From beaa1ffe551c330d8ea23de158432ecaad6c0410 Mon Sep 17 00:00:00 2001
From: Yunying Sun <yunying.sun@intel.com>
Date: Wed, 16 Nov 2022 16:22:21 +0800
Subject: [PATCH 1/8] clocksource: Print clocksource name when clocksource is
tested unstable
Some "TSC fall back to HPET" messages appear on systems having more than
2 NUMA nodes:
clocksource: timekeeping watchdog on CPU168: hpet read-back delay of 4296200ns, attempt 4, marking unstable
The "hpet" here is misleading the clocksource watchdog is really
doing repeated reads of "hpet" in order to check for unrelated delays.
Therefore, print the name of the clocksource under test, prefixed by
"wd-" and suffixed by "-wd", for example, "wd-tsc-wd".
Signed-off-by: Yunying Sun <yunying.sun@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/time/clocksource.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 9cf32ccda715d..4a2c3bb92e2e9 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -257,8 +257,8 @@ static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow,
goto skip_test;
}
- pr_warn("timekeeping watchdog on CPU%d: %s read-back delay of %lldns, attempt %d, marking unstable\n",
- smp_processor_id(), watchdog->name, wd_delay, nretries);
+ pr_warn("timekeeping watchdog on CPU%d: wd-%s-wd read-back delay of %lldns, attempt %d, marking unstable\n",
+ smp_processor_id(), cs->name, wd_delay, nretries);
return WD_READ_UNSTABLE;
skip_test:
From c37e85c135cead4256dc8860073c468d8925c3df Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paulmck@kernel.org>
Date: Tue, 6 Dec 2022 19:36:10 -0800
Subject: [PATCH 2/8] clocksource: Loosen clocksource watchdog constraints
Currently, MAX_SKEW_USEC is set to 100 microseconds, which has worked
reasonably well. However, NTP is willing to tolerate 500 microseconds
of skew per second, and a clocksource that is good enough for NTP should
be good enough for the clocksource watchdog. The watchdog's skew is
controlled by MAX_SKEW_USEC and the CLOCKSOURCE_WATCHDOG_MAX_SKEW_US
Kconfig option. However, these values are doubled before being associated
with a clocksource's ->uncertainty_margin, and the ->uncertainty_margin
values of the pair of clocksource's being compared are summed before
checking against the skew.
Therefore, set both MAX_SKEW_USEC and the default for the
CLOCKSOURCE_WATCHDOG_MAX_SKEW_US Kconfig option to 125 microseconds of
skew per second, resulting in 500 microseconds of skew per second in
the clocksource watchdog's skew comparison.
Suggested-by Rik van Riel <riel@surriel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/time/Kconfig | 6 +++++-
kernel/time/clocksource.c | 15 +++++++++------
2 files changed, 14 insertions(+), 7 deletions(-)
diff --git a/kernel/time/Kconfig b/kernel/time/Kconfig
index a41753be1a2bf..bae8f11070bef 100644
--- a/kernel/time/Kconfig
+++ b/kernel/time/Kconfig
@@ -200,10 +200,14 @@ config CLOCKSOURCE_WATCHDOG_MAX_SKEW_US
int "Clocksource watchdog maximum allowable skew (in μs)"
depends on CLOCKSOURCE_WATCHDOG
range 50 1000
- default 100
+ default 125
help
Specify the maximum amount of allowable watchdog skew in
microseconds before reporting the clocksource to be unstable.
+ The default is based on a half-second clocksource watchdog
+ interval and NTP's maximum frequency drift of 500 parts
+ per million. If the clocksource is good enough for NTP,
+ it is good enough for the clocksource watchdog!
endmenu
endif
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index 4a2c3bb92e2e9..a3d19f6660ac7 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -95,6 +95,11 @@ static char override_name[CS_NAME_LEN];
static int finished_booting;
static u64 suspend_start;
+/*
+ * Interval: 0.5sec.
+ */
+#define WATCHDOG_INTERVAL (HZ >> 1)
+
/*
* Threshold: 0.0312s, when doubled: 0.0625s.
* Also a default for cs->uncertainty_margin when registering clocks.
