From:   Thomas Gleixner <tglx@linutronix.de>
To:     "Paul E. McKenney" <paulmck@kernel.org>
Cc:     linux-kernel@vger.kernel.org, john.stultz@linaro.org,
        sboyd@kernel.org, corbet@lwn.net, Mark.Rutland@arm.com,
        maz@kernel.org, kernel-team@meta.com, neeraju@codeaurora.org,
        ak@linux.intel.com, feng.tang@intel.com, zhengjun.xing@intel.com,
        "Paul E. McKenney" <paulmck@kernel.org>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        Waiman Long <longman@redhat.com>, x86@kernel.org
Subject: Re: [PATCH v2 clocksource 6/7] clocksource: Verify HPET and PMTMR
 when TSC unverified
In-Reply-To: <20230125002730.1471349-6-paulmck@kernel.org>
References: <20230125002708.GA1471122@paulmck-ThinkPad-P17-Gen-1>
 <20230125002730.1471349-6-paulmck@kernel.org>
Date:   Wed, 01 Feb 2023 11:24:14 +0100
Message-ID: <87wn51znsh.ffs@tglx>
MIME-Version: 1.0
Content-Type: text/plain
Precedence: bulk

Paul!

On Tue, Jan 24 2023 at 16:27, Paul E. McKenney wrote:
> 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 production-level 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
> sometimes can 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.

I'm more than unhappy about this. We finally have a point where the TSC
watchdog overhead can go away without adding TSC=reliable to the kernel
commandline.

Now you add an unconditionally enforce the watchdog again in a way which
even cannot be disabled on the kernel command line.

Patently bad idea, no cookies for you!

Thanks,

        tglx