On Mon, Oct 9, 2023 at 11:43 AM Yajun Deng <yajun.d...@linux.dev> wrote: > > > On 2023/10/9 17:30, Eric Dumazet wrote: > > On Mon, Oct 9, 2023 at 10:36 AM Yajun Deng <yajun.d...@linux.dev> wrote: > >> > >> On 2023/10/9 16:20, Eric Dumazet wrote: > >>> On Mon, Oct 9, 2023 at 10:14 AM Yajun Deng <yajun.d...@linux.dev> wrote: > >>>> On 2023/10/9 15:53, Eric Dumazet wrote: > >>>>> On Mon, Oct 9, 2023 at 5:07 AM Yajun Deng <yajun.d...@linux.dev> wrote: > >>>>> > >>>>>> 'this_cpu_read + this_cpu_write' and 'pr_info + this_cpu_inc' will make > >>>>>> the trace work well. > >>>>>> > >>>>>> They all have 'pop' instructions in them. This may be the key to making > >>>>>> the trace work well. > >>>>>> > >>>>>> Hi all, > >>>>>> > >>>>>> I need your help on percpu and ftrace. > >>>>>> > >>>>> I do not think you made sure netdev_core_stats_inc() was never inlined. > >>>>> > >>>>> Adding more code in it is simply changing how the compiler decides to > >>>>> inline or not. > >>>> Yes, you are right. It needs to add the 'noinline' prefix. The > >>>> disassembly code will have 'pop' > >>>> > >>>> instruction. > >>>> > >>> The function was fine, you do not need anything like push or pop. > >>> > >>> The only needed stuff was the call __fentry__. > >>> > >>> The fact that the function was inlined for some invocations was the > >>> issue, because the trace point > >>> is only planted in the out of line function. > >> > >> But somehow the following code isn't inline? They didn't need to add the > >> 'noinline' prefix. > >> > >> + field = (unsigned long *)((void *)this_cpu_ptr(p) + > >> offset); > >> + WRITE_ONCE(*field, READ_ONCE(*field) + 1); > >> > >> Or > >> + (*(unsigned long *)((void *)this_cpu_ptr(p) + offset))++; > >> > > I think you are very confused. > > > > You only want to trace netdev_core_stats_inc() entry point, not > > arbitrary pieces of it. > > > Yes, I will trace netdev_core_stats_inc() entry point. I mean to replace > > + field = (__force unsigned long > __percpu *)((__force void *)p + offset); > + this_cpu_inc(*field); > > with > > + field = (unsigned long *)((void *)this_cpu_ptr(p) + offset); > + WRITE_ONCE(*field, READ_ONCE(*field) + 1); > > Or > + (*(unsigned long *)((void *)this_cpu_ptr(p) + offset))++; > > The netdev_core_stats_inc() entry point will work fine even if it doesn't > have 'noinline' prefix. > > I don't know why this code needs to add 'noinline' prefix. > + field = (__force unsigned long __percpu *)((__force void *)p > + offset); > + this_cpu_inc(*field); >
C compiler decides to inline or not, depending on various factors. The most efficient (and small) code is generated by this_cpu_inc() version, allowing the compiler to inline it. If you copy/paste this_cpu_inc() twenty times, then the compiler would not inline the function anymore.
