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Date:   Fri, 20 May 2022 17:04:24 -0700
From:   Nathan Chancellor <nathan@kernel.org>
To:     Saravana Kannan <saravanak@google.com>
Cc:     Jonathan Corbet <corbet@lwn.net>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Rafael J. Wysocki" <rafael@kernel.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>,
        Rob Herring <robh@kernel.org>,
        Linus Walleij <linus.walleij@linaro.org>,
        Will Deacon <will@kernel.org>,
        Ulf Hansson <ulf.hansson@linaro.org>,
        Kevin Hilman <khilman@kernel.org>,
        Thierry Reding <treding@nvidia.com>,
        Mark Brown <broonie@kernel.org>, Pavel Machek <pavel@ucw.cz>,
        Geert Uytterhoeven <geert@linux-m68k.org>,
        Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>,
        Paul Kocialkowski <paul.kocialkowski@bootlin.com>,
        linux-gpio@vger.kernel.org, linux-pm@vger.kernel.org,
        iommu@lists.linux-foundation.org, kernel-team@android.com,
        linux-doc@vger.kernel.org, linux-kernel@vger.kernel.org,
        linux-s390@vger.kernel.org
Subject: Re: [PATCH v1] driver core: Extend deferred probe timeout on driver
 registration
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On Fri, May 20, 2022 at 04:49:48PM -0700, Saravana Kannan wrote:
> On Fri, May 20, 2022 at 4:30 PM Nathan Chancellor <nathan@kernel.org> wrote:
> >
> > Hi Saravana,
> >
> > On Fri, Apr 29, 2022 at 03:09:32PM -0700, Saravana Kannan wrote:
> > > The deferred probe timer that's used for this currently starts at
> > > late_initcall and runs for driver_deferred_probe_timeout seconds. The
> > > assumption being that all available drivers would be loaded and
> > > registered before the timer expires. This means, the
> > > driver_deferred_probe_timeout has to be pretty large for it to cover the
> > > worst case. But if we set the default value for it to cover the worst
> > > case, it would significantly slow down the average case. For this
> > > reason, the default value is set to 0.
> > >
> > > Also, with CONFIG_MODULES=y and the current default values of
> > > driver_deferred_probe_timeout=0 and fw_devlink=on, devices with missing
> > > drivers will cause their consumer devices to always defer their probes.
> > > This is because device links created by fw_devlink defer the probe even
> > > before the consumer driver's probe() is called.
> > >
> > > Instead of a fixed timeout, if we extend an unexpired deferred probe
> > > timer on every successful driver registration, with the expectation more
> > > modules would be loaded in the near future, then the default value of
> > > driver_deferred_probe_timeout only needs to be as long as the worst case
> > > time difference between two consecutive module loads.
> > >
> > > So let's implement that and set the default value to 10 seconds when
> > > CONFIG_MODULES=y.
> > >
> > > Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
> > > Cc: "Rafael J. Wysocki" <rjw@rjwysocki.net>
> > > Cc: Rob Herring <robh@kernel.org>
> > > Cc: Linus Walleij <linus.walleij@linaro.org>
> > > Cc: Will Deacon <will@kernel.org>
> > > Cc: Ulf Hansson <ulf.hansson@linaro.org>
> > > Cc: Kevin Hilman <khilman@kernel.org>
> > > Cc: Thierry Reding <treding@nvidia.com>
> > > Cc: Mark Brown <broonie@kernel.org>
> > > Cc: Pavel Machek <pavel@ucw.cz>
> > > Cc: Geert Uytterhoeven <geert@linux-m68k.org>
> > > Cc: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
> > > Cc: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
> > > Cc: linux-gpio@vger.kernel.org
> > > Cc: linux-pm@vger.kernel.org
> > > Cc: iommu@lists.linux-foundation.org
> > > Signed-off-by: Saravana Kannan <saravanak@google.com>
> >
> > I bisected a boot hang with ARCH=s390 defconfig in QEMU down to this
> > change as commit 2b28a1a84a0e ("driver core: Extend deferred probe
> > timeout on driver registration") in next-20220520 (bisect log below).
> >
> > $ make -skj"$(nproc)" ARCH=s390 CROSS_COMPILE=s390x-linux-gnu- defconfig bzImage
> >
> > $ timeout --foreground 15m stdbuf -oL -eL \
> > qemu-system-s390x \
> > -initrd ... \
> > -M s390-ccw-virtio \
> > -display none \
> > -kernel arch/s390/boot/bzImage \
> > -m 512m \
> > -nodefaults \
> > -serial mon:stdio
> > ...
> > [    2.077303] In-situ OAM (IOAM) with IPv6
> > [    2.077639] NET: Registered PF_PACKET protocol family
> > [    2.078063] bridge: filtering via arp/ip/ip6tables is no longer available by default. Update your scripts to load br_netfilter if you need this.
> > [    2.078795] Key type dns_resolver registered
> > [    2.079317] cio: Channel measurement facility initialized using format extended (mode autodetected)
> > [    2.081494] Discipline DIAG cannot be used without z/VM
> > [  260.626363] random: crng init done
> > qemu-system-s390x: terminating on signal 15 from pid 3815762 (timeout)
> >
> > We have a simple rootfs available if necessary:
> >
> > https://github.com/ClangBuiltLinux/boot-utils/raw/bc0d17785eb67f1edd0ee0a134970a807895f741/images/s390/rootfs.cpio.zst
> >
> > If there is any other information I can provide, please let me know!
> 
> Hmm... strange. Can you please try the following command line options
> and tell me which of these has the issue and which don't?

Sure thing!

> 1) deferred_probe_timeout=0

No issue.

> 2) deferred_probe_timeout=1
> 3) deferred_probe_timeout=300

Both of these appear to hang in the same way, I let each sit for five
minutes.

Cheers,
Nathan