@@ -106,11 +111,14 @@ static u64 suspend_start;
* clocksource surrounding a read of the clocksource being validated.
* This delay could be due to SMIs, NMIs, or to VCPU preemptions. Used as
* a lower bound for cs->uncertainty_margin values when registering clocks.
+ *
+ * The default of 500 parts per million is based on NTP's limits.
+ * If a clocksource is good enough for NTP, it is good enough for us!
*/
#ifdef CONFIG_CLOCKSOURCE_WATCHDOG_MAX_SKEW_US
#define MAX_SKEW_USEC CONFIG_CLOCKSOURCE_WATCHDOG_MAX_SKEW_US
#else
-#define MAX_SKEW_USEC 100
+#define MAX_SKEW_USEC (125 * WATCHDOG_INTERVAL / HZ)
#endif
#define WATCHDOG_MAX_SKEW (MAX_SKEW_USEC * NSEC_PER_USEC)
@@ -140,11 +148,6 @@ static inline void clocksource_watchdog_unlock(unsigned long *flags)
static int clocksource_watchdog_kthread(void *data);
static void __clocksource_change_rating(struct clocksource *cs, int rating);
-/*
- * Interval: 0.5sec.
- */
-#define WATCHDOG_INTERVAL (HZ >> 1)
-
static void clocksource_watchdog_work(struct work_struct *work)
{
/*
From f092eb34b33043152bfb8a4ca01db9a06728261d Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paulmck@kernel.org>
Date: Tue, 13 Dec 2022 13:57:28 -0800
Subject: [PATCH 3/8] clocksource: Improve read-back-delay message
When cs_watchdog_read() is unable to get a qualifying clocksource read
within the limit set by max_cswd_read_retries, it prints a message
and marks the clocksource under test as unstable. But that message is
unclear to anyone unfamiliar with the code:
clocksource: timekeeping watchdog on CPU13: wd-tsc-wd read-back delay 1000614ns, attempt 3, marking unstable
Therefore, add some context so that the message appears as follows:
clocksource: timekeeping watchdog on CPU13: wd-tsc-wd excessive read-back delay of 1000614ns vs. limit of 125000ns, wd-wd read-back delay only 27ns, attempt 3, marking tsc unstable
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: John Stultz <jstultz@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Feng Tang <feng.tang@intel.com>
---
kernel/time/clocksource.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index a3d19f6660ac7..b59914953809f 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -260,8 +260,8 @@ static enum wd_read_status cs_watchdog_read(struct clocksource *cs, u64 *csnow,
goto skip_test;
}
- pr_warn("timekeeping watchdog on CPU%d: wd-%s-wd read-back delay of %lldns, attempt %d, marking unstable\n",
- smp_processor_id(), cs->name, wd_delay, nretries);
+ pr_warn("timekeeping watchdog on CPU%d: wd-%s-wd excessive read-back delay of %lldns vs. limit of %ldns, wd-wd read-back delay only %lldns, attempt %d, marking %s unstable\n",
+ smp_processor_id(), cs->name, wd_delay, WATCHDOG_MAX_SKEW, wd_seq_delay, nretries, cs->name);
return WD_READ_UNSTABLE;
skip_test:
From dd029269947a32047b8ce1f8513b0b3b13f0df32 Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paulmck@kernel.org>
Date: Tue, 13 Dec 2022 16:42:15 -0800
Subject: [PATCH 4/8] clocksource: Improve "skew is too large" messages
When clocksource_watchdog() detects excessive clocksource skew compared
to the watchdog clocksource, it marks the clocksource under test as
unstable and prints several lines worth of message. But that message
is unclear to anyone unfamiliar with the code:
clocksource: timekeeping watchdog on CPU2: Marking clocksource 'wdtest-ktime' as unstable because the skew is too large:
clocksource: 'kvm-clock' wd_nsec: 400744390 wd_now: 612625c2c wd_last: 5fa7f7c66 mask: ffffffffffffffff
clocksource: 'wdtest-ktime' cs_nsec: 600744034 cs_now: 173081397a292d4f cs_last: 17308139565a8ced mask: ffffffffffffffff
clocksource: 'kvm-clock' (not 'wdtest-ktime') is current clocksource.
