Re: [patch v3 5/6] sched, ttwu_queue: queue remote wakeups only when crossing cache domains

[Mike Galbraith <efault@gmx.de>]

On Thu, 2011-12-01 at 17:07 -0800, Suresh Siddha wrote:
> plain text document attachment
> (use_ttwu_queue_when_crossing_cache_domains.patch)
> From: Mike Galbraith <efault@gmx.de>
> 
> Context-switch intensive microbenchmark on a 8-socket system had
> ~600K times more resched IPI's on each logical CPU because of the
> TTWU_QUEUE sched feature, which queues the task on the remote cpu's
> queue and completes the wakeup locally using an IPI.
> 
> As the TTWU_QUEUE sched feature is for minimizing the cache-misses
> associated with the remote wakeups, use the IPI only when the local and
> the remote cpu's are from different cache domains. Otherwise use the
> traditional remote wakeup.

FYI, Peter has already (improved and) queued this patch.

> With this, context-switch microbenchmark performed 5 times better on the
> 8-socket NHM-EX system.
> 
> Signed-off-by: Mike Galbraith <efault@gmx.de>
> Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
> ---
>  kernel/sched/core.c |   25 ++++++++++++++++++++++++-
>  1 file changed, 24 insertions(+), 1 deletion(-)
> 
> Index: tip/kernel/sched/core.c
> ===================================================================
> --- tip.orig/kernel/sched/core.c
> +++ tip/kernel/sched/core.c
> @@ -1481,12 +1481,35 @@ static int ttwu_activate_remote(struct t
>  #endif /* __ARCH_WANT_INTERRUPTS_ON_CTXSW */
>  #endif /* CONFIG_SMP */
>  
> +static int ttwu_share_cache(int this_cpu, int cpu)
> +{
> +#ifndef CONFIG_X86
> +	struct sched_domain *sd;
> +	int ret = 0;
> +
> +	rcu_read_lock();
> +	for_each_domain(this_cpu, sd) {
> +		if (!cpumask_test_cpu(cpu, sched_domain_span(sd)))
> +			continue;
> +
> +		ret = (sd->flags & SD_SHARE_PKG_RESOURCES);
> +		break;
> +	}
> +	rcu_read_unlock();
> +
> +	return ret;
> +#else
> +	return per_cpu(cpu_llc_id, this_cpu) == per_cpu(cpu_llc_id, cpu);
> +#endif
> +}
> +
>  static void ttwu_queue(struct task_struct *p, int cpu)
>  {
>  	struct rq *rq = cpu_rq(cpu);
>  
>  #if defined(CONFIG_SMP)
> -	if (sched_feat(TTWU_QUEUE) && cpu != smp_processor_id()) {
> +	if (sched_feat(TTWU_QUEUE) &&
> +	    !ttwu_share_cache(smp_processor_id(), cpu)) {
>  		sched_clock_cpu(cpu); /* sync clocks x-cpu */
>  		ttwu_queue_remote(p, cpu);
>  		return;
> 
>