[Xen-devel] [PATCH v2 1/3] xen: RCU: let the RCU idle timer handler run

[Dario Faggioli <dario.faggioli@xxxxxxxxxx>]

If stop_timer() is called between when the RCU
idle timer's interrupt arrives (and TIMER_SOFTIRQ is
raised) and when softirqs are checked and handled, the
timer is deactivated, and the handler never runs.

This happens to the RCU idle timer because stop_timer()
is called on it during the wakeup from idle (e.g., C-states,
on x86) path.

To fix that, we avoid calling stop_timer(), in case we see
that the timer itself is:
- still active,
- expired (i.e., it's expiry time is in the past).
In fact, that indicates (for this particular timer) that
it has fired, and we are just about to handle the TIMER_SOFTIRQ
(which will perform the timer deactivation and run its handler).

Signed-off-by: Dario Faggioli <dario.faggioli@xxxxxxxxxx>
---
Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Cc: George Dunlap <George.Dunlap@xxxxxxxxxxxxx>
Cc: Jan Beulich <jbeulich@xxxxxxxx>
Cc: Stefano Stabellini <sstabellini@xxxxxxxxxx>
Cc: Julien Grall <julien.grall@xxxxxxx>
Cc: Tim Deegan <tim@xxxxxxx>
---
Changes from v1:
- logic changed completely: instead of avoiding deactivate_timer() in
  stop_timer(), we avoid stop_timer() in rcu_idle_timer_stop() (if
  appropriate, of course).
---
 xen/common/rcupdate.c   |   16 +++++++++++++++-
 xen/common/timer.c      |   17 +++++++++++++++++
 xen/include/xen/timer.h |    3 +++
 3 files changed, 35 insertions(+), 1 deletion(-)

diff --git a/xen/common/rcupdate.c b/xen/common/rcupdate.c
index 871936f..4a02cdd 100644
--- a/xen/common/rcupdate.c
+++ b/xen/common/rcupdate.c
@@ -465,7 +465,21 @@ void rcu_idle_timer_stop()
         return;
 
     rdp->idle_timer_active = false;
-    stop_timer(&rdp->idle_timer);
+
+    /*
+     * In general, as the CPU is becoming active again, we don't need the
+     * idle timer, and so we want to stop it.
+     *
+     * However, in case we are here because idle_timer has (just) fired and
+     * has woken up the CPU, we skip stop_timer() now. In fact, if we stop
+     * it, then the TIMER_SOFTIRQ handler wouldn't find idle_timer among the
+     * active timers any longer, and hence won't call rcu_idle_timer_handler().
+     *
+     * Therefore, if we see that the timer is expired already, leave it alone.
+     * It will be finally deactiveted by the TIMER_SOFTIRQ handler.
+     */
+    if ( !timer_is_expired(&rdp->idle_timer, NOW()) )
+        stop_timer(&rdp->idle_timer);
 }
 
 static void rcu_idle_timer_handler(void* data)
diff --git a/xen/common/timer.c b/xen/common/timer.c
index d9ff669..a768aa3 100644
--- a/xen/common/timer.c
+++ b/xen/common/timer.c
@@ -332,6 +332,23 @@ void stop_timer(struct timer *timer)
 }
 
 
+bool timer_is_expired(struct timer *timer, s_time_t now)
+{
+    unsigned long flags;
+    bool ret = false;
+
+    if ( !timer_lock_irqsave(timer, flags) )
+        return ret;
+
+    if ( active_timer(timer) && timer->expires <= now )
+        ret = true;
+
+    timer_unlock_irqrestore(timer, flags);
+
+    return ret;
+}
+
+
 void migrate_timer(struct timer *timer, unsigned int new_cpu)
 {
     unsigned int old_cpu;
diff --git a/xen/include/xen/timer.h b/xen/include/xen/timer.h
index 9531800..a095007 100644
--- a/xen/include/xen/timer.h
+++ b/xen/include/xen/timer.h
@@ -70,6 +70,9 @@ void set_timer(struct timer *timer, s_time_t expires);
  */
 void stop_timer(struct timer *timer);
 
+/* True if a timer is active, but with its expiry time before now. */
+bool timer_is_expired(struct timer *timer, s_time_t now);
+
 /* Migrate a timer to a different CPU. The timer may be currently active. */
 void migrate_timer(struct timer *timer, unsigned int new_cpu);
 


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