Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 5 Apr 2019 11:57:15 -0500
From:   Josh Poimboeuf <jpoimboe@redhat.com>
To:     Kairui Song <kasong@redhat.com>
Cc:     Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Dave Young <dyoung@redhat.com>
Subject: Re: [RFC PATCH] perf/x86: make perf callchain work without
 CONFIG_FRAME_POINTER
Message-ID: <20190405165715.fpgh4ggkmnqdtfwm@treble>
References: <20190404172545.20928-1-kasong@redhat.com>
 <20190405140929.pycfea7drnpb2sug@treble>
 <CACPcB9f7v4SU37YWKmUr=+iFWqL8hRapp1GPX+H8ixvGwzHsHA@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=utf-8
Content-Disposition: inline
In-Reply-To: <CACPcB9f7v4SU37YWKmUr=+iFWqL8hRapp1GPX+H8ixvGwzHsHA@mail.gmail.com>
User-Agent: NeoMutt/20180716
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

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