Re: [PATCH 1/3] riscv: Add perf callchain support

Mao Han
2019-04-29T08:40:35Z

On Thu, Apr 25, 2019 at 02:11:00PM -0700, Palmer Dabbelt wrote:
> On Thu, 11 Apr 2019 00:53:48 PDT (-0700), han_mao@c-sky.com wrote:
> >+ if (!kstack_end((void *)frame->fp) &&
> >+ !((unsigned long)frame->fp & 0x3) &&
> >+ ((unsigned long)frame->fp >= TASK_SIZE)) {
> >+ frame->ra = ((struct stackframe *)frame->fp - 1)->ra;
> >+ frame->fp = ((struct stackframe *)frame->fp - 1)->fp;
>
> It looks like this depends on having a frame pointer? In that case, shouldn't
> we add some Kconfig logic to make CONFIG_PERF_EVENTS select
> -fno-omit-frame-pointer? Frame pointers aren't enabled by default on RISC-V
> and therefor are unlikely to exist at all.
>

Yes, frame pointer is required for kernel backtrace, -fno-omit-frame-pointer
should be added if frame pointer is not enabled by default on RISC-V.
When I testing the callchain support with buildroot chunk
git://git.busybox.net/buildroot
cbf1d861fadb491eee6e3686019d8f67d7f4ad85
both kernel and user program have fp without adding any extra option,
so I thought frame pointer was enabled by default.

ffffffe0000009ee <kernel_init_freeable>:
ffffffe0000009ee: 711d addi sp,sp,-96
ffffffe0000009f0: ec86 sd ra,88(sp)
ffffffe0000009f2: e8a2 sd s0,80(sp)
ffffffe0000009f4: e4a6 sd s1,72(sp)

void test_3(void)
{
10498: fe010113 addi sp,sp,-32
1049c: 00113c23 sd ra,24(sp)
104a0: 00813823 sd s0,16(sp)
104a4: 02010413 addi s0,sp,32

> >+ /* make sure CONFIG_FUNCTION_GRAPH_TRACER is turned on */
>
> Should that also be mandated by a Kconfig?
>

Yes, it is required for ftrace_graph_ret_addr support.
It's already default for ARCH_RV64I in Kconfig.

> >+/*
> >+ * This will be called when the target is in user mode
> >+ * This function will only be called when we use
> >+ * "PERF_SAMPLE_CALLCHAIN" in
> >+ * kernel/events/core.c:perf_prepare_sample()
> >+ *
> >+ * How to trigger perf_callchain_[user/kernel] :
> >+ * $ perf record -e cpu-clock --call-graph fp ./program
> >+ * $ perf report --call-graph
> >+ *
> >+ * On RISC-V platform, the program being sampled and the C library
> >+ * need to be compiled with * -mbacktrace, otherwise the user
>
> What is "-mbacktrace"? I don't remember that ever being a RISC-V GCC option,
> and my compiler doesn't undersand it. It understands "-fbacktrace" but that
> doesn't produce a frame pointer.
>

It's a csky specific option, I forget to modify the comment.
So it should be -fno-omit-frame-pointer here.

Thanks,
Mao Han