[Xen-changelog] [xen master] x86/hpet: Improve handling of timer_deadline

[patchbot@xxxxxxx]

commit 9e051a840d7f7121118470a55d2d600b46be97f7
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Wed May 31 14:56:26 2017 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Tue Aug 15 15:08:57 2017 +0100

    x86/hpet: Improve handling of timer_deadline
    
    timer_deadline is only ever updated via this_cpu() in 
timer_softirq_action(),
    so is not going to change behind the back of the currently running cpu.
    
    Update hpet_broadcast_{enter,exit}() to cache the value in a local variable 
to
    avoid the repeated RELOC_HIDE() penalty.
    
    handle_hpet_broadcast() reads the timer_deadlines of remote cpus, but there 
is
    no need to force the read for cpus which are not present in the mask.  One
    requirement is that we only sample the value once (which happens as a side
    effect of RELOC_HIDE()), but is made more explicit with ACCESS_ONCE().
    
    Bloat-o-meter shows a modest improvement:
    
      add/remove: 0/0 grow/shrink: 0/3 up/down: 0/-144 (-144)
      function                                     old     new   delta
      hpet_broadcast_exit                          335     313     -22
      hpet_broadcast_enter                         327     278     -49
      handle_hpet_broadcast                        572     499     -73
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/arch/x86/hpet.c | 22 +++++++++++++---------
 1 file changed, 13 insertions(+), 9 deletions(-)

diff --git a/xen/arch/x86/hpet.c b/xen/arch/x86/hpet.c
index 46f4c42..8229c63 100644
--- a/xen/arch/x86/hpet.c
+++ b/xen/arch/x86/hpet.c
@@ -189,12 +189,11 @@ again:
     {
         s_time_t deadline;
 
-        rmb();
-        deadline = per_cpu(timer_deadline, cpu);
-        rmb();
         if ( !cpumask_test_cpu(cpu, ch->cpumask) )
             continue;
 
+        deadline = ACCESS_ONCE(per_cpu(timer_deadline, cpu));
+
         if ( deadline <= now )
             __cpumask_set_cpu(cpu, &mask);
         else if ( deadline < next_event )
@@ -697,8 +696,9 @@ void hpet_broadcast_enter(void)
 {
     unsigned int cpu = smp_processor_id();
     struct hpet_event_channel *ch = per_cpu(cpu_bc_channel, cpu);
+    s_time_t deadline = per_cpu(timer_deadline, cpu);
 
-    if ( per_cpu(timer_deadline, cpu) == 0 )
+    if ( deadline == 0 )
         return;
 
     if ( !ch )
@@ -714,9 +714,12 @@ void hpet_broadcast_enter(void)
     cpumask_set_cpu(cpu, ch->cpumask);
 
     spin_lock(&ch->lock);
-    /* reprogram if current cpu expire time is nearer */
-    if ( per_cpu(timer_deadline, cpu) < ch->next_event )
-        reprogram_hpet_evt_channel(ch, per_cpu(timer_deadline, cpu), NOW(), 1);
+    /*
+     * Reprogram if current cpu expire time is nearer.  deadline is never
+     * written by a remote cpu, so the value read earlier is still valid.
+     */
+    if ( deadline < ch->next_event )
+        reprogram_hpet_evt_channel(ch, deadline, NOW(), 1);
     spin_unlock(&ch->lock);
 }
 
@@ -724,8 +727,9 @@ void hpet_broadcast_exit(void)
 {
     unsigned int cpu = smp_processor_id();
     struct hpet_event_channel *ch = per_cpu(cpu_bc_channel, cpu);
+    s_time_t deadline = per_cpu(timer_deadline, cpu);
 
-    if ( per_cpu(timer_deadline, cpu) == 0 )
+    if ( deadline == 0 )
         return;
 
     if ( !ch )
@@ -733,7 +737,7 @@ void hpet_broadcast_exit(void)
 
     /* Reprogram the deadline; trigger timer work now if it has passed. */
     enable_APIC_timer();
-    if ( !reprogram_timer(per_cpu(timer_deadline, cpu)) )
+    if ( !reprogram_timer(deadline) )
         raise_softirq(TIMER_SOFTIRQ);
 
     cpumask_clear_cpu(cpu, ch->cpumask);
--
generated by git-patchbot for /home/xen/git/xen.git#master

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