Re: [patch 2/2] timekeeping: Always check for negative motion

[Guenter Roeck]

On 11/27/24 14:08, John Stultz wrote:
> On Sun, Nov 24, 2024 at 4:48 PM Guenter Roeck <linux@xxxxxxxxxxxx> wrote:
> > On Thu, Oct 31, 2024 at 01:04:08PM +0100, Thomas Gleixner wrote:
> > > clocksource_delta() has two variants. One with a check for negative motion,
> > > which is only selected by x86. This is a historic leftover as this function
> > > was previously used in the time getter hot paths.
> > >
> > > Since 135225a363ae timekeeping_cycles_to_ns() has unconditional protection
> > > against this as a by-product of the protection against 64bit math overflow.
> > >
> > > clocksource_delta() is only used in the clocksource watchdog and in
> > > timekeeping_advance(). The extra conditional there is not hurting anyone.
> > >
> > > Remove the config option and unconditionally prevent negative motion of the
> > > readout.
> > >
> > > Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> >
> > This patch causes the kuda-bmc qemu emulation to stall. Reverting it fixes
> > the problem.
>
> I'm not familiar with kuda-bmc and I'm not finding too many details
> searching on it.

Sorry, I keep misspelling it. kudo-bmc.

> > From other qemu bmc reults I'm guessing this is an arm32 architecture?
Yes.

> Do you have any more details about where it's stalling?  Also dmesg
> details might help illuminate what clocksource was used, etc.
> I'm wondering if the clocksource mask value is incorrect for the
> clocksource being used here?

An example log is at [1]. It says

clocksource: npcm7xx-timer1: mask: 0xffffff max_cycles: 0xffffff, max_idle_ns: 597268854 ns
...
clocksource: jiffies: mask: 0xffffffff max_cycles: 0xffffffff, max_idle_ns: 19112604462750000 ns
...
clocksource: Switched to clocksource npcm7xx-timer1

I don't know where exactly it stalls; sometime after handover to userspace.
I'll be happy to do some more debugging, but you'll nee to let me know what
to look for.

Thanks,
Guenter

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[1] https://kerneltests.org/builders/qemu-arm-v7-master/builds/324/steps/qemubuildcommand/logs/stdio