From: Miroslav Lichvar <mlichvar@redhat.com>
To: David Woodhouse <dwmw2@infradead.org>
Cc: "Richard Cochran" <richardcochran@gmail.com>,
"Wen Gu" <guwen@linux.alibaba.com>,
"Andrew Lunn" <andrew+netdev@lunn.ch>,
"David S. Miller" <davem@davemloft.net>,
"Eric Dumazet" <edumazet@google.com>,
"Jakub Kicinski" <kuba@kernel.org>,
"Paolo Abeni" <pabeni@redhat.com>,
"John Stultz" <jstultz@google.com>,
"Thomas Gleixner" <tglx@kernel.org>,
"Stephen Boyd" <sboyd@kernel.org>,
"Anna-Maria Behnsen" <anna-maria@linutronix.de>,
"Frederic Weisbecker" <frederic@kernel.org>,
"Shuah Khan" <shuah@kernel.org>,
"Peter Zijlstra" <peterz@infradead.org>,
"Thomas Weißschuh" <thomas.weissschuh@linutronix.de>,
"Arnd Bergmann" <arnd@arndb.de>,
"Julien Ridoux" <ridouxj@amazon.com>,
"Ryan Luu" <rluu@amazon.com>,
linux-kernel@vger.kernel.org,
"David Woodhouse" <dwmw@amazon.co.uk>
Subject: Re: [RFC PATCH v2 3/8] timekeeping: Clamp time_offset delta to prevent infinite tail
Date: Tue, 19 May 2026 15:25:57 +0200 [thread overview]
Message-ID: <agxk5WnCCcCEsz4Z@localhost> (raw)
In-Reply-To: <20260517220326.4625-4-dwmw2@infradead.org>
On Sun, May 17, 2026 at 10:25:40PM +0100, David Woodhouse wrote:
> From: David Woodhouse <dwmw@amazon.co.uk>
>
> ntp_offset_chunk() computes delta as time_offset >> (SHIFT_PLL +
> time_constant), which exponentially decays toward zero but never
> reaches it. This means time_offset asymptotically approaches zero
> without ever completing — the clock never fully converges.
That's how the NTP PLL was designed to work. It is an infinite impulse
response filter.
> Fix by clamping delta:
> - Minimum: 20ns/sec (NTP_OFFSET_DELTA_MIN), ensuring the tail
> converges in finite time
I don't think that is an acceptable change of the filter. The impact
could be measured on a sufficiently stable clock.
To me that looks like using a wrong tool for the job.
--
Miroslav Lichvar
next prev parent reply other threads:[~2026-05-19 13:26 UTC|newest]
Thread overview: 50+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-05-17 21:25 [RFC PATCH v2 0/8] timekeeping: Fix draft tracking precision and add feed-forward discipline via vmclock David Woodhouse
2026-05-17 21:25 ` [RFC PATCH v2 1/8] timekeeping: Remove xtime_remainder from ntp_error accumulation David Woodhouse
2026-05-17 21:25 ` [RFC PATCH v2 2/8] timekeeping: Account for clawback adjustment in ntp_error David Woodhouse
2026-05-19 1:59 ` John Stultz
2026-05-19 10:04 ` David Woodhouse
2026-05-19 19:28 ` John Stultz
2026-05-20 10:47 ` Miroslav Lichvar
2026-05-20 12:37 ` David Woodhouse
2026-05-17 21:25 ` [RFC PATCH v2 3/8] timekeeping: Clamp time_offset delta to prevent infinite tail David Woodhouse
2026-05-19 13:25 ` Miroslav Lichvar [this message]
2026-05-19 13:31 ` David Woodhouse
2026-05-19 14:17 ` Miroslav Lichvar
2026-05-19 15:06 ` David Woodhouse
2026-05-17 21:25 ` [RFC PATCH v2 4/8] timekeeping: Add absolute reference for feed-forward clock discipline David Woodhouse
2026-05-19 2:09 ` John Stultz
2026-05-19 11:07 ` David Woodhouse
2026-05-17 21:25 ` [RFC PATCH v2 5/8] ptp_vmclock: Feed reference to timekeeping for feed-forward discipline David Woodhouse
2026-05-17 21:25 ` [RFC PATCH v2 6/8] timekeeping: Guard against divide-by-zero in timekeeping_adjust David Woodhouse
2026-05-17 21:25 ` [RFC PATCH v2 7/8] timekeeping: Drive time_offset skew via per-tick ntp_error transfer David Woodhouse
2026-05-17 21:25 ` [RFC PATCH v2 8/8] WIP: kernel/time: Add /dev/vmclock_host miscdev David Woodhouse
2026-05-19 13:16 ` [RFC PATCH v2 0/8] timekeeping: Fix draft tracking precision and add feed-forward discipline via vmclock Miroslav Lichvar
2026-05-19 15:50 ` David Woodhouse
2026-05-20 10:39 ` Miroslav Lichvar
2026-05-20 12:21 ` David Woodhouse
2026-05-21 6:35 ` Miroslav Lichvar
2026-05-21 9:54 ` David Woodhouse
2026-05-25 8:08 ` Miroslav Lichvar
2026-05-25 9:14 ` David Woodhouse
2026-05-26 7:10 ` Miroslav Lichvar
2026-05-26 10:00 ` David Woodhouse
2026-05-27 7:46 ` Miroslav Lichvar
2026-05-27 12:28 ` David Woodhouse
2026-05-21 18:30 ` Thomas Gleixner
2026-05-21 21:06 ` David Woodhouse
2026-05-22 8:02 ` Thomas Gleixner
2026-05-22 10:01 ` David Woodhouse
2026-05-22 15:28 ` Thomas Gleixner
2026-05-22 16:23 ` David Woodhouse
2026-05-24 12:36 ` Thomas Gleixner
2026-05-24 13:13 ` David Woodhouse
2026-05-24 15:05 ` Thomas Gleixner
2026-05-25 8:06 ` Arthur Kiyanovski
2026-05-25 8:41 ` David Woodhouse
2026-05-26 14:12 ` Thomas Gleixner
2026-05-22 16:50 ` David Woodhouse
2026-05-24 15:15 ` Thomas Gleixner
2026-05-24 15:37 ` Thomas Gleixner
2026-05-24 15:48 ` Thomas Gleixner
2026-05-24 16:36 ` Thomas Gleixner
2026-05-24 16:42 ` David Woodhouse
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