Re: [PATCHv2 2/2] powerpc: implement arch_scale_smt_power for Power7

Benjamin Herrenschmidt
2010-01-27T00:55:26Z

On Tue, 2010-01-26 at 17:28 -0600, Joel Schopp wrote:
> On Power7 processors running in SMT4 mode with 2, 3, or 4 idle threads
> there is performance benefit to idling the higher numbered threads in
> the core.
>
> This patch implements arch_scale_smt_power to dynamically update smt
> thread power in these idle cases in order to prefer threads 0,1 over
> threads 2,3 within a core.
>
> v2 - Same functionality as v1, better coding style.

Better. Some more comments...

> Signed-off-by: Joel Schopp <jschopp@austin.ibm.com>
> ---
> Version 2 addresses style and optimization, same basic functionality
> Index: linux-2.6.git/arch/powerpc/kernel/smp.c
> ===================================================================
> --- linux-2.6.git.orig/arch/powerpc/kernel/smp.c
> +++ linux-2.6.git/arch/powerpc/kernel/smp.c
> @@ -620,3 +620,55 @@ void __cpu_die(unsigned int cpu)
> smp_ops->cpu_die(cpu);
> }
> #endif
> +
> +unsigned long arch_scale_smt_power(struct sched_domain *sd, int cpu)
> +{
> + int sibling;
> + int idle_count = 0;
> + int thread;
> +
> + struct cpumask *sibling_map = sched_domain_span(sd);

What about an early exit if !cpu_has_feature(CPU_FTR_SMT) ? That would
de-facto compile it out for 32-bit CPU platforms that don't support SMT
at all and avoid some overhead on POWER3,4,970...

> + unsigned long weight = cpumask_weight(sibling_map);
> + unsigned long smt_gain = sd->smt_gain;
> +
> + if (cpu_has_feature(CPU_FTR_ASYNC_SMT4) && weight == 4) {

So that will only handle the case where all 4 threads are online right ?
There is no provision for the case where the user play tricks like
offlining thread, in which case it will stop trying to "push down"
processes right ? Not a big deal per-se I suppose, just something to be
aware of.

Also, can you add a comment as to why this is done in the code itself ?
above the if (cpu_has_feature(...)) statement.

> + for_each_cpu(sibling, sibling_map) {
> + if (idle_cpu(sibling))
> + idle_count++;
> + }
> +
> + /* the following section attempts to tweak cpu power based
> + * on current idleness of the threads dynamically at runtime
> + */
> + if (idle_count > 1) {
> + thread = cpu_thread_in_core(cpu);
> + if (thread < 2) {
> + /* add 75 % to thread power */
> + smt_gain += (smt_gain >> 1) + (smt_gain >> 2);
> + } else {
> + /* subtract 75 % to thread power */
> + smt_gain = smt_gain >> 2;
> + }
> + }
> + }
> +
> + /* default smt gain is 1178, weight is # of SMT threads */
> + switch (weight) {
> + case 1:
> + /*divide by 1, do nothing*/
> + break;
> + case 2:
> + smt_gain = smt_gain >> 1;
> + break;
> + case 4:
> + smt_gain = smt_gain >> 2;
> + break;
> + default:
> + smt_gain /= weight;
> + break;
> + }
> +
> + return smt_gain;
> +}

Appart from that, it looks allright to me.

Cheers,
Ben.