Sorry, for the late reply. Just got back from traveling.
On Thu, 23 Feb 2017 18:54:38 +0100
Peter Zijlstra <pet...@infradead.org> wrote:
> On Thu, Feb 23, 2017 at 06:45:05PM +0100, Peter Zijlstra wrote:
> > Hurm.. maybe we should do what Steve initially suggested. The
> > alternative is link order trickery, and I'm not sure we want to do that.
>
> That is, given:
>
> kernel/sched/Makefile: obj-y += idle_task.o fair.o rt.o deadline.o stop_task.o
>
> results in:
>
> readelf -s defconfig-build/vmlinux | awk '/sched_class/ {print $2 " " $8}' |
> sort -n
> 00000000602c93c0 idle_sched_class
> 00000000602c9480 fair_sched_class
> 00000000602c9580 rt_sched_class
> 00000000602c96c0 dl_sched_class
> 00000000602c97c0 stop_sched_class
>
> we can do this, but yuck!
>
> ---
> kernel/sched/core.c | 12 +++++++++---
> 1 file changed, 9 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 8f972df76eb2..eebe6729ceb7 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3285,10 +3285,16 @@ pick_next_task(struct rq *rq, struct task_struct
> *prev, struct rq_flags *rf)
> struct task_struct *p;
>
> /*
> - * Optimization: we know that if all tasks are in
> - * the fair class we can call that function directly:
> + * Optimization: we know that if all tasks are in the fair class we can
> + * call that function directly, but only if the @prev task wasn't of a
> + * higher scheduling class, because otherwise those loose the
> + * opportinity to pull in more work from other CPUs.
> + *
> + * Depends on link order in kernel/sched/Makefile.
> */
> - if (likely(rq->nr_running == rq->cfs.h_nr_running)) {
> + if (likely(rq->nr_running == rq->cfs.h_nr_running &&
> + prev->sched_class <= &fair_sched_class)) {
If we go this route, I would suggest that we hardcode the classes in
vmlinux.lds.h.
-- Steve
> +
> p = fair_sched_class.pick_next_task(rq, prev, rf);
> if (unlikely(p == RETRY_TASK))
> goto again;
