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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>,
        paulmck@kernel.org
Cc:     John Stultz <jstultz@google.com>, Stephen Boyd <sboyd@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Sebastian Andrzej Siewior <bigeasy@linutronix.de>,
        x86@kernel.org, joel@joelfernandes.org
Subject: Re: [PATCH] clocksource: disable irq when holding watchdog_lock.
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Date:   Thu, 19 Oct 2023 14:14:34 +0200
Message-ID: <87r0lq3j2t.ffs@tglx>
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On Thu, Oct 19 2023 at 18:30, Tetsuo Handa wrote:
> On 2023/10/17 23:10, Paul E. McKenney wrote:
>>> But I'm hitting something different (but might be timer/scheduler related) problem.
>>> What config option would cause taking more than 2 minutes to bring up only 8 CPUs?
>>> (This environment is Oracle VM VirtualBox on Windows 11 host.)
>>>
>> The timing is about the same in both cases.  Does this happen in kernels
>> built with CONFIG_PROVE_LOCKING=n?
>
> Disabling all options in "Lock Debugging (spinlocks, mutexes, etc...)" does not help.
>
>> 
>> Either way, what mainline or -stable version is this?
>
> The kernel is latest commit of upstream linux.git tree.
> I'm seeing this slowdown (almost hung-up) problem for many releases,
> but nobody else seems to be reporting this problem.

Maybe because of this:
>>> (This environment is Oracle VM VirtualBox on Windows 11 host.)

> The only reliable workaround is to specify "nosmp" kernel command line option.

Which makes the kernel keep TSC as clocksource because there is no
synchronization problem between CPU0 and CPU0 :)

What is the fallback clocksource when the TSC is discarded on SMP?

From the dmesg snippets I assume it is the ACPI PMTIMER, which is I/O
port based and presumably takes a VMEXIT on every read and depending on
the emulation this might be slooooow.

So you could boot with "nosmp clocksource=acpi_pm" on the command line
and compare that against a "nosmp" boot.

If my assumption is right, then on UP the ACPI PM variant won't see the
massive slowdown SMP observes, but there should be still an observable
difference.

Thanks,

        tglx