Re: [PATCH bpf-next] selftests/bpf: Switch test_vmlinux to use hrtimer_range_start_ns.

From: Hao Luo <haoluo@google.com>
To: Yonghong Song <yhs@fb.com>
Cc: Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	linux-kernel@vger.kernel.org, clang-built-linux@googlegroups.com,
	linux-kselftest@vger.kernel.org,
	Stanislav Fomichev <sdf@google.com>,
	Shuah Khan <shuah@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Andrii Nakryiko <andriin@fb.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Martin KaFai Lau <kafai@fb.com>, Song Liu <songliubraving@fb.com>,
	John Fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@chromium.org>, Bill Wendling <morbo@google.com>
Subject: Re: [PATCH bpf-next] selftests/bpf: Switch test_vmlinux to use hrtimer_range_start_ns.
Date: Tue, 30 Jun 2020 17:10:42 -0700	[thread overview]
Message-ID: <CA+khW7jNqVMqq2dzf6Dy0pWCZYjHrG7Vm_sUEKKLS-L-ptzEtQ@mail.gmail.com> (raw)
In-Reply-To: <CA+khW7iJu2tzcz36XzL6gBq4poq+5Qt0vbrmPRdYuvC-c5U4_A@mail.gmail.com>

Ok, with the help of my colleague Ian Rogers, I think we solved the
mystery. Clang actually inlined hrtimer_nanosleep() inside
SyS_nanosleep(), so there is no call to that function throughout the
path of the nanosleep syscall. I've been looking at the function body
of hrtimer_nanosleep for quite some time, but clearly overlooked the
caller of hrtimer_nanosleep. hrtimer_nanosleep is pretty short and
there are many constants, inlining would not be too surprising.
Sigh...

Hao

On Tue, Jun 30, 2020 at 3:48 PM Hao Luo <haoluo@google.com> wrote:
>
> On Tue, Jun 30, 2020 at 1:37 PM Yonghong Song <yhs@fb.com> wrote:
> >
> > On 6/30/20 11:49 AM, Hao Luo wrote:
> > > The test_vmlinux test uses hrtimer_nanosleep as hook to test tracing
> > > programs. But it seems Clang may have done an aggressive optimization,
> > > causing fentry and kprobe to not hook on this function properly on a
> > > Clang build kernel.
> >
> > Could you explain why it does not on clang built kernel? How did you
> > build the kernel? Did you use [thin]lto?
> >
> > hrtimer_nanosleep is a global function who is called in several
> > different files. I am curious how clang optimization can make
> > function disappear, or make its function signature change, or
> > rename the function?
> >
>
> Yonghong,
>
> We didn't enable LTO. It also puzzled me. But I can confirm those
> fentry/kprobe test failures via many different experiments I've done.
> After talking to my colleague on kernel compiling tools (Bill, cc'ed),
> we suspected this could be because of clang's aggressive inlining. We
> also noticed that all the callsites of hrtimer_nanosleep() are tail
> calls.
>
> For a better explanation, I can reach out to the people who are more
> familiar to clang in the compiler team to see if they have any
> insights. This may not be of high priority for them though.
>
> Hao