linux-kernel - Re: [RFC PATCH] clocksource: Suspend the watchdog temporarily when high read lantency detected

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Date:   Wed, 21 Dec 2022 21:55:15 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Feng Tang <feng.tang@...el.com>, John Stultz <jstultz@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Stephen Boyd <sboyd@...nel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        linux-kernel@...r.kernel.org, Tim Chen <tim.c.chen@...el.com>
Subject: Re: [RFC PATCH] clocksource: Suspend the watchdog temporarily when
 high read lantency detected

On Wed, Dec 21, 2022 at 10:39:53PM -0500, Waiman Long wrote:
> On 12/21/22 19:40, Paul E. McKenney wrote:
> > commit 199dfa2ba23dd0d650b1482a091e2e15457698b7
> > Author: Paul E. McKenney<paulmck@...nel.org>
> > Date:   Wed Dec 21 16:20:25 2022 -0800
> > 
> >      clocksource: Verify HPET and PMTMR when TSC unverified
> >      On systems with two or fewer sockets, when the boot CPU has CONSTANT_TSC,
> >      NONSTOP_TSC, and TSC_ADJUST, clocksource watchdog verification of the
> >      TSC is disabled.  This works well much of the time, but there is the
> >      occasional system that meets all of these criteria, but which still
> >      has a TSC that skews significantly from atomic-clock time.  This is
> >      usually attributed to a firmware or hardware fault.  Yes, the various
> >      NTP daemons do express their opinions of userspace-to-atomic-clock time
> >      skew, but they put them in various places, depending on the daemon and
> >      distro in question.  It would therefore be good for the kernel to have
> >      some clue that there is a problem.
> >      The old behavior of marking the TSC unstable is a non-starter because a
> >      great many workloads simply cannot tolerate the overheads and latencies
> >      of the various non-TSC clocksources.  In addition, NTP-corrected systems
> >      often seem to be able to tolerate significant kernel-space time skew as
> >      long as the userspace time sources are within epsilon of atomic-clock
> >      time.
> >      Therefore, when watchdog verification of TSC is disabled, enable it for
> >      HPET and PMTMR (AKA ACPI PM timer).  This provides the needed in-kernel
> >      time-skew diagnostic without degrading the system's performance.
> >      Signed-off-by: Paul E. McKenney<paulmck@...nel.org>
> >      Cc: Thomas Gleixner<tglx@...utronix.de>
> >      Cc: Ingo Molnar<mingo@...hat.com>
> >      Cc: Borislav Petkov<bp@...en8.de>
> >      Cc: Dave Hansen<dave.hansen@...ux.intel.com>
> >      Cc: "H. Peter Anvin"<hpa@...or.com>
> >      Cc: Daniel Lezcano<daniel.lezcano@...aro.org>
> >      Cc: Feng Tang<feng.tang@...el.com>
> >      Cc: Waiman Long <longman@...hat.com
> >      Cc:<x86@...nel.org>
> 
> As I currently understand, you are trying to use TSC as a watchdog to check
> against HPET and PMTMR. I do have 2 questions about this patch.
> 
> First of all, why you need to use both HPET and PMTMR? Can you just use one
> of those that are available. Secondly, is it possible to enable this
> time-skew diagnostic for a limit amount of time instead running
> indefinitely? The running of the clocksource watchdog itself will still
> consume a tiny amount of CPU cycles.

I could certainly do something so that only the first of HPET and PMTMR
is checked.  Could you give me a quick run-through of the advantages of
using only one?  I would need to explain that in the commit log.

Would it make sense to have a kernel boot variable giving the number of
minutes for which the watchdog was to run, with a default of zero
meaning "indefinitely"?

							Thanx, Paul