Therefore, add the following line near the end of that message:
Clocksource 'wdtest-ktime' skewed 199999644 ns (199 ms) over watchdog 'kvm-clock' interval of 400744390 ns (400 ms)
This new line clearly indicates the amount of skew between the two
clocksources, along with the duration of the time interval over which
the skew occurred, both in nanoseconds and milliseconds.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: John Stultz <jstultz@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Feng Tang <feng.tang@intel.com>
---
kernel/time/clocksource.c | 8 ++++++++
1 file changed, 8 insertions(+)
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index b59914953809f..fc486cd972635 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -446,12 +446,20 @@ static void clocksource_watchdog(struct timer_list *unused)
/* Check the deviation from the watchdog clocksource. */
md = cs->uncertainty_margin + watchdog->uncertainty_margin;
if (abs(cs_nsec - wd_nsec) > md) {
+ u64 cs_wd_msec;
+ u64 wd_msec;
+ u32 wd_rem;
+
pr_warn("timekeeping watchdog on CPU%d: Marking clocksource '%s' as unstable because the skew is too large:\n",
smp_processor_id(), cs->name);
pr_warn(" '%s' wd_nsec: %lld wd_now: %llx wd_last: %llx mask: %llx\n",
watchdog->name, wd_nsec, wdnow, wdlast, watchdog->mask);
pr_warn(" '%s' cs_nsec: %lld cs_now: %llx cs_last: %llx mask: %llx\n",
cs->name, cs_nsec, csnow, cslast, cs->mask);
+ cs_wd_msec = div_u64_rem(cs_nsec - wd_nsec, 1000U * 1000U, &wd_rem);
+ wd_msec = div_u64_rem(wd_nsec, 1000U * 1000U, &wd_rem);
+ pr_warn(" Clocksource '%s' skewed %lld ns (%lld ms) over watchdog '%s' interval of %lld ns (%lld ms)\n",
+ cs->name, cs_nsec - wd_nsec, cs_wd_msec, watchdog->name, wd_nsec, wd_msec);
if (curr_clocksource == cs)
pr_warn(" '%s' is current clocksource.\n", cs->name);
else if (curr_clocksource)
From b7082cdfc464bf9231300605d03eebf943dda307 Mon Sep 17 00:00:00 2001
From: Feng Tang <feng.tang@intel.com>
Date: Tue, 20 Dec 2022 16:25:12 +0800
Subject: [PATCH 5/8] clocksource: Suspend the watchdog temporarily when high
read latency detected
Bugs have been reported on 8 sockets x86 machines in which the TSC was
wrongly disabled when the system is under heavy workload.
[ 818.380354] clocksource: timekeeping watchdog on CPU336: hpet wd-wd read-back delay of 1203520ns
[ 818.436160] clocksource: wd-tsc-wd read-back delay of 181880ns, clock-skew test skipped!
[ 819.402962] clocksource: timekeeping watchdog on CPU338: hpet wd-wd read-back delay of 324000ns
[ 819.448036] clocksource: wd-tsc-wd read-back delay of 337240ns, clock-skew test skipped!
[ 819.880863] clocksource: timekeeping watchdog on CPU339: hpet read-back delay of 150280ns, attempt 3, marking unstable
[ 819.936243] tsc: Marking TSC unstable due to clocksource watchdog
[ 820.068173] TSC found unstable after boot, most likely due to broken BIOS. Use 'tsc=unstable'.
[ 820.092382] sched_clock: Marking unstable (818769414384, 1195404998)
[ 820.643627] clocksource: Checking clocksource tsc synchronization from CPU 267 to CPUs 0,4,25,70,126,430,557,564.
[ 821.067990] clocksource: Switched to clocksource hpet
This can be reproduced by running memory intensive 'stream' tests,
or some of the stress-ng subcases such as 'ioport'.
The reason for these issues is the when system is under heavy load, the
read latency of the clocksources can be very high. Even lightweight TSC
reads can show high latencies, and latencies are much worse for external
clocksources such as HPET or the APIC PM timer. These latencies can
result in false-positive clocksource-unstable determinations.
These issues were initially reported by a customer running on a production
system, and this problem was reproduced on several generations of Xeon
servers, especially when running the stress-ng test. These Xeon servers
were not production systems, but they did have the latest steppings
and firmware.
