[PATCH] backport rcu_needs_cpu and call it from stop_hz_timer

[Harry Butterworth <harry@hebutterworth.freeserve.co.uk>]

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I have now tested this against xen-testing 9735.  Without the patch, and
with a domU kernel configured with CONFIG_DEBUG_SOFTLOCKUP disabled
(this debug code masks the problem), and smp and cpu hotplug enabled, a
script which does lots of cpu hotplug/removal will hang (cpu
hotplug/removal stresses the RCU synchronize_sched function).  With the
patch, the script doing cpu hotplug/removal runs for as long as I have
let it so far (thousands of hotplugs).

Please apply.

Signed-off-by: Harry Butterworth <butterwo@uk.ibm.com>

On Fri, 2006-06-09 at 21:21 +0100, Harry Butterworth wrote:
> There is a problem with the current implementation of stop_hz_timer in
> arch/i386/kernel/time-xen.c where the hz timer can be stopped on a CPU
> which has RCU callbacks pending.
> 
> This patch backports a new RCU API created to fix this problem for the
> s390 implementation of stop_hz_timer and also updates the time-xen.c
> implementation of stop_hz_timer to call the new API.
> 
> Signed-off-by: Harry Butterworth <butterwo@uk.ibm.com>
> 
> _______________________________________________
> Xen-devel mailing list
> Xen-devel@lists.xensource.com
> http://lists.xensource.com/xen-devel

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diff -r 8c64169a05d3 -r 8ef455f268b3 linux-2.6-xen-sparse/arch/i386/kernel/time-xen.c
--- a/linux-2.6-xen-sparse/arch/i386/kernel/time-xen.c	Thu Jun  8 08:52:04 2006
+++ b/linux-2.6-xen-sparse/arch/i386/kernel/time-xen.c	Fri Jun  9 19:47:18 2006
@@ -978,12 +978,19 @@
 	unsigned int cpu = smp_processor_id();
 	unsigned long j;
 
-	/* We must do this /before/ checking rcu_pending(). */
 	cpu_set(cpu, nohz_cpu_mask);
+
+	/* See matching smp_mb in rcu_start_batch in rcupdate.c.  These mbs  */
+	/* ensure that if __rcu_pending (nested in rcu_needs_cpu) fetches a  */
+	/* value of rcp->cur that matches rdp->quiescbatch and allows us to  */
+	/* stop the hz timer then the cpumasks created for subsequent values */
+	/* of cur in rcu_start_batch are guaranteed to pick up the updated   */
+	/* nohz_cpu_mask and so will not depend on this cpu.                 */
+
 	smp_mb();
 
 	/* Leave ourselves in 'tick mode' if rcu or softirq pending. */
-	if (rcu_pending(cpu) || local_softirq_pending()) {
+	if (rcu_needs_cpu(cpu) || local_softirq_pending()) {
 		cpu_clear(cpu, nohz_cpu_mask);
 		j = jiffies + 1;
 	} else {
diff -r 8c64169a05d3 -r 8ef455f268b3 patches/linux-2.6.16.13/rcu_needs_cpu.patch
--- /dev/null	Thu Jun  8 08:52:04 2006
+++ b/patches/linux-2.6.16.13/rcu_needs_cpu.patch	Fri Jun  9 19:47:18 2006
@@ -0,0 +1,33 @@
+--- ../pristine-linux-2.6.16.13/kernel/rcupdate.c	2006-05-02 22:38:44.000000000 +0100
++++ ./kernel/rcupdate.c	2006-06-09 20:27:45.000000000 +0100
+@@ -485,6 +485,20 @@ int rcu_pending(int cpu)
+ 		__rcu_pending(&rcu_bh_ctrlblk, &per_cpu(rcu_bh_data, cpu));
+ }
+ 
++/*
++ * Check to see if any future RCU-related work will need to be done
++ * by the current CPU, even if none need be done immediately, returning
++ * 1 if so.  This function is part of the RCU implementation; it is -not-
++ * an exported member of the RCU API.
++ */
++int rcu_needs_cpu(int cpu)
++{
++	struct rcu_data *rdp = &per_cpu(rcu_data, cpu);
++	struct rcu_data *rdp_bh = &per_cpu(rcu_bh_data, cpu);
++
++	return (!!rdp->curlist || !!rdp_bh->curlist || rcu_pending(cpu));
++}
++
+ void rcu_check_callbacks(int cpu, int user)
+ {
+ 	if (user || 
+--- ../pristine-linux-2.6.16.13/include/linux/rcupdate.h	2006-05-02 22:38:44.000000000 +0100
++++ ./include/linux/rcupdate.h	2006-06-09 20:28:57.000000000 +0100
+@@ -134,6 +134,7 @@ static inline void rcu_bh_qsctr_inc(int 
+ }
+ 
+ extern int rcu_pending(int cpu);
++extern int rcu_needs_cpu(int cpu);
+ 
+ /**
+  * rcu_read_lock - mark the beginning of an RCU read-side critical section.

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