linux-kernel - Re: [GIT PULL] isolation: 1Hz residual tick offloading v4

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Date:   Mon, 29 Jan 2018 02:10:26 +0100
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Luiz Capitulino <lcapitulino@...hat.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Chris Metcalf <cmetcalf@...lanox.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Christoph Lameter <cl@...ux.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Wanpeng Li <kernellwp@...il.com>,
        Mike Galbraith <efault@....de>, Rik van Riel <riel@...hat.com>
Subject: Re: [GIT PULL] isolation: 1Hz residual tick offloading v4

On Wed, Jan 24, 2018 at 10:46:08AM -0500, Luiz Capitulino wrote:
> On Fri, 19 Jan 2018 01:02:14 +0100
> Frederic Weisbecker <frederic@...nel.org> wrote:
> 
> > Ingo,
> > 
> > Please pull the sched/0hz-v2 branch that can be found at:
> > 
> > git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
> > 	sched/0hz-v2
> > 
> > HEAD: 9b14d5204490f9acd03998a5e406ecadb87cddba
> > 
> > Changes in v4:
> > 
> > * Remove the nohz_offload option, just stick with the existing interface,
> >   the change is transparent. Suggested by Luiz.
> > 
> > * Automatically pin workqueues to housekeepers.
> 
> I've been testing this series and the tick doesn't go completely away
> for me: it ticks at around 8 seconds interval.
> 
> I've debugged this down to the clocksource_watchdog() timer, which is
> created by clocksource_start_watchdog(). This timer cycles over all online
> CPUs. I couldn't find a way to disable it. It seems to be always enabled
> for x86 by CONFIG_CLOCKSOURCE_WATCHDOG since commit 6471b825c4.
> 
> Since the 1Hz tick offload worked for you, I must be missing a way
> to disable this timer or the kernel is thinking my CPU has unstable
> TSC (which it doesn't AFAIK).

It's beyond the scope of this patchset but indeed that's right, I run my
kernels with tsc=reliable because my CPUs don't have the TSC_RELIABLE flag.
That's the only way I found to shutdown the tick completely on my test
machine, otherwise I keep having that clocksource watchdog.

You can try "tsc=reliable" but that's at your own risks and it's hard
to tell what exactly are those risks depending on your CPU model (and
perhaps BIOS?).

You likely already had that watchdog timer before this patchset but didn't
notice because the 1Hz was a more frequent annoyance.

Thanks.