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From:   Song Liu <songliubraving@fb.com>
To:     "kasong@redhat.com" <kasong@redhat.com>,
        lkml <linux-kernel@vger.kernel.org>
CC:     Peter Zijlstra <peterz@infradead.org>,
        Kernel Team <Kernel-team@fb.com>
Subject: Getting empty callchain from perf_callchain_kernel() 
Thread-Topic: Getting empty callchain from perf_callchain_kernel() 
Thread-Index: AQHVDEJXYtrotGS6bU+cxwR7BKWjbA==
Date:   Thu, 16 May 2019 23:51:55 +0000
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Hi,=20

We found a failure with selftests/bpf/tests_prog in test_stacktrace_map (on=
 bpf/master
branch).=20

After digging into the code, we found that perf_callchain_kernel() is givin=
g 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.=20

I found the following change fixes the selftest. But I am not very sure, it=
 is=20
the best solution here.=20

Please share comments and suggestions on this.=20

Thanks in advance!

Song


diff --git i/arch/x86/events/core.c w/arch/x86/events/core.c
index f315425d8468..7b8a9eb4d5fd 100644
--- i/arch/x86/events/core.c
+++ w/arch/x86/events/core.c
@@ -2402,9 +2402,9 @@ perf_callchain_kernel(struct perf_callchain_entry_ctx=
 *entry, struct pt_regs *re
                return;
        }

+       if (perf_callchain_store(entry, regs->ip))
+               return;
        if (perf_hw_regs(regs)) {
-               if (perf_callchain_store(entry, regs->ip))
-                       return;
                unwind_start(&state, current, regs, NULL);
        } else {
                unwind_start(&state, current, NULL, (void *)regs->sp);