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[RFC PATCH 20/23] watchdog/hardlockup/hpet: Rotate interrupt among all monitored CPUs

In order to detect hardlockups in all the monitored CPUs, move the
interrupt to the next monitored CPU when handling the NMI interrupt; wrap
around when reaching the highest CPU in the mask. This rotation is achieved
by setting the affinity mask to only contain the next CPU to monitor.

In order to prevent our interrupt to be reassigned to another CPU, flag

The cpumask monitored_mask keeps track of the CPUs that the watchdog
should monitor. This structure is updated when the NMI watchdog is
enabled or disabled in a specific CPU. As this mask can change
concurrently as CPUs are put online or offline and the watchdog is
disabled or enabled, a lock is required to protect the monitored_mask.

Cc: Ashok Raj <ashok.raj@xxxxxxxxx>
Cc: Andi Kleen <andi.kleen@xxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Jacob Pan <jacob.jun.pan@xxxxxxxxx>
Cc: "Rafael J. Wysocki" <rafael.j.wysocki@xxxxxxxxx>
Cc: Don Zickus <dzickus@xxxxxxxxxx>
Cc: Nicholas Piggin <npiggin@xxxxxxxxx>
Cc: Michael Ellerman <mpe@xxxxxxxxxxxxxx>
Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
Cc: Babu Moger <babu.moger@xxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Philippe Ombredanne <pombredanne@xxxxxxxx>
Cc: Colin Ian King <colin.king@xxxxxxxxxxxxx>
Cc: Byungchul Park <byungchul.park@xxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: "Luis R. Rodriguez" <mcgrof@xxxxxxxxxx>
Cc: Waiman Long <longman@xxxxxxxxxx>
Cc: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
Cc: Randy Dunlap <rdunlap@xxxxxxxxxxxxx>
Cc: Davidlohr Bueso <dave@xxxxxxxxxxxx>
Cc: Christoffer Dall <cdall@xxxxxxxxxx>
Cc: Marc Zyngier <marc.zyngier@xxxxxxx>
Cc: Kai-Heng Feng <kai.heng.feng@xxxxxxxxxxxxx>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>
Cc: David Rientjes <rientjes@xxxxxxxxxx>
Cc: "Ravi V. Shankar" <ravi.v.shankar@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: iommu@xxxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@xxxxxxxxxxxxxxx>
 kernel/watchdog_hld_hpet.c | 28 ++++++++++++++++++++++++----
 1 file changed, 24 insertions(+), 4 deletions(-)

diff --git a/kernel/watchdog_hld_hpet.c b/kernel/watchdog_hld_hpet.c
index 857e051..c40acfd 100644
--- a/kernel/watchdog_hld_hpet.c
+++ b/kernel/watchdog_hld_hpet.c
@@ -10,6 +10,7 @@
 #include <linux/nmi.h>
 #include <linux/hpet.h>
 #include <asm/hpet.h>
+#include <asm/cpumask.h>
 #include <asm/irq_remapping.h>
 #undef pr_fmt
@@ -199,8 +200,8 @@ static irqreturn_t hardlockup_detector_irq_handler(int irq, void *data)
  * @regs:	Register values as seen when the NMI was asserted
  * When an NMI is issued, look for hardlockups. If the timer is not periodic,
- * kick it. The interrupt is always handled when if delivered via the
- * Front-Side Bus.
+ * kick it. Move the interrupt to the next monitored CPU. The interrupt is
+ * always handled when if delivered via the Front-Side Bus.
  * Returns:
@@ -211,7 +212,7 @@ static int hardlockup_detector_nmi_handler(unsigned int val,
 					   struct pt_regs *regs)
 	struct hpet_hld_data *hdata = hld_data;
-	unsigned int use_fsb;
+	unsigned int use_fsb, cpu;
 	 * If FSB delivery mode is used, the timer interrupt is programmed as
@@ -222,8 +223,27 @@ static int hardlockup_detector_nmi_handler(unsigned int val,
 	if (!use_fsb && !is_hpet_wdt_interrupt(hdata))
 		return NMI_DONE;
+	/* There are no CPUs to monitor. */
+	if (!cpumask_weight(&hdata->monitored_mask))
+		return NMI_HANDLED;
+	/*
+	 * Target a new CPU. Keep trying until we find a monitored CPU. CPUs
+	 * are addded and removed to this mask at cpu_up() and cpu_down(),
+	 * respectively. Thus, the interrupt should be able to be moved to
+	 * the next monitored CPU.
+	 */
+	spin_lock(&hld_data->lock);
+	for_each_cpu_wrap(cpu, &hdata->monitored_mask, smp_processor_id() + 1) {
+		if (!irq_set_affinity(hld_data->irq, cpumask_of(cpu)))
+			break;
+		pr_err("Could not assign interrupt to CPU %d. Trying with next present CPU.\n",
+		       cpu);
+	}
+	spin_unlock(&hld_data->lock);
 	if (!(hdata->flags & HPET_DEV_PERI_CAP))
@@ -336,7 +356,7 @@ static int setup_hpet_irq(struct hpet_hld_data *hdata)
 	 * Request an interrupt to activate the irq in all the needed domains.
 	ret = request_irq(hwirq, hardlockup_detector_irq_handler,
 			  "hpet_hld", hdata);
 	if (ret)
 		unregister_nmi_handler(NMI_LOCAL, "hpet_hld");