Messages in this thread | | | Date | Mon, 9 Oct 2023 02:33:09 +0800 | From | wuqiang <> | Subject | Re: [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 >> > >
| |