Re: [PATCH v11 2/2] sched/rt: Add support for SD_PREFER_SIBLING on find_lowest_rq()

[Byungchul Park <byungchul.park@lge.com>]

On Fri, Aug 25, 2017 at 03:40:51PM +0900, Byungchul Park wrote:
> It would be better to try to check other siblings first if
> SD_PREFER_SIBLING is flaged when pushing tasks - migration.

Any opinions?

> Suggested-by: Peter Zijlstra <peterz@infradead.org>
> Signed-off-by: Byungchul Park <byungchul.park@lge.com>
> Reviewed-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> ---
>  kernel/sched/rt.c | 80 ++++++++++++++++++++++++++++++++++++++++++++++++++++---
>  1 file changed, 77 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 979b734..a32e36d 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1618,12 +1618,35 @@ static struct task_struct *pick_highest_pushable_task(struct rq *rq, int cpu)
>  
>  static DEFINE_PER_CPU(cpumask_var_t, local_cpu_mask);
>  
> +/*
> + * Find the first cpu in: mask & sd & ~prefer
> + */
> +static int find_cpu(const struct cpumask *mask,
> +		    const struct sched_domain *sd,
> +		    const struct sched_domain *prefer)
> +{
> +	const struct cpumask *sds = sched_domain_span(sd);
> +	const struct cpumask *ps  = prefer ? sched_domain_span(prefer) : NULL;
> +	int cpu;
> +
> +	for_each_cpu(cpu, mask) {
> +		if (!cpumask_test_cpu(cpu, sds))
> +			continue;
> +		if (ps && cpumask_test_cpu(cpu, ps))
> +			continue;
> +		break;
> +	}
> +
> +	return cpu;
> +}
> +
>  static int find_lowest_rq(struct task_struct *task)
>  {
> -	struct sched_domain *sd;
> +	struct sched_domain *sd, *prefer = NULL;
>  	struct cpumask *lowest_mask = this_cpu_cpumask_var_ptr(local_cpu_mask);
>  	int this_cpu = smp_processor_id();
>  	int cpu      = task_cpu(task);
> +	int fallback_cpu = -1;
>  
>  	/* Make sure the mask is initialized first */
>  	if (unlikely(!lowest_mask))
> @@ -1668,9 +1691,37 @@ static int find_lowest_rq(struct task_struct *task)
>  				return this_cpu;
>  			}
>  
> -			best_cpu = cpumask_first_and(lowest_mask,
> -						     sched_domain_span(sd));
> +			/*
> +			 * If a cpu exists that is in the lowest_mask and
> +			 * the current sd span, but not in the prefer sd
> +			 * span, then that becomes our choice.
> +			 *
> +			 * Of course, the lowest possible cpu is already
> +			 * under consideration through lowest_mask.
> +			 */
> +			best_cpu = find_cpu(lowest_mask, sd, prefer);
> +
>  			if (best_cpu < nr_cpu_ids) {
> +				/*
> +				 * If current domain is SD_PREFER_SIBLING
> +				 * flaged, we have to try to check other
> +				 * siblings first.
> +				 */
> +				if (sd->flags & SD_PREFER_SIBLING) {
> +					prefer = sd;
> +
> +					/*
> +					 * fallback_cpu should be one
> +					 * in the closest domain among
> +					 * SD_PREFER_SIBLING domains,
> +					 * in case that more than one
> +					 * SD_PREFER_SIBLING domains
> +					 * exist in the hierachy.
> +					 */
> +					if (fallback_cpu == -1)
> +						fallback_cpu = best_cpu;
> +					continue;
> +				}
>  				rcu_read_unlock();
>  				return best_cpu;
>  			}
> @@ -1679,6 +1730,29 @@ static int find_lowest_rq(struct task_struct *task)
>  	rcu_read_unlock();
>  
>  	/*
> +	 * If fallback_cpu is valid, all our guesses failed *except* for
> +	 * SD_PREFER_SIBLING domain. Now, we can return the fallback cpu.
> +	 *
> +	 * XXX: Consider the following example, 4 cores SMT2 system:
> +	 *
> +	 *    LLC [0       -        7]
> +	 *    SMT [0 1][2 3][4 5][6 7]
> +	 *         o x  o x  x x  x x
> +	 *
> +	 *    where 'o': occupied and 'x': empty.
> +	 *
> +	 * A wakeup on cpu0 will exclude cpu1 and choose cpu3, since
> +	 * cpu1 is in a SD_PREFER_SIBLING sd and cpu3 is not. However,
> +	 * in this case, cpu4 would have been a better choice, since
> +	 * cpu3 is a (SMT) thread of an already loaded core.
> +	 *
> +	 * Doing it 'right' is difficult and expensive. The current
> +	 * solution is an acceptable approximation.
> +	 */
> +	if (fallback_cpu != -1)
> +		return fallback_cpu;
> +
> +	/*
>  	 * And finally, if there were no matches within the domains
>  	 * just give the caller *something* to work with from the compatible
>  	 * locations.
> -- 
> 1.9.1