Given that the clocksource watchdog is a continual diagnostic check with
frequency of twice a second, there is no need to rush it when the system
is under heavy load. Therefore, when high clocksource read latencies
are detected, suspend the watchdog timer for 5 minutes.
Signed-off-by: Feng Tang <feng.tang@intel.com>
Acked-by: Waiman Long <longman@redhat.com>
Cc: John Stultz <jstultz@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Stephen Boyd <sboyd@kernel.org>
Cc: Feng Tang <feng.tang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/time/clocksource.c | 45 ++++++++++++++++++++++++++++-----------
1 file changed, 32 insertions(+), 13 deletions(-)
diff --git a/kernel/time/clocksource.c b/kernel/time/clocksource.c
index fc486cd972635..91836b727cef5 100644
--- a/kernel/time/clocksource.c
+++ b/kernel/time/clocksource.c
@@ -387,6 +387,15 @@ void clocksource_verify_percpu(struct clocksource *cs)
}
EXPORT_SYMBOL_GPL(clocksource_verify_percpu);
+static inline void clocksource_reset_watchdog(void)
+{
+ struct clocksource *cs;
+
+ list_for_each_entry(cs, &watchdog_list, wd_list)
+ cs->flags &= ~CLOCK_SOURCE_WATCHDOG;
+}
+
+
static void clocksource_watchdog(struct timer_list *unused)
{
u64 csnow, wdnow, cslast, wdlast, delta;
@@ -394,6 +403,7 @@ static void clocksource_watchdog(struct timer_list *unused)
int64_t wd_nsec, cs_nsec;
struct clocksource *cs;
enum wd_read_status read_ret;
+ unsigned long extra_wait = 0;
u32 md;
spin_lock(&watchdog_lock);
@@ -413,13 +423,30 @@ static void clocksource_watchdog(struct timer_list *unused)
read_ret = cs_watchdog_read(cs, &csnow, &wdnow);
- if (read_ret != WD_READ_SUCCESS) {
- if (read_ret == WD_READ_UNSTABLE)
- /* Clock readout unreliable, so give it up. */
- __clocksource_unstable(cs);
+ if (read_ret == WD_READ_UNSTABLE) {
+ /* Clock readout unreliable, so give it up. */
+ __clocksource_unstable(cs);
continue;
}
+ /*
+ * When WD_READ_SKIP is returned, it means the system is likely
+ * under very heavy load, where the latency of reading
+ * watchdog/clocksource is very big, and affect the accuracy of
+ * watchdog check. So give system some space and suspend the
+ * watchdog check for 5 minutes.
+ */
+ if (read_ret == WD_READ_SKIP) {
+ /*
+ * As the watchdog timer will be suspended, and
+ * cs->last could keep unchanged for 5 minutes, reset
+ * the counters.
+ */
+ clocksource_reset_watchdog();
+ extra_wait = HZ * 300;
+ break;
+ }
+
/* Clocksource initialized ? */
if (!(cs->flags & CLOCK_SOURCE_WATCHDOG) ||
atomic_read(&watchdog_reset_pending)) {
@@ -523,7 +550,7 @@ static void clocksource_watchdog(struct timer_list *unused)
* pair clocksource_stop_watchdog() clocksource_start_watchdog().
*/
if (!timer_pending(&watchdog_timer)) {
- watchdog_timer.expires += WATCHDOG_INTERVAL;
+ watchdog_timer.expires += WATCHDOG_INTERVAL + extra_wait;
add_timer_on(&watchdog_timer, next_cpu);
}
out:
@@ -548,14 +575,6 @@ static inline void clocksource_stop_watchdog(void)
watchdog_running = 0;
}
-static inline void clocksource_reset_watchdog(void)
-{
- struct clocksource *cs;
-
- list_for_each_entry(cs, &watchdog_list, wd_list)
- cs->flags &= ~CLOCK_SOURCE_WATCHDOG;
-}
-
static void clocksource_resume_watchdog(void)
{
atomic_inc(&watchdog_reset_pending);
From a7ec817d55421ac214cac9d3e5ebb65d848198dc Mon Sep 17 00:00:00 2001
From: Feng Tang <feng.tang@intel.com>
Date: Wed, 4 Jan 2023 16:19:38 +0800
Subject: [PATCH 6/8] x86/tsc: Add option to force frequency recalibration with
HW timer
The kernel assumes that the TSC frequency which is provided by the
hardware / firmware via MSRs or CPUID(0x15) is correct after applying
a few basic consistency checks. This disables the TSC recalibration
against HPET or PM timer.
