Message-ID: <387b120b-d68a-45e8-b6ab-768cd95d11c2@roeck-us.net>
Date: Sun, 24 Nov 2024 16:48:40 -0800
From: Guenter Roeck <linux@...ck-us.net>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>, John Stultz <jstultz@...gle.com>,
	Anna-Maria Behnsen <anna-maria@...utronix.de>,
	Frederic Weisbecker <frederic@...nel.org>,
	Stephen Boyd <sboyd@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [patch 2/2] timekeeping: Always check for negative motion

Hi,

On Thu, Oct 31, 2024 at 01:04:08PM +0100, Thomas Gleixner wrote:
> clocksource_delta() has two variants. One with a check for negative motion,
> which is only selected by x86. This is a historic leftover as this function
> was previously used in the time getter hot paths.
> 
> Since 135225a363ae timekeeping_cycles_to_ns() has unconditional protection
> against this as a by-product of the protection against 64bit math overflow.
> 
> clocksource_delta() is only used in the clocksource watchdog and in
> timekeeping_advance(). The extra conditional there is not hurting anyone.
> 
> Remove the config option and unconditionally prevent negative motion of the
> readout.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

This patch causes the kuda-bmc qemu emulation to stall. Reverting it fixes
the problem.

Bisecting the problem was a bit difficult. I attached two bisect results.
The first is on mainline, the second from timers-core-2024-11-18 after
rebasing it on top of mainline. The second log shows how the problem is
introduced.

Guenter

---
bisect on mainline:

# bad: [06afb0f36106ecb839c5e2509905e68c1e2677de] Merge tag 'trace-v6.13' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace
# good: [adc218676eef25575469234709c2d87185ca223a] Linux 6.12
git bisect start 'HEAD' 'v6.12'
# bad: [6e95ef0258ff4ee23ae3b06bf6b00b33dbbd5ef7] Merge tag 'bpf-next-6.13' of git://git.kernel.org/pub/scm/linux/kernel/git/bpf/bpf-next
git bisect bad 6e95ef0258ff4ee23ae3b06bf6b00b33dbbd5ef7
# bad: [aad3a0d084513e811233ad48bf234fbfcfcd0a14] Merge tag 'ftrace-v6.13' of git://git.kernel.org/pub/scm/linux/kernel/git/trace/linux-trace
git bisect bad aad3a0d084513e811233ad48bf234fbfcfcd0a14
# good: [ba1f9c8fe3d443a78814cdf8ac8f9829b5ca7095] Merge tag 'arm64-upstream' of git://git.kernel.org/pub/scm/linux/kernel/git/arm64/linux
git bisect good ba1f9c8fe3d443a78814cdf8ac8f9829b5ca7095
# good: [9d7d4ad222aea8ab482e78858d03b10221c7fe78] Merge tag 'objtool-core-2024-11-18' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
git bisect good 9d7d4ad222aea8ab482e78858d03b10221c7fe78
# good: [035238752319a58244d86facd442c5f40b0e97e2] Merge tag 'timers-vdso-2024-11-18' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
git bisect good 035238752319a58244d86facd442c5f40b0e97e2
# bad: [e5cfc0989d9a2849c51c720a16b90b2c061a1aeb] clocksource/drivers/timer-ti-dm: Fix child node refcount handling
git bisect bad e5cfc0989d9a2849c51c720a16b90b2c061a1aeb
# bad: [147ba943024e564e89d9ac265d6a07a0d2c03988] timekeeping: Merge timekeeping_update_staged() and timekeeping_update()
git bisect bad 147ba943024e564e89d9ac265d6a07a0d2c03988
# bad: [f36eb171410839325fff9cd9b7b7400f7e606962] timers: Update function descriptions of sleep/delay related functions
git bisect bad f36eb171410839325fff9cd9b7b7400f7e606962
# bad: [9d7130dfc0e1c53112fcbed4b9f566d0f6fbc949] ntp: Move pps_jitter into ntp_data
git bisect bad 9d7130dfc0e1c53112fcbed4b9f566d0f6fbc949
# bad: [ec93ec22aa10fb5311c0f068ee66c5b6d39788fe] ntp: Move tick_length* into ntp_data
git bisect bad ec93ec22aa10fb5311c0f068ee66c5b6d39788fe
# bad: [38007dc032bd90920463c5d2e6a27d89f7617d6d] ntp: Cleanup formatting of code
git bisect bad 38007dc032bd90920463c5d2e6a27d89f7617d6d
# bad: [66606a93849bfe3cbe9f0b801b40f60b87c54e11] ntp: Make tick_usec static
git bisect bad 66606a93849bfe3cbe9f0b801b40f60b87c54e11
# bad: [a849a0273d0f73a252d14d31c5003a8059ea51fc] ntp: Remove unused tick_nsec
git bisect bad a849a0273d0f73a252d14d31c5003a8059ea51fc
# first bad commit: [a849a0273d0f73a252d14d31c5003a8059ea51fc] ntp: Remove unused tick_nsec

---
bisect on timers-core-2024-11-18 after rebasing it on top of mainline
prior to the merge:

# bad: [997be8ee5090ece72d32e0386400a3e0046d29c7] posix-timers: Fix spurious warning on double enqueue versus do_exit()
# good: [035238752319a58244d86facd442c5f40b0e97e2] Merge tag 'timers-vdso-2024-11-18' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
git bisect start 'HEAD' 'bf9aa14fc523~1'
# good: [820128b09e6443aa6e2fc4efa1488e6c6c9d41d6] signal: Confine POSIX_TIMERS properly
git bisect good 820128b09e6443aa6e2fc4efa1488e6c6c9d41d6
# bad: [3349e563d11aa9fdb957be9ccd1a3f0db771a748] signal: Provide ignored_posix_timers list
git bisect bad 3349e563d11aa9fdb957be9ccd1a3f0db771a748
# good: [6e882ecdf76efe3846d3d327c5e17f7ca1252397] clocksource/drivers/timer-tegra: Remove clockevents shutdown call on offlining
git bisect good 6e882ecdf76efe3846d3d327c5e17f7ca1252397
# bad: [5c6b73e3bf36c30005948f072e9d5d351db4d7e0] posix-timers: Add a refcount to struct k_itimer
git bisect bad 5c6b73e3bf36c30005948f072e9d5d351db4d7e0
# bad: [6ab61583280a183828024101549d69b28ca1be9e] posix-cpu-timers: Correctly update timer status in posix_cpu_timer_del()
git bisect bad 6ab61583280a183828024101549d69b28ca1be9e
# good: [1c4cf0acb9a94d3d22ae24ba5a22b9892bf6d025] timekeeping: Remove CONFIG_DEBUG_TIMEKEEPING
git bisect good 1c4cf0acb9a94d3d22ae24ba5a22b9892bf6d025
# bad: [cc9811d2b494c3e12bab01a79be8f2a0c16b32c8] timekeeping: Always check for negative motion
git bisect bad cc9811d2b494c3e12bab01a79be8f2a0c16b32c8
# first bad commit: [cc9811d2b494c3e12bab01a79be8f2a0c16b32c8] timekeeping: Always check for negative motion

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