Re: [PATCH v9 0/5] lib,kprobes: kretprobe scalability improvement

[wuqiang]

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@xxxxxxxxxxxxx> 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