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

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Date:   Wed, 21 Dec 2022 22:39:53 -0500
From:   Waiman Long <longman@...hat.com>
To:     paulmck@...nel.org
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 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.

Cheers,
Longman