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SubjectRe: [PATCH 6/7] RT: Select tasks based on relative affinity
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On Fri, 2007-10-12 at 20:16 -0400, Gregory Haskins wrote:
> In theory, tasks will be most efficient if they are allowed to re-wake to
> the CPU that they last ran on due to cache affinity. Short of that, it is
> cheaper to wake up the current CPU. If neither of those two are options,
> than the lowest CPU will do.
>
> Signed-off-by: Gregory Haskins <ghaskins@novell.com>
> ---
>
> kernel/sched.c | 72 +++++++++++++++++++++++++++++++++++++++++---------------
> 1 files changed, 53 insertions(+), 19 deletions(-)
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 3c71156..b79b968 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -1511,6 +1511,15 @@ static int double_lock_balance(struct rq *this_rq, struct rq *busiest);
> /* Only try this algorithm three times */
> #define RT_PUSH_MAX_TRIES 3
>
> +static int non_rt_cpu(cpumask_t *mask, int cpu)
> +{
> + if (!rt_prio(cpu_rq(cpu)->curr_prio) &&
> + cpu_isset(cpu, *mask))
> + return 1;
> +
> + return 0;
> +}
> +
> /* Will lock the rq it finds */
> static int find_lowest_cpu(cpumask_t *cpu_mask, struct task_struct *task,
> struct rq *this_rq)
> @@ -1519,32 +1528,57 @@ static int find_lowest_cpu(cpumask_t *cpu_mask, struct task_struct *task,
> int dst_cpu = -1;
> int cpu;
> int tries;
> + int this_cpu = smp_processor_id();
>
> for (tries = 0; tries < RT_PUSH_MAX_TRIES; tries++) {
> +
> /*
> - * Scan each rq for the lowest prio.
> + * We select a CPU in the following priority:
> + *
> + * task_cpu, this_cpu, first_cpu
> + *
> + * for efficiency.
> + *
> + * - task_cpu preserves cache affinity

After thinking about this over the weekend, the task_cpu optimization
doesn't make sense. It made sense in my original design before I
integrated with Steve's series, but not here. I think the second
optimization (this_cpu) still makes sense though, so I will update this
patch for the next drop.

> + * - this_cpu is (presumably) cheaper to preempt
> + * (note that sometimes task_cpu and this_cpu
> + * are the same).
> + * - Finally, we will take whatever is available
> + * if the first two don't pan out by scanning.
> */
> - for_each_cpu_mask(cpu, *cpu_mask) {
> - struct rq *rq = &per_cpu(runqueues, cpu);
> -
> - if (cpu == smp_processor_id())
> - continue;
> -
> - /* We look for lowest RT prio or non-rt CPU */
> - if (rq->curr_prio >= MAX_RT_PRIO) {
> - lowest_rq = rq;
> - dst_cpu = cpu;
> - break;
> - }
> + if (non_rt_cpu(cpu_mask, task_cpu(task))) {
> + lowest_rq = task_rq(task);
> + dst_cpu = lowest_rq->cpu;
> + } else if (non_rt_cpu(cpu_mask, this_cpu)) {
> + dst_cpu = this_cpu;
> + lowest_rq = cpu_rq(this_cpu);
> + } else {
>
> - /* no locking for now */
> - if (rq->curr_prio > task->prio &&
> - (!lowest_rq || rq->curr_prio < lowest_rq->curr_prio)) {
> - dst_cpu = cpu;
> - lowest_rq = rq;
> + /*
> + * Scan each rq for the lowest prio.
> + */
> + for_each_cpu_mask(cpu, *cpu_mask) {
> + struct rq *rq = &per_cpu(runqueues, cpu);
> +
> + if (cpu == this_cpu)
> + continue;
> +
> + /* We look for lowest RT prio or non-rt CPU */
> + if (rq->curr_prio >= MAX_RT_PRIO) {
> + lowest_rq = rq;
> + dst_cpu = cpu;
> + break;
> + }
> +
> + /* no locking for now */
> + if (rq->curr_prio > task->prio &&
> + (!lowest_rq || rq->curr_prio < lowest_rq->curr_prio)) {
> + dst_cpu = cpu;
> + lowest_rq = rq;
> + }
> }
> }
> -
> +
> if (!lowest_rq) {
> dst_cpu = -1;
> break;
>

-
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