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

Peter Zijlstra
2011-01-31T11:25:20Z

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?