[Xen-devel] [PATCH 1/2] hrtimers: provide a hrtimers_late_resume() call

[David Vrabel <david.vrabel@xxxxxxxxxx>]

From: David Vrabel <david.vrabel@xxxxxxxxxx>

Xen suspends (and resumes) without disabling non-boot CPUs as doing so
adds considerable delay to live migrations.  A 4 VCPU guest had more
than 200 ms of additional downtime if disable_nonboot_cpus() was
called prior to suspending.

As a consequence, only high resolution timers on the current CPU are
retriggered when resuming.  The Xen resume path worked around this
with a call to clock_was_set() to retrigger timers on all the CPUs.

A subsequent change will make clock_was_set() internal to hrtimers so
add an new call (hrtimers_late_resume()) to do the same job.

Signed-off-by: David Vrabel <david.vrabel@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
 drivers/xen/manage.c    |    8 ++++++--
 include/linux/hrtimer.h |    1 +
 kernel/hrtimer.c        |    9 +++++++++
 3 files changed, 16 insertions(+), 2 deletions(-)

diff --git a/drivers/xen/manage.c b/drivers/xen/manage.c
index 412b96c..75bc2d5 100644
--- a/drivers/xen/manage.c
+++ b/drivers/xen/manage.c
@@ -166,8 +166,12 @@ out_resume:
 
        dpm_resume_end(si.cancelled ? PMSG_THAW : PMSG_RESTORE);
 
-       /* Make sure timer events get retriggered on all CPUs */
-       clock_was_set();
+       /*
+        * syscore_resume() ends up calling hrtimer_resume() but this
+        * only retriggers timer events on the current CPU.  We need
+        * to retrigger the events on all the other CPUS.
+        */
+       hrtimers_late_resume();
 
 out_thaw:
 #ifdef CONFIG_PREEMPT
diff --git a/include/linux/hrtimer.h b/include/linux/hrtimer.h
index d19a5c2..13df0fa 100644
--- a/include/linux/hrtimer.h
+++ b/include/linux/hrtimer.h
@@ -323,6 +323,7 @@ extern void timerfd_clock_was_set(void);
 static inline void timerfd_clock_was_set(void) { }
 #endif
 extern void hrtimers_resume(void);
+extern void hrtimers_late_resume(void);
 
 extern ktime_t ktime_get(void);
 extern ktime_t ktime_get_real(void);
diff --git a/kernel/hrtimer.c b/kernel/hrtimer.c
index fd4b13b..34384b4 100644
--- a/kernel/hrtimer.c
+++ b/kernel/hrtimer.c
@@ -784,6 +784,15 @@ void hrtimers_resume(void)
        timerfd_clock_was_set();
 }
 
+/*
+ * If non-boot CPUs were online during resume, we need to retrigger
+ * the events for all the non-boot CPUs.
+ */
+void hrtimers_late_resume(void)
+{
+       clock_was_set();
+}
+
 static inline void timer_stats_hrtimer_set_start_info(struct hrtimer *timer)
 {
 #ifdef CONFIG_TIMER_STATS
-- 
1.7.2.5


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