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Date:   Fri, 17 May 2019 09:46:00 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Song Liu <songliubraving@fb.com>
Cc:     "kasong@redhat.com" <kasong@redhat.com>,
        lkml <linux-kernel@vger.kernel.org>,
        Kernel Team <Kernel-team@fb.com>,
        Josh Poimboeuf <jpoimboe@redhat.com>
Subject: Re: Getting empty callchain from perf_callchain_kernel()
Message-ID: <20190517074600.GJ2623@hirez.programming.kicks-ass.net>
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On Thu, May 16, 2019 at 11:51:55PM +0000, Song Liu wrote:
> Hi, 
> 
> We found a failure with selftests/bpf/tests_prog in test_stacktrace_map (on bpf/master
> branch). 
> 
> After digging into the code, we found that perf_callchain_kernel() is giving empty
> callchain for tracepoint sched/sched_switch. And it seems related to commit
> 
> d15d356887e770c5f2dcf963b52c7cb510c9e42d
> ("perf/x86: Make perf callchains work without CONFIG_FRAME_POINTER")
> 
> Before this commit, perf_callchain_kernel() returns callchain with regs->ip. With
> this commit, regs->ip is not sent for !perf_hw_regs(regs) case. 

So while I think the below is indeed right; we should store regs->ip
regardless of the unwind path chosen, I still think there's something
fishy if this results in just the 1 entry.

The sched/sched_switch event really should have a non-trivial stack.

Let me see if I can reproduce with just perf.