Re: [RFC PATCH] perf/x86: make perf callchain work without CONFIG_FRAME_POINTER

[Josh Poimboeuf]

On Fri, Apr 05, 2019 at 11:13:02PM +0800, Kairui Song wrote:
> Hi Josh, thanks for the review, I tried again, using latest upstream
> kernel commit ea2cec24c8d429ee6f99040e4eb6c7ad627fe777:
> # uname -a
> Linux localhost.localdomain 5.1.0-rc3+ #29 SMP Fri Apr 5 22:53:05 CST
> 2019 x86_64 x86_64 x86_64 GNU/Linux
> 
> Having following config:
> > CONFIG_UNWINDER_ORC=y
> > # CONFIG_UNWINDER_FRAME_POINTER is not set
> and CONFIG_FRAME_POINTER is off too.
> 
> Then record something with perf (also latest upstream version):
> ./perf record -g -e kmem:* -c 1
> 
> Interrupt it, then view the output:
> perf script | less
> 
> Then I notice the stacktrace in kernle is incomplete like following.
> Did I miss anything?
> --------------
> lvmetad   617 [000]    55.600786:                     kmem:kfree:
> call_site=ffffffffb219e269 ptr=(nil)
>         ffffffffb22b2d1c kfree+0x11c (/lib/modules/5.1.0-rc3+/build/vmlinux)
>             7fba7e58fd0f __select+0x5f (/usr/lib64/libc-2.28.so)
> 
> kworker/u2:5-rp   171 [000]    55.628529:
> kmem:kmem_cache_alloc: call_site=ffffffffb20e963d
> ptr=0xffffa07f39c581e0 bytes_req=80 bytes_alloc=80
> gfp_flags=GFP_ATOMIC
>         ffffffffb22b0dec kmem_cache_alloc+0x13c
> (/lib/modules/5.1.0-rc3+/build/vmlinux)
> -------------
> 
> And for the patch, I debugged the problem, and found how it happend:
> The reason is that we use following code for fetching the registers on
> a trace point:
> ...snip...
> #define perf_arch_fetch_caller_regs(regs, __ip) { \
> (regs)->ip = (__ip); \
> (regs)->bp = caller_frame_pointer(); \
> (regs)->cs = __KERNEL_CS;
> ...snip...

Thanks, I was able to recreate.  It only happens when unwinding from a
tracepoint.  I haven't investigated yet, but
perf_arch_fetch_caller_regs() looks highly suspect, since it's doing
(regs)->bp = caller_frame_pointer(), even for ORC.

My only explanation for how your patch works is that RBP just happens to
point to somewhere higher on the stack, causing the unwinder to start at
a semi-random location.  I suspect the real "fix" is that you're no
longer passing the regs to unwind_start().

-- 
Josh