As a result there is no mechanism to validate that frequency in cases
where a firmware or hardware defect is suspected. And there was case
that some user used atomic clock to measure the TSC frequency and
reported an inaccuracy issue, which was later fixed in firmware.
Add an option 'recalibrate' for 'tsc' kernel parameter to force the
tsc freq recalibration with HPET or PM timer, and warn if the
deviation from previous value is more than about 500 PPM, which
provides a way to verify the data from hardware / firmware.
There is no functional change to existing work flow.
Recently there was a real-world case: "The 40ms/s divergence between
TSC and HPET was observed on hardware that is quite recent" [1], on
that platform the TSC frequence 1896 MHz was got from CPUID(0x15),
and the force-reclibration with HPET/PMTIMER both calibrated out
value of 1975 MHz, which also matched with check from software
'chronyd', indicating it's a problem of BIOS or firmware.
[Thanks tglx for helping improving the commit log]
[ paulmck: Wordsmith Kconfig help text. ]
[1]. https://lore.kernel.org/lkml/20221117230910.GI4001@paulmck-ThinkPad-P17-Gen-1/
Signed-off-by: Feng Tang <feng.tang@intel.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: <x86@kernel.org>
Cc: <linux-doc@vger.kernel.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
.../admin-guide/kernel-parameters.txt | 4 +++
arch/x86/kernel/tsc.c | 34 ++++++++++++++++---
2 files changed, 34 insertions(+), 4 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 6cfa6e3996cf7..95f0d104c2322 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6369,6 +6369,10 @@
in situations with strict latency requirements (where
interruptions from clocksource watchdog are not
acceptable).
+ [x86] recalibrate: force recalibration against a HW timer
+ (HPET or PM timer) on systems whose TSC frequency was
+ obtained from HW or FW using either an MSR or CPUID(0x15).
+ Warn if the difference is more than 500 ppm.
tsc_early_khz= [X86] Skip early TSC calibration and use the given
value instead. Useful when the early TSC frequency discovery
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index a78e73da4a74b..92bbc4a6b3fc3 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -48,6 +48,8 @@ static DEFINE_STATIC_KEY_FALSE(__use_tsc);
int tsc_clocksource_reliable;
+static int __read_mostly tsc_force_recalibrate;
+
static u32 art_to_tsc_numerator;
static u32 art_to_tsc_denominator;
static u64 art_to_tsc_offset;
@@ -303,6 +305,8 @@ static int __init tsc_setup(char *str)
mark_tsc_unstable("boot parameter");
if (!strcmp(str, "nowatchdog"))
no_tsc_watchdog = 1;
+ if (!strcmp(str, "recalibrate"))
+ tsc_force_recalibrate = 1;
return 1;
}
@@ -1374,6 +1378,25 @@ static void tsc_refine_calibration_work(struct work_struct *work)
else
freq = calc_pmtimer_ref(delta, ref_start, ref_stop);
+ /* Will hit this only if tsc_force_recalibrate has been set */
+ if (boot_cpu_has(X86_FEATURE_TSC_KNOWN_FREQ)) {
+
+ /* Warn if the deviation exceeds 500 ppm */
+ if (abs(tsc_khz - freq) > (tsc_khz >> 11)) {
+ pr_warn("Warning: TSC freq calibrated by CPUID/MSR differs from what is calibrated by HW timer, please check with vendor!!\n");
+ pr_info("Previous calibrated TSC freq:\t %lu.%03lu MHz\n",
+ (unsigned long)tsc_khz / 1000,
+ (unsigned long)tsc_khz % 1000);
