linux-kernel - Re: [PATCH] tick: prefer a lower rating device only if it's CPU local device

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Date:   Tue, 3 Jul 2018 08:04:37 -0700
From:   Kevin Hilman <khilman@...libre.com>
To:     Sudeep Holla <sudeep.holla@....com>
Cc:     lkml <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>, fweisbec@...il.com,
        Arnd Bergmann <arnd@...db.de>,
        Martin Blumenstingl <martin.blumenstingl@...glemail.com>
Subject: Re: [PATCH] tick: prefer a lower rating device only if it's CPU local device

On Tue, Jul 3, 2018 at 3:54 AM Sudeep Holla <sudeep.holla@....com> wrote:
>
> On Mon, Jul 02, 2018 at 04:44:33PM -0700, Kevin Hilman wrote:
> > Hi Sudeep,
> >
> > On Wed, May 9, 2018 at 9:02 AM Sudeep Holla <sudeep.holla@....com> wrote:
> > >
> > > Checking the equality of cpumask for both new and old tick device doesn't
> > > ensure that it's CPU local device. This will cause issue if a low rating
> > > clockevent tick device is registered first followed by the registration
> > > of higher rating clockevent tick device.
> > >
> > > In such case, clockevents_released list will never get emptied as both
> > > the devices get selected as preferred one and we will loop forever in
> > > clockevents_notify_released.
> > >
> > > Cc: Frederic Weisbecker <fweisbec@...il.com>
> > > Cc: Thomas Gleixner <tglx@...utronix.de>
> > > Signed-off-by: Sudeep Holla <sudeep.holla@....com>
> >
> > I've got a arm32 board (meson8b-odroidc1) that's been failing in
> > kernelCI.org since the merge window (boot log[1]), and I finally got
> > around to bisecting it[2].  Unfortunately, the bisect pointed at a
> > merge commit, but with some trial and error (and a suggestion by Arnd)
> > I was able to test that revering $SUBJECT commit[3], my problem goes
> > away.
> >
>
> Interesting. Sorry for causing the regression.
>
> > Another interesting data point is that disabling SMP (either by
> > "nosmp" on the command-line or CONFIG_SMP=n) also makes the problem go
> > away, without needing to revert this patch.
> >
>
> I am not sure of nosmp, but with CONFIG_SMP=n, TICK_BROADCAST also gets
> disabled. dummy_timer won't be registered I assume.
>
> I am not sure if dummy_timer is selected as it's per_cpu but the rating
> is low anyways.

> > AFAICT, this platform, is using a single timer as a clocksource
> > ("amlogic,meson6-timer") which is not a per-CPU timer.
> >
>
> Yes that's what I could gather from DT. But this is A5 right ? It may
> have per CPU TWD(watchdof timer) but DT doesn't specify it, so should be
> fine.
>
> > I ran out of time to keep digging on this issue, and I'm still not
> > sure exactly what's going on, but I wanted to report it in case anyone
> > else has any ideas, and so we can hopefully get it fixed during the
> > -rc cycle.
> >
>
> From the log, it looks like the platform has booted to userspace. Any chance
> we can have a look at:
> $ grep "" /sys/devices/system/clock*/{broadcast,clock*}/{available,current}_*

In the failing case, it doesn't boot to a shell, so I can't do that,
but after I revert the patch, I have this:

/ # ls -l /sys/devices/system/clocksource
total 0
drwxr-xr-x    3 root     root             0 Jan  1 00:00 clocksource0
drwxr-xr-x    2 root     root             0 Jan  1 00:00 power
-rw-r--r--    1 root     root          4096 Jan  1 00:00 uevent
/ # cat /sys/devices/system/clocksource/clocksource0/available_clocksource
timer jiffies
/ # cat /sys/devices/system/clocksource/clocksource0/current_clocksource
timer

/ # cat /sys/devices/system/clockevents/broadcast/current_device
meson6_tick
/ # cat /sys/devices/system/clockevents/clockevent0/current_device
dummy_timer
/ # cat /sys/devices/system/clockevents/clockevent1/current_device
dummy_timer
/ # cat /sys/devices/system/clockevents/clockevent2/current_device
dummy_timer

Kevin