[PATCH v2] clocksource: tsc: respect tsc bootparam for clocksource_tsc_early

[PATCH v2] clocksource: tsc: respect tsc bootparam for clocksource_tsc_early

[Michael Zhivich]

Introduction of clocksource_tsc_early broke functionality of "tsc=reliable"
and "tsc=nowatchdog" bootparams, since clocksource_tsc_early is *always*
registered with CLOCK_SOURCE_MUST_VERIFY and thus put on the watchdog list.

This frequently causes TSC to be declared unstable during boot:

  clocksource: timekeeping watchdog on CPU0: Marking clocksource
               'tsc-early' as unstable because the skew is too large:
  clocksource: 'refined-jiffies' wd_now: fffb7018 wd_last: fffb6e9d 
               mask: ffffffff
  clocksource: 'tsc-early' cs_now: 68a6a7070f6a0 cs_last: 68a69ab6f74d6 
               mask: ffffffffffffffff
  tsc: Marking TSC unstable due to clocksource watchdog

The corresponding elapsed times are cs_nsec=1224152026 and wd_nsec=378942392, so
watchdog differs from TSC by 0.84 seconds.  Since jiffies is not guaranteed
to provide reliable timekeeping due to possibility of missing timer
interrupts, this is a false positive.

This issue affects machines running with HPET disabled; otherwise, HPET would
be the watchdog for tsc-early clocksource, and this situation would be avoided. 

Restore previous behavior by respecting "tsc=reliable" and "tsc=nowatchdog"
boot params when registering clocksource_tsc_early.

Fixes: aa83c45762a24 ("x86/tsc: Introduce early tsc clocksource")
Signed-off-by: Michael Zhivich <mzhivich@akamai.com>
---
 arch/x86/kernel/tsc.c | 3 +++
 1 file changed, 3 insertions(+)

 Changes from v1: updated commit message, added "Fixes" tag.

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index c59454c382fd..7e322e2daaf5 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1505,6 +1505,9 @@ void __init tsc_init(void)
 		return;
 	}
 
+	if (tsc_clocksource_reliable || no_tsc_watchdog)
+		clocksource_tsc_early.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
+
 	clocksource_register_khz(&clocksource_tsc_early, tsc_khz);
 	detect_art();
 }
-- 
2.17.1

[tip: x86/urgent] x86/tsc: Respect tsc command line paraemeter for clocksource_tsc_early

[tip-bot2 for Michael Zhivich]

The following commit has been merged into the x86/urgent branch of tip:

Commit-ID:     63ec58b44fcc05efd1542045abd7faf056ac27d9
Gitweb:        https://git.kernel.org/tip/63ec58b44fcc05efd1542045abd7faf056ac27d9
Author:        Michael Zhivich <mzhivich@akamai.com>
AuthorDate:    Thu, 24 Oct 2019 13:59:45 -04:00
Committer:     Thomas Gleixner <tglx@linutronix.de>
CommitterDate: Tue, 05 Nov 2019 01:24:56 +01:00

x86/tsc: Respect tsc command line paraemeter for clocksource_tsc_early

The introduction of clocksource_tsc_early broke the functionality of
"tsc=reliable" and "tsc=nowatchdog" command line parameters, since
clocksource_tsc_early is unconditionally registered with
CLOCK_SOURCE_MUST_VERIFY and thus put on the watchdog list.

This can cause the TSC to be declared unstable during boot:

  clocksource: timekeeping watchdog on CPU0: Marking clocksource
               'tsc-early' as unstable because the skew is too large:
  clocksource: 'refined-jiffies' wd_now: fffb7018 wd_last: fffb6e9d
               mask: ffffffff
  clocksource: 'tsc-early' cs_now: 68a6a7070f6a0 cs_last: 68a69ab6f74d6
               mask: ffffffffffffffff
  tsc: Marking TSC unstable due to clocksource watchdog

The corresponding elapsed times are cs_nsec=1224152026 and wd_nsec=378942392, so
the watchdog differs from TSC by 0.84 seconds.

This happens when HPET is not available and jiffies are used as the TSC
watchdog instead and the jiffies update is not happening due to lost timer
interrupts in periodic mode, which can happen e.g. with expensive debug
mechanisms enabled or under massive overload conditions in virtualized
environments.

Before the introduction of the early TSC clocksource the command line
parameters "tsc=reliable" and "tsc=nowatchdog" could be used to work around
this issue.

Restore the behaviour by disabling the watchdog if requested on the kernel
command line.

[ tglx: Clarify changelog ]

Fixes: aa83c45762a24 ("x86/tsc: Introduce early tsc clocksource")
Signed-off-by: Michael Zhivich <mzhivich@akamai.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20191024175945.14338-1-mzhivich@akamai.com
---
 arch/x86/kernel/tsc.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index c59454c..7e322e2 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1505,6 +1505,9 @@ void __init tsc_init(void)
 		return;
 	}
 
+	if (tsc_clocksource_reliable || no_tsc_watchdog)
+		clocksource_tsc_early.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
+
 	clocksource_register_khz(&clocksource_tsc_early, tsc_khz);
 	detect_art();
 }