rethook.c | 90 +++--
>> lib/Kconfig.debug | 11 +
>> lib/Makefile | 4 +-
>> lib/objpool.c | 338 +++++++++++++++++++
>> lib/test_objpool.c | 689 +++++++++++++++++++++++++++++++++++++++
>> 12 files changed, 1320 insertions(+), 274 deletions(-)
>> delete mode 100644 include/linux/freelist.h
>> create mode 100644 include/linux/objpool.h
>> create mode 100644 lib/objpool.c
>> create mode 100644 lib/test_objpool.c
>>
>> --
>> 2.40.1
>>
>
>


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