Re: [PATCH v4 05/10] signal: Introduce TRAP_PERF si_code and si_perf to siginfo

[Marco Elver <elver@google.com>]

On Wed, 21 Apr 2021 at 12:57, Marek Szyprowski <m.szyprowski@samsung.com> wrote:
>
> On 21.04.2021 11:35, Marek Szyprowski wrote:
> > On 21.04.2021 10:11, Marco Elver wrote:
> >> On Wed, 21 Apr 2021 at 09:35, Marek Szyprowski
> >> <m.szyprowski@samsung.com> wrote:
> >>> On 21.04.2021 08:21, Marek Szyprowski wrote:
> >>>> On 21.04.2021 00:42, Marco Elver wrote:
> >>>>> On Tue, 20 Apr 2021 at 23:26, Marek Szyprowski
> >>>>> <m.szyprowski@samsung.com> wrote:
> >>>>>> On 08.04.2021 12:36, Marco Elver wrote:
> >>>>>>> Introduces the TRAP_PERF si_code, and associated siginfo_t field
> >>>>>>> si_perf. These will be used by the perf event subsystem to send
> >>>>>>> signals
> >>>>>>> (if requested) to the task where an event occurred.
> >>>>>>>
> >>>>>>> Acked-by: Geert Uytterhoeven <geert@linux-m68k.org> # m68k
> >>>>>>> Acked-by: Arnd Bergmann <arnd@arndb.de> # asm-generic
> >>>>>>> Signed-off-by: Marco Elver <elver@google.com>
> >>>>>> This patch landed in linux-next as commit fb6cc127e0b6 ("signal:
> >>>>>> Introduce TRAP_PERF si_code and si_perf to siginfo"). It causes
> >>>>>> regression on my test systems (arm 32bit and 64bit). Most systems
> >>>>>> fails
> >>>>>> to boot in the given time frame. I've observed that there is a
> >>>>>> timeout
> >>>>>> waiting for udev to populate /dev and then also during the network
> >>>>>> interfaces configuration. Reverting this commit, together with
> >>>>>> 97ba62b27867 ("perf: Add support for SIGTRAP on perf events") to
> >>>>>> let it
> >>>>>> compile, on top of next-20210420 fixes the issue.
> >>>>> Thanks, this is weird for sure and nothing in particular stands out.
> >>>>>
> >>>>> I have questions:
> >>>>> -- Can you please share your config?
> >>>> This happens with standard multi_v7_defconfig (arm) or just defconfig
> >>>> for arm64.
> >>>>
> >>>>> -- Also, can you share how you run this? Can it be reproduced in
> >>>>> qemu?
> >>>> Nothing special. I just boot my test systems and see that they are
> >>>> waiting lots of time during the udev populating /dev and network
> >>>> interfaces configuration. I didn't try with qemu yet.
> >>>>> -- How did you derive this patch to be at fault? Why not just
> >>>>> 97ba62b27867, given you also need to revert it?
> >>>> Well, I've just run my boot tests with automated 'git bisect' and that
> >>>> was its result. It was a bit late in the evening, so I didn't analyze
> >>>> it further, I've just posted a report about the issue I've found. It
> >>>> looks that bisecting pointed to a wrong commit somehow.
> >>>>> If you are unsure which patch exactly it is, can you try just
> >>>>> reverting 97ba62b27867 and see what happens?
> >>>> Indeed, this is a real faulty commit. Initially I've decided to revert
> >>>> it to let kernel compile (it uses some symbols introduced by this
> >>>> commit). Reverting only it on top of linux-next 20210420 also fixes
> >>>> the issue. I'm sorry for the noise in this thread. I hope we will find
> >>>> what really causes the issue.
> >>> This was a premature conclusion. It looks that during the test I've did
> >>> while writing that reply, the modules were not deployed properly and a
> >>> test board (RPi4) booted without modules. In that case the board booted
> >>> fine and there was no udev timeout. After deploying kernel modules, the
> >>> udev timeout is back.
> >> I'm confused now. Can you confirm that the problem is due to your
> >> kernel modules, or do you think it's still due to 97ba62b27867? Or
> >> fb6cc127e0b6 (this patch)?
> >
> > I don't use any custom kernel modules. I just deploy all modules that
> > are being built from the given kernel defconfig (arm
> > multi_v7_defconfig or arm64 default) and they are automatically loaded
> > during the boot by udev. I've checked again and bisect was right. The
> > kernel built from fb6cc127e0b6 suffers from the described issue, while
> > the one build from the previous commit (2e498d0a74e5) works fine.
>
> I've managed to reproduce this issue with qemu. I've compiled the kernel
> for arm 32bit with multi_v7_defconfig and used some older Debian rootfs
> image. The log and qemu parameters are here:
> https://paste.debian.net/1194526/
>
> Check the timestamp for the 'EXT4-fs (vda): re-mounted' message and
> 'done (timeout)' status for the 'Waiting for /dev to be fully populated'
> message. This happens only when kernel modules build from the
> multi_v7_defconfig are deployed on the rootfs.

Still hard to say what is going on and what is at fault. But being
able to repro this in qemu helps debug quicker -- would you also be
able to share the precise rootfs.img, i.e. upload it somewhere I can
fetch it? And just to be sure, please also share your .config, as it
might have compiler-version dependent configuration that might help
repro (unlikely, but you never know).

Thanks,
-- Marco