+ }
+
+ pr_info("TSC freq recalibrated by [%s]:\t %lu.%03lu MHz\n",
+ hpet ? "HPET" : "PM_TIMER",
+ (unsigned long)freq / 1000,
+ (unsigned long)freq % 1000);
+
+ return;
+ }
+
/* Make sure we're within 1% */
if (abs(tsc_khz - freq) > tsc_khz/100)
goto out;
@@ -1407,8 +1430,10 @@ static int __init init_tsc_clocksource(void)
if (!boot_cpu_has(X86_FEATURE_TSC) || !tsc_khz)
return 0;
- if (tsc_unstable)
- goto unreg;
+ if (tsc_unstable) {
+ clocksource_unregister(&clocksource_tsc_early);
+ return 0;
+ }
if (boot_cpu_has(X86_FEATURE_NONSTOP_TSC_S3))
clocksource_tsc.flags |= CLOCK_SOURCE_SUSPEND_NONSTOP;
@@ -1421,9 +1446,10 @@ static int __init init_tsc_clocksource(void)
if (boot_cpu_has(X86_FEATURE_ART))
art_related_clocksource = &clocksource_tsc;
clocksource_register_khz(&clocksource_tsc, tsc_khz);
-unreg:
clocksource_unregister(&clocksource_tsc_early);
- return 0;
+
+ if (!tsc_force_recalibrate)
+ return 0;
}
schedule_delayed_work(&tsc_irqwork, 0);
From efc8b329c7fdc30921a7dfc109237523e1e5b1cc Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paulmck@kernel.org>
Date: Wed, 21 Dec 2022 16:20:25 -0800
Subject: [PATCH 7/8] 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 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.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
Cc: Waiman Long <longman@redhat.com>
Cc: <x86@kernel.org>
Tested-by: Feng Tang <feng.tang@intel.com>
---
arch/x86/include/asm/time.h | 1 +
arch/x86/kernel/hpet.c | 2 ++
arch/x86/kernel/tsc.c | 5 +++++
drivers/clocksource/acpi_pm.c | 6 ++++--
4 files changed, 12 insertions(+), 2 deletions(-)
diff --git a/arch/x86/include/asm/time.h b/arch/x86/include/asm/time.h
index 8ac563abb567b..a53961c64a567 100644
--- a/arch/x86/include/asm/time.h
+++ b/arch/x86/include/asm/time.h
@@ -8,6 +8,7 @@
extern void hpet_time_init(void);
extern void time_init(void);
extern bool pit_timer_init(void);
+extern bool tsc_clocksource_watchdog_disabled(void);
extern struct clock_event_device *global_clock_event;
diff --git a/arch/x86/kernel/hpet.c b/arch/x86/kernel/hpet.c
index 71f336425e58a..c8eb1ac5125ab 100644
--- a/arch/x86/kernel/hpet.c
+++ b/arch/x86/kernel/hpet.c
@@ -1091,6 +1091,8 @@ int __init hpet_enable(void)
if (!hpet_counting())
goto out_nohpet;
+ if (tsc_clocksource_watchdog_disabled())
+ clocksource_hpet.flags |= CLOCK_SOURCE_MUST_VERIFY;
clocksource_register_hz(&clocksource_hpet, (u32)hpet_freq);
if (id & HPET_ID_LEGSUP) {
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 92bbc4a6b3fc3..a5371c6d4b64b 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1190,6 +1190,11 @@ static void __init tsc_disable_clocksource_watchdog(void)
clocksource_tsc.flags &= ~CLOCK_SOURCE_MUST_VERIFY;
}
+bool tsc_clocksource_watchdog_disabled(void)
+{
+ return !(clocksource_tsc.flags & CLOCK_SOURCE_MUST_VERIFY);
+}
+
static void __init check_system_tsc_reliable(void)
{
#if defined(CONFIG_MGEODEGX1) || defined(CONFIG_MGEODE_LX) || defined(CONFIG_X86_GENERIC)
diff --git a/drivers/clocksource/acpi_pm.c b/drivers/clocksource/acpi_pm.c
index 279ddff81ab49..82338773602ca 100644
--- a/drivers/clocksource/acpi_pm.c
+++ b/drivers/clocksource/acpi_pm.c
@@ -23,6 +23,7 @@
#include <linux/pci.h>
#include <linux/delay.h>
#include <asm/io.h>
+#include <asm/time.h>
/*
* The I/O port the PMTMR resides at.
