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From: Peter Zijlstra <peterz@infradead.org>
To: Kairui Song <kasong@redhat.com>
Cc: Song Liu <songliubraving@fb.com>,
	lkml <linux-kernel@vger.kernel.org>,
	Kernel Team <Kernel-team@fb.com>,
	Josh Poimboeuf <jpoimboe@redhat.com>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	"bpf@vger.kernel.org" <bpf@vger.kernel.org>
Subject: Re: Getting empty callchain from perf_callchain_kernel()
Date: Wed, 22 May 2019 16:02:33 +0200	[thread overview]
Message-ID: <20190522140233.GC16275@worktop.programming.kicks-ass.net> (raw)
In-Reply-To: <CACPcB9cpNp5CBqoRs+XMCwufzAFa8Pj-gbmj9fb+g5wVdue=ig@mail.gmail.com>

On Mon, May 20, 2019 at 02:06:54AM +0800, Kairui Song wrote:
> On Fri, May 17, 2019 at 5:10 PM Peter Zijlstra <peterz@infradead.org> wrote:
> >
> > On Fri, May 17, 2019 at 04:15:39PM +0800, Kairui Song wrote:
> > > Hi, I think the actual problem is that bpf_get_stackid_tp (and maybe
> > > some other bfp functions) is now broken, or, strating an unwind
> > > directly inside a bpf program will end up strangely. It have following
> > > kernel message:
> >
> > Urgh, what is that bpf_get_stackid_tp() doing to get the regs? I can't
> > follow.
> 
> bpf_get_stackid_tp will just use the regs passed to it from the trace
> point. And then it will eventually call perf_get_callchain to get the
> call chain.
> With a tracepoint we have the fake regs, so unwinder will start from
> where it is called, and use the fake regs as the indicator of the
> target frame it want, and keep unwinding until reached the actually
> callsite.
> 
> But if the stack trace is started withing a bpf func call then it's broken...

I'm confused.. how is this broken? Surely we should eventually find the
original stack frame and be good again, right?

> If the unwinder could trace back through the bpf func call then there
> will be no such problem.

Why couldn't it trace back through the bpf stuff? And how can we fix
that?

  parent reply	other threads:[~2019-05-22 14:02 UTC|newest]

Thread overview: 45+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2019-05-16 23:51 Getting empty callchain from perf_callchain_kernel() Song Liu
2019-05-17  7:46 ` Peter Zijlstra
2019-05-17  8:10   ` Peter Zijlstra
2019-05-17  8:15     ` Kairui Song
2019-05-17  8:32       ` Kairui Song
2019-05-17 16:22         ` Song Liu
2019-05-17  9:10       ` Peter Zijlstra
2019-05-17 18:40         ` Song Liu
2019-05-17 21:06           ` Alexei Starovoitov
2019-05-17 21:48             ` Song Liu
2019-05-19 18:07               ` Kairui Song
2019-05-20 17:22                 ` Song Liu
2019-05-22 13:51                   ` Peter Zijlstra
2019-05-19 18:06         ` Kairui Song
2019-05-20 17:16           ` Song Liu
2019-05-20 17:19           ` Song Liu
2019-05-22 14:02           ` Peter Zijlstra [this message]
2019-05-22 14:49             ` Alexei Starovoitov
2019-05-22 17:45               ` Josh Poimboeuf
2019-05-22 23:46                 ` Josh Poimboeuf
2019-05-23  6:48                   ` Kairui Song
2019-05-23  8:27                     ` Song Liu
2019-05-23  9:11                       ` Kairui Song
2019-05-23 13:32                     ` Josh Poimboeuf
2019-05-23 14:50                       ` Kairui Song
2019-05-23 15:24                         ` Josh Poimboeuf
2019-05-23 16:41                           ` Kairui Song
2019-05-23 17:27                             ` Josh Poimboeuf
2019-05-24  2:20                               ` Kairui Song
2019-05-24 23:23                                 ` Josh Poimboeuf
2019-05-27 11:57                                   ` Kairui Song
2019-06-06 16:04                                     ` Song Liu
2019-06-06 23:58                                       ` Josh Poimboeuf
2019-06-11 21:03                                       ` Josh Poimboeuf
2019-05-24  8:53                           ` Peter Zijlstra
2019-05-24 13:05                             ` Josh Poimboeuf
2019-06-12  3:05                             ` Josh Poimboeuf
2019-06-12  8:54                               ` Peter Zijlstra
2019-06-12 14:50                                 ` Josh Poimboeuf
2019-06-13 20:26                                   ` Josh Poimboeuf
2019-06-12 13:10                               ` Steven Rostedt
2019-06-12 14:26                                 ` Josh Poimboeuf
2019-05-22 18:07       ` Josh Poimboeuf
2019-05-22 21:55         ` Alexei Starovoitov
2019-05-17 16:32     ` Song Liu

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