Hi,
On 23/05/17 11:00, Byungchul Park wrote:
> When cpudl_find() returns any among free_cpus, the cpu might not be
> closer than others, considering sched domain. For example:
>
> this_cpu: 15
> free_cpus: 0, 1,..., 14 (== later_mask)
> best_cpu: 0
>
> topology:
>
> 0 --+
> +--+
> 1 --+ |
> +-- ... --+
> 2 --+ | |
> +--+ |
> 3 --+ |
>
> ... ...
>
> 12 --+ |
> +--+ |
> 13 --+ | |
> +-- ... -+
> 14 --+ |
> +--+
> 15 --+
>
> In this case, it would be best to select 14 since it's a free cpu and
> closest to 15(this_cpu). However, currently the code select 0(best_cpu)
> even though that's just any among free_cpus. Fix it.
>
> Signed-off-by: Byungchul Park <[email protected]>
The patch looks essentially all right to me: makes sense and it aligns
behavior with RT.
However...
> ---
> kernel/sched/deadline.c | 27 ++++++++++++++-------------
> 1 file changed, 14 insertions(+), 13 deletions(-)
>
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index a2ce590..9d997d9 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -1324,7 +1324,7 @@ static int find_later_rq(struct task_struct *task)
> struct sched_domain *sd;
> struct cpumask *later_mask = this_cpu_cpumask_var_ptr(local_cpu_mask_dl);
> int this_cpu = smp_processor_id();
> - int best_cpu, cpu = task_cpu(task);
> + int cpu = task_cpu(task);
>
> /* Make sure the mask is initialized first */
> if (unlikely(!later_mask))
> @@ -1337,17 +1337,14 @@ static int find_later_rq(struct task_struct *task)
> * We have to consider system topology and task affinity
> * first, then we can look for a suitable cpu.
> */
> - best_cpu = cpudl_find(&task_rq(task)->rd->cpudl,
> - task, later_mask);
> - if (best_cpu == -1)
> + if (cpudl_find(&task_rq(task)->rd->cpudl, task, later_mask) == -1)
> return -1;
>
> /*
> - * If we are here, some target has been found,
> - * the most suitable of which is cached in best_cpu.
> - * This is, among the runqueues where the current tasks
> - * have later deadlines than the task's one, the rq
> - * with the latest possible one.
> + * If we are here, some targets have been found, including
> + * the most suitable which is, among the runqueues where the
> + * current tasks have later deadlines than the task's one, the
> + * rq with the latest possible one.
> *
> * Now we check how well this matches with task's
> * affinity and system topology.
> @@ -1367,6 +1364,7 @@ static int find_later_rq(struct task_struct *task)
> rcu_read_lock();
> for_each_domain(cpu, sd) {
> if (sd->flags & SD_WAKE_AFFINE) {
This is orthogonal to the proposed change, but I'm wondering if it make
sense to do the following only for SD_WAKE_AFFINE domains. The
consideration applies to RT as well, actually. Also, find_later_rq gets
called when trying to push tasks away as well and in that case checking
for this flag seems inappropriate? Peter, Steve?
Thanks,
- Juri
On Wed, Jul 12, 2017 at 02:13:36PM +0100, Juri Lelli wrote:
> > @@ -1367,6 +1364,7 @@ static int find_later_rq(struct task_struct *task)
> > rcu_read_lock();
> > for_each_domain(cpu, sd) {
> > if (sd->flags & SD_WAKE_AFFINE) {
Hi,
> This is orthogonal to the proposed change, but I'm wondering if it make
> sense to do the following only for SD_WAKE_AFFINE domains. The
Actually I also wonder it..
> consideration applies to RT as well, actually. Also, find_later_rq gets
> called when trying to push tasks away as well and in that case checking
> for this flag seems inappropriate? Peter, Steve?
>
> Thanks,
>
> - Juri