@@ -210,8 +211,9 @@ static int __init init_acpi_pm_clocksource(void)
return -ENODEV;
}
- return clocksource_register_hz(&clocksource_acpi_pm,
- PMTMR_TICKS_PER_SEC);
+ if (tsc_clocksource_watchdog_disabled())
+ clocksource_acpi_pm.flags |= CLOCK_SOURCE_MUST_VERIFY;
+ return clocksource_register_hz(&clocksource_acpi_pm, PMTMR_TICKS_PER_SEC);
}
/* We use fs_initcall because we want the PCI fixups to have run
From 0051293c533017e2a860e0a0a33517bc40240fff Mon Sep 17 00:00:00 2001
From: "Paul E. McKenney" <paulmck@kernel.org>
Date: Wed, 1 Feb 2023 13:53:07 -0800
Subject: [PATCH 8/8] clocksource: Enable TSC watchdog checking of HPET and
PMTMR only when requested
Unconditionally enabling TSC watchdog checking of the HPET and PMTMR
clocksources can degrade latency and performance. Therefore, provide
a new "watchdog" option to the tsc= boot parameter that opts into such
checking. Note that tsc=watchdog is overridden by a tsc=nowatchdog
regardless of their relative positions in the list of boot parameters.
Reported-by: Thomas Gleixner <tglx@linutronix.de>
Reported-by: Waiman Long <longman@redhat.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Acked-by: Waiman Long <longman@redhat.com>
---
.../admin-guide/kernel-parameters.txt | 6 ++++++
arch/x86/kernel/tsc.c | 18 ++++++++++++++++--
2 files changed, 22 insertions(+), 2 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 95f0d104c2322..7b4df6d89d3c3 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -6373,6 +6373,12 @@
(HPET or PM timer) on systems whose TSC frequency was
obtained from HW or FW using either an MSR or CPUID(0x15).
Warn if the difference is more than 500 ppm.
+ [x86] watchdog: Use TSC as the watchdog clocksource with
+ which to check other HW timers (HPET or PM timer), but
+ only on systems where TSC has been deemed trustworthy.
+ This will be suppressed by an earlier tsc=nowatchdog and
+ can be overridden by a later tsc=nowatchdog. A console
+ message will flag any such suppression or overriding.
tsc_early_khz= [X86] Skip early TSC calibration and use the given
value instead. Useful when the early TSC frequency discovery
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index a5371c6d4b64b..306c233c98d84 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -294,6 +294,7 @@ __setup("notsc", notsc_setup);
static int no_sched_irq_time;
static int no_tsc_watchdog;
+static int tsc_as_watchdog;
static int __init tsc_setup(char *str)
{
@@ -303,10 +304,22 @@ static int __init tsc_setup(char *str)
no_sched_irq_time = 1;
if (!strcmp(str, "unstable"))
mark_tsc_unstable("boot parameter");
- if (!strcmp(str, "nowatchdog"))
+ if (!strcmp(str, "nowatchdog")) {
no_tsc_watchdog = 1;
+ if (tsc_as_watchdog)
+ pr_alert("%s: Overriding earlier tsc=watchdog with tsc=nowatchdog\n",
+ __func__);
+ tsc_as_watchdog = 0;
+ }
if (!strcmp(str, "recalibrate"))
tsc_force_recalibrate = 1;
+ if (!strcmp(str, "watchdog")) {
+ if (no_tsc_watchdog)
+ pr_alert("%s: tsc=watchdog overridden by earlier tsc=nowatchdog\n",
+ __func__);
+ else
+ tsc_as_watchdog = 1;
+ }
return 1;
}
@@ -1192,7 +1205,8 @@ static void __init tsc_disable_clocksource_watchdog(void)
bool tsc_clocksource_watchdog_disabled(void)
{
- return !(clocksource_tsc.flags & CLOCK_SOURCE_MUST_VERIFY);
+ return !(clocksource_tsc.flags & CLOCK_SOURCE_MUST_VERIFY) &&
+ tsc_as_watchdog && !no_tsc_watchdog;
}
static void __init check_system_tsc_reliable(void)