Commit 7edaeb68 authored by Thomas Gleixner's avatar Thomas Gleixner
Browse files

kernel/watchdog: Prevent false positives with turbo modes 



The hardlockup detector on x86 uses a performance counter based on unhalted
CPU cycles and a periodic hrtimer. The hrtimer period is about 2/5 of the
performance counter period, so the hrtimer should fire 2-3 times before the
performance counter NMI fires. The NMI code checks whether the hrtimer
fired since the last invocation. If not, it assumess a hard lockup.

The calculation of those periods is based on the nominal CPU
frequency. Turbo modes increase the CPU clock frequency and therefore
shorten the period of the perf/NMI watchdog. With extreme Turbo-modes (3x
nominal frequency) the perf/NMI period is shorter than the hrtimer period
which leads to false positives.

A simple fix would be to shorten the hrtimer period, but that comes with
the side effect of more frequent hrtimer and softlockup thread wakeups,
which is not desired.

Implement a low pass filter, which checks the perf/NMI period against
kernel time. If the perf/NMI fires before 4/5 of the watchdog period has
elapsed then the event is ignored and postponed to the next perf/NMI.

That solves the problem and avoids the overhead of shorter hrtimer periods
and more frequent softlockup thread wakeups.

Fixes: 58687acb ("lockup_detector: Combine nmi_watchdog and softlockup detector")
Reported-and-tested-by: default avatarKan Liang <Kan.liang@intel.com>
Signed-off-by: default avatarThomas Gleixner <tglx@linutronix.de>
Cc: dzickus@redhat.com
Cc: prarit@redhat.com
Cc: ak@linux.intel.com
Cc: babu.moger@oracle.com
Cc: peterz@infradead.org
Cc: eranian@google.com
Cc: acme@redhat.com
Cc: stable@vger.kernel.org
Cc: atomlin@redhat.com
Cc: akpm@linux-foundation.org
Cc: torvalds@linux-foundation.org
Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1708150931310.1886@nanos
parent ef954844

arch/x86/Kconfig

+1 −0
Original line number Diff line number Diff line
@@ -100,6 +100,7 @@ config X86 
	select GENERIC_STRNCPY_FROM_USER

	select GENERIC_STRNLEN_USER

	select GENERIC_TIME_VSYSCALL

	select HARDLOCKUP_CHECK_TIMESTAMP	if X86_64

	select HAVE_ACPI_APEI			if ACPI

	select HAVE_ACPI_APEI_NMI		if ACPI

	select HAVE_ALIGNED_STRUCT_PAGE		if SLUB

include/linux/nmi.h

+8 −0
Original line number Diff line number Diff line
@@ -168,6 +168,14 @@ extern int sysctl_hardlockup_all_cpu_backtrace; 
#define sysctl_softlockup_all_cpu_backtrace 0

#define sysctl_hardlockup_all_cpu_backtrace 0

#endif



#if defined(CONFIG_HARDLOCKUP_CHECK_TIMESTAMP) && \

    defined(CONFIG_HARDLOCKUP_DETECTOR)

void watchdog_update_hrtimer_threshold(u64 period);

#else

static inline void watchdog_update_hrtimer_threshold(u64 period) { }

#endif



extern bool is_hardlockup(void);

struct ctl_table;

extern int proc_watchdog(struct ctl_table *, int ,

kernel/watchdog.c

+1 −0
Original line number Diff line number Diff line
@@ -240,6 +240,7 @@ static void set_sample_period(void) 
	 * hardlockup detector generates a warning

	 */

	sample_period = get_softlockup_thresh() * ((u64)NSEC_PER_SEC / 5);

	watchdog_update_hrtimer_threshold(sample_period);

}



/* Commands for resetting the watchdog */

kernel/watchdog_hld.c

+59 −0
Original line number Diff line number Diff line
@@ -37,6 +37,62 @@ void arch_touch_nmi_watchdog(void) 
}

EXPORT_SYMBOL(arch_touch_nmi_watchdog);



#ifdef CONFIG_HARDLOCKUP_CHECK_TIMESTAMP

static DEFINE_PER_CPU(ktime_t, last_timestamp);

static DEFINE_PER_CPU(unsigned int, nmi_rearmed);

static ktime_t watchdog_hrtimer_sample_threshold __read_mostly;



void watchdog_update_hrtimer_threshold(u64 period)

{

	/*

	 * The hrtimer runs with a period of (watchdog_threshold * 2) / 5

	 *

	 * So it runs effectively with 2.5 times the rate of the NMI

	 * watchdog. That means the hrtimer should fire 2-3 times before

	 * the NMI watchdog expires. The NMI watchdog on x86 is based on

	 * unhalted CPU cycles, so if Turbo-Mode is enabled the CPU cycles

	 * might run way faster than expected and the NMI fires in a

	 * smaller period than the one deduced from the nominal CPU

	 * frequency. Depending on the Turbo-Mode factor this might be fast

	 * enough to get the NMI period smaller than the hrtimer watchdog

	 * period and trigger false positives.

	 *

	 * The sample threshold is used to check in the NMI handler whether

	 * the minimum time between two NMI samples has elapsed. That

	 * prevents false positives.

	 *

	 * Set this to 4/5 of the actual watchdog threshold period so the

	 * hrtimer is guaranteed to fire at least once within the real

	 * watchdog threshold.

	 */

	watchdog_hrtimer_sample_threshold = period * 2;

}



static bool watchdog_check_timestamp(void)

{

	ktime_t delta, now = ktime_get_mono_fast_ns();



	delta = now - __this_cpu_read(last_timestamp);

	if (delta < watchdog_hrtimer_sample_threshold) {

		/*

		 * If ktime is jiffies based, a stalled timer would prevent

		 * jiffies from being incremented and the filter would look

		 * at a stale timestamp and never trigger.

		 */

		if (__this_cpu_inc_return(nmi_rearmed) < 10)

			return false;

	}

	__this_cpu_write(nmi_rearmed, 0);

	__this_cpu_write(last_timestamp, now);

	return true;

}

#else

static inline bool watchdog_check_timestamp(void)

{

	return true;

}

#endif



static struct perf_event_attr wd_hw_attr = {

	.type		= PERF_TYPE_HARDWARE,

	.config		= PERF_COUNT_HW_CPU_CYCLES,
@@ -61,6 +117,9 @@ static void watchdog_overflow_callback(struct perf_event *event, 
		return;

	}



	if (!watchdog_check_timestamp())

		return;



	/* check for a hardlockup

	 * This is done by making sure our timer interrupt

	 * is incrementing.  The timer interrupt should have

lib/Kconfig.debug

+7 −0
Original line number Diff line number Diff line
@@ -797,6 +797,13 @@ config HARDLOCKUP_DETECTOR_PERF 
	bool

	select SOFTLOCKUP_DETECTOR



#

# Enables a timestamp based low pass filter to compensate for perf based

# hard lockup detection which runs too fast due to turbo modes.

#

config HARDLOCKUP_CHECK_TIMESTAMP

	bool



#

# arch/ can define HAVE_HARDLOCKUP_DETECTOR_ARCH to provide their own hard

# lockup detector rather than the perf based detector.

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