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From:   Guo Ren <guoren@kernel.org>
Date:   Wed, 22 Jul 2020 16:39:53 +0800
Message-ID: <CAJF2gTTM6fsK5A9CKFkEhCC3hdsM8bCfBh613ohvM50fTLc5cw@mail.gmail.com>
Subject: Re: [PATCH v3 6/7] riscv: Add KPROBES_ON_FTRACE supported
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Palmer Dabbelt <palmerdabbelt@google.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Oleg Nesterov <oleg@redhat.com>,
        linux-riscv <linux-riscv@lists.infradead.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Anup Patel <anup@brainfault.org>, linux-csky@vger.kernel.org,
        Greentime Hu <greentime.hu@sifive.com>,
        Zong Li <zong.li@sifive.com>,
        =?UTF-8?Q?Patrick_St=C3=A4hlin?= <me@packi.ch>,
        =?UTF-8?B?QmrDtnJuIFTDtnBlbA==?= <bjorn.topel@gmail.com>,
        Guo Ren <guoren@linux.alibaba.com>,
        Pekka Enberg <penberg@kernel.org>
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Hi Masami,

On Tue, Jul 21, 2020 at 9:27 PM Masami Hiramatsu <mhiramat@kernel.org> wrot=
e:
>
> On Wed, 15 Jul 2020 00:24:54 +0800
> Guo Ren <guoren@kernel.org> wrote:
>
> > Thx Masami,
> >
> > On Tue, Jul 14, 2020 at 7:38 PM Masami Hiramatsu <mhiramat@kernel.org> =
wrote:
> > >
> > > On Mon, 13 Jul 2020 23:39:21 +0000
> > > guoren@kernel.org wrote:
> > >
> > > > From: Guo Ren <guoren@linux.alibaba.com>
> > > >
> > > > This patch adds support for kprobes on ftrace call sites to avoids
> > > > much of the overhead with regular kprobes. Try it with simple
> > > > steps:
> > > >
> > > > 1. Get _do_fork ftrace call site.
> > > > Dump of assembler code for function _do_fork:
> > > >    0xffffffe00020af64 <+0>:     addi    sp,sp,-128
> > > >    0xffffffe00020af66 <+2>:     sd      s0,112(sp)
> > > >    0xffffffe00020af68 <+4>:     sd      ra,120(sp)
> > > >    0xffffffe00020af6a <+6>:     addi    s0,sp,128
> > > >    0xffffffe00020af6c <+8>:     sd      s1,104(sp)
> > > >    0xffffffe00020af6e <+10>:    sd      s2,96(sp)
> > > >    0xffffffe00020af70 <+12>:    sd      s3,88(sp)
> > > >    0xffffffe00020af72 <+14>:    sd      s4,80(sp)
> > > >    0xffffffe00020af74 <+16>:    sd      s5,72(sp)
> > > >    0xffffffe00020af76 <+18>:    sd      s6,64(sp)
> > > >    0xffffffe00020af78 <+20>:    sd      s7,56(sp)
> > > >    0xffffffe00020af7a <+22>:    mv      s4,a0
> > > >    0xffffffe00020af7c <+24>:    mv      a0,ra
> > > >    0xffffffe00020af7e <+26>:    nop   <<<<<<<< here!
> > > >    0xffffffe00020af82 <+30>:    nop
> > > >    0xffffffe00020af86 <+34>:    ld      s3,0(s4)
> > > >
> > > > 2. Set _do_fork+26 as the kprobe.
> > > >   echo 'p:myprobe _do_fork+26 dfd=3D%a0 filename=3D%a1 flags=3D%a2 =
mode=3D+4($stack)' > /sys/kernel/debug/tracing/kprobe_events
> > > >   echo 1 > /sys/kernel/debug/tracing/events/kprobes/enable
> > > >   cat /sys/kernel/debug/tracing/trace
> > > >   tracer: nop
> > > >
> > > >   entries-in-buffer/entries-written: 3/3   #P:1
> > > >
> > > >                                _-----=3D> irqs-off
> > > >                               / _----=3D> need-resched
> > > >                              | / _---=3D> hardirq/softirq
> > > >                              || / _--=3D> preempt-depth
> > > >                              ||| /     delay
> > > >             TASK-PID   CPU#  ||||    TIMESTAMP  FUNCTION
> > > >                | |       |   ||||       |         |
> > > >               sh-87    [000] ....   551.557031: myprobe: (_do_fork+=
0x1a/0x2e6) dfd=3D0xffffffe00020af7e filename=3D0xffffffe00020b34e flags=3D=
0xffffffe00101e7c0 mode=3D0x20af86ffffffe0
> > > >
> > > >   cat /sys/kernel/debug/kprobes/list
> > > > ffffffe00020af7e  k  _do_fork+0x1a    [FTRACE]
> > > >                                        ^^^^^^
> > >
> > > Hmm, this seems fentry is not supported on RISC-V yet. But anyway,
> > > it will be useful for users (if they can find the offset).
> >
> > Seems only x86 & =E2=AC=86=EF=B8=8F90 use fentry=EF=BC=8Ccan you elabor=
ate more about fentry's
> > benefit and how the user could set kprobe on ftrace call site without
> > disassemble?
>
> On x86, the fentry replaces the mcount with just one call instruction, wi=
thout
> saving any arguments. This means all probes which are puts on the address=
 of
> target symbol, are automatically using ftrace. IOW, all probes on _do_for=
k+0
> will use ftrace. We don't need any disassembling.
>
> I think if RISC-V already support "-fpatchable-function-entry=3D2" option=
 on
> GCC, you can easily enable it as same as arm64. See https://lkml.org/lkml=
/2019/6/18/648
the link is:
[PATCH 0/7] powerpc/ftrace: Patch out -mprofile-kernel instructions

Is that right?

--=20
Best Regards
 Guo Ren

ML: https://lore.kernel.org/linux-csky/