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Date:   Tue, 30 Jun 2020 17:10:42 -0700
From:   Hao Luo <haoluo@...gle.com>
To:     Yonghong Song <yhs@...com>
Cc:     Networking <netdev@...r.kernel.org>, bpf <bpf@...r.kernel.org>,
        linux-kernel@...r.kernel.org, clang-built-linux@...glegroups.com,
        linux-kselftest@...r.kernel.org,
        Stanislav Fomichev <sdf@...gle.com>,
        Shuah Khan <shuah@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Andrii Nakryiko <andriin@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Martin KaFai Lau <kafai@...com>,
        Song Liu <songliubraving@...com>,
        John Fastabend <john.fastabend@...il.com>,
        KP Singh <kpsingh@...omium.org>,
        Bill Wendling <morbo@...gle.com>
Subject: Re: [PATCH bpf-next] selftests/bpf: Switch test_vmlinux to use hrtimer_range_start_ns.

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@...gle.com> wrote:
>
> On Tue, Jun 30, 2020 at 1:37 PM Yonghong Song <yhs@...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

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