On 2014/9/4 23:32, Nicolas Pitre wrote:
> The code in find_idlest_cpu() looks for the CPU with the smallest load.
> However, if multiple CPUs are idle, the first idle CPU is selected
> irrespective of the depth of its idle state.
>
> Among the idle CPUs we should pick the one with with the shallowest idle
> state, or the latest to have gone idle if all idle CPUs are in the same
> state. The later applies even when cpuidle is configured out.
>
> This patch doesn't cover the following issues:
>
> - The idle exit latency of a CPU might be larger than the time needed
> to migrate the waking task to an already running CPU with sufficient
> capacity, and therefore performance would benefit from task packing
> in such case (in most cases task packing is about power saving).
>
> - Some idle states have a non negligible and non abortable entry latency
> which needs to run to completion before the exit latency can start.
> A concurrent patch series is making this info available to the cpuidle
> core. Once available, the entry latency with the idle timestamp could
> determine when the exit latency may be effective.
>
> Those issues will be handled in due course. In the mean time, what
> is implemented here should improve things already compared to the current
> state of affairs.
>
> Based on an initial patch from Daniel Lezcano.
>
> Signed-off-by: Nicolas Pitre <n...@linaro.org>
> ---
> kernel/sched/fair.c | 43 ++++++++++++++++++++++++++++++++++++-------
> 1 file changed, 36 insertions(+), 7 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index bfa3c86d0d..416329e1a6 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -23,6 +23,7 @@
> #include <linux/latencytop.h>
> #include <linux/sched.h>
> #include <linux/cpumask.h>
> +#include <linux/cpuidle.h>
> #include <linux/slab.h>
> #include <linux/profile.h>
> #include <linux/interrupt.h>
> @@ -4428,20 +4429,48 @@ static int
> find_idlest_cpu(struct sched_group *group, struct task_struct *p, int
> this_cpu)
> {
> unsigned long load, min_load = ULONG_MAX;
> - int idlest = -1;
> + unsigned int min_exit_latency = UINT_MAX;
> + u64 latest_idle_timestamp = 0;
> + int least_loaded_cpu = this_cpu;
> + int shallowest_idle_cpu = -1;
> int i;
>
> /* Traverse only the allowed CPUs */
> for_each_cpu_and(i, sched_group_cpus(group), tsk_cpus_allowed(p)) {
> - load = weighted_cpuload(i);
> -
> - if (load < min_load || (load == min_load && i == this_cpu)) {
> - min_load = load;
> - idlest = i;
> + if (idle_cpu(i)) {
> + struct rq *rq = cpu_rq(i);
> + struct cpuidle_state *idle = idle_get_state(rq);
> + if (idle && idle->exit_latency < min_exit_latency) {
> + /*
> + * We give priority to a CPU whose idle state
> + * has the smallest exit latency irrespective
> + * of any idle timestamp.
> + */
> + min_exit_latency = idle->exit_latency;
> + latest_idle_timestamp = rq->idle_stamp;
> + shallowest_idle_cpu = i;
> + } else if ((!idle || idle->exit_latency ==
> min_exit_latency) &&
> + rq->idle_stamp > latest_idle_timestamp) {
> + /*
> + * If equal or no active idle state, then
> + * the most recently idled CPU might have
> + * a warmer cache.
> + */
> + latest_idle_timestamp = rq->idle_stamp;
> + shallowest_idle_cpu = i;
> + }
> + cpuidle_put_state(rq);
> + } else {
I think we needn't test no idle cpus after find an idle cpu.
And what about this?
} else if (shallowest_idle_cpu == -1) {
> + load = weighted_cpuload(i);
> + if (load < min_load ||
> + (load == min_load && i == this_cpu)) {
> + min_load = load;
> + least_loaded_cpu = i;
> + }
> }
> }
>
> - return idlest;
> + return shallowest_idle_cpu != -1 ? shallowest_idle_cpu :
> least_loaded_cpu;
> }
>
> /*
--
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