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SubjectRe: [PATCH 11/15] sched: Pass unlimited __cpu_power information to upper domain level groups
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On Thu, 2009-08-20 at 15:41 +0200, Andreas Herrmann wrote:
> For performance reasons __cpu_power in a sched_group might be limited
> such that the group can handle only one task. To correctly calculate
> the capacity in upper domain level groups the unlimited power
> information is required. This patch stores unlimited __cpu_power
> information in sched_groups.orig_power and uses this when calculating
> __cpu_power in upper domain level groups.

OK, so this tries to fix the cpu_power wreckage?

ok, so let me try this with an example:


Suppose we have a dual-core with shared cache and SMT

0-3 MC
0-1 2-3 SMT

Then both levels fancy setting SHARED_RESOURCES and both levels end up
normalizing the cpu_power to 1, so when we unplug cpu 2, load-balancing
gets all screwy because the whole system doesn't get normalized
properly.

What you propose here is every time we muck with cpu_power we keