Messages in this thread | | | Date | Mon, 9 Oct 2023 18:58:27 +0800 | Subject | Re: [PATCH net-next v7] net/core: Introduce netdev_core_stats_inc() | From | Yajun Deng <> |
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On 2023/10/9 18:16, Eric Dumazet wrote: > On Mon, Oct 9, 2023 at 11:43 AM Yajun Deng <yajun.deng@linux.dev> wrote: >> >> On 2023/10/9 17:30, Eric Dumazet wrote: >>> On Mon, Oct 9, 2023 at 10:36 AM Yajun Deng <yajun.deng@linux.dev> wrote: >>>> On 2023/10/9 16:20, Eric Dumazet wrote: >>>>> On Mon, Oct 9, 2023 at 10:14 AM Yajun Deng <yajun.deng@linux.dev> wrote: >>>>>> On 2023/10/9 15:53, Eric Dumazet wrote: >>>>>>> On Mon, Oct 9, 2023 at 5:07 AM Yajun Deng <yajun.deng@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.
Got it. Thank you.
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