Re: [PATCH] UNTESTED backport rcu_needs_cpu and call it from stop_hz_timer UNTESTED

[David F Barrera <dfbp@us.ibm.com>]

Harry Butterworth wrote:
> THIS PATCH FOR REVIEW ONLY IT'S UNTESTED!!!!!!  Ill test on Monday if
> no-one has done it by then.
>
> 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>
>
>   
> ------------------------------------------------------------------------
>
> 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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>   
Harry,

I am doing some quick testing of your patch. So far, the results are 
consistent with the mainline testing results.

-- 

Regards,

David F Barrera
Linux Technology Center
Systems and Technology Group, IBM

"The wisest men follow their own direction. "
	
                          Euripides