Re: [PATCH v2 5/6] KVM-GST: adjust scheduler cpu power

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2011-01-28 at 14:52 -0500, Glauber Costa wrote:

> +#ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
> +static DEFINE_PER_CPU(u64, cpu_steal_time);
> +
> +#ifndef CONFIG_64BIT
> +static DEFINE_PER_CPU(seqcount_t, steal_time_seq);
> +
> +static inline void steal_time_write_begin(void)
> +{
> +	__this_cpu_inc(steal_time_seq.sequence);
> +	smp_wmb();
> +}
> +
> +static inline void steal_time_write_end(void)
> +{
> +	smp_wmb();
> +	__this_cpu_inc(steal_time_seq.sequence);
> +}
> +
> +static inline u64 steal_time_read(int cpu)
> +{
> +	u64 steal_time;
> +	unsigned seq;
> +
> +	do {
> +		seq = read_seqcount_begin(&per_cpu(steal_time_seq, cpu));
> +		steal_time = per_cpu(cpu_steal_time, cpu);
> +	} while (read_seqcount_retry(&per_cpu(steal_time_seq, cpu), seq));
> +
> +	return steal_time;
> +}
> +#else /* CONFIG_64BIT */
> +static inline void steal_time_write_begin(void)
> +{
> +}
> +
> +static inline void steal_time_write_end(void)
> +{
> +}
> +
> +static inline u64 steal_time_read(int cpu)
> +{
> +	return per_cpu(cpu_steal_time, cpu);
> +}


> @@ -3536,6 +3592,11 @@ static int touch_steal_time(int is_idle)
>  
>  	if (st) {
>  		account_steal_time(st);
> +#ifdef CONFIG_PARAVIRT_TIME_ACCOUNTING
> +		steal_time_write_begin();
> +		__this_cpu_add(cpu_steal_time, steal);
> +		steal_time_write_end();
> +#endif
>  		return 1;
>  	}
>  	return 0;


Why replicate all logic you've already got in patch 4? That too is
reading steal time in a loop in kvm_account_steal_time(), why not extend
that interface to take a cpu argument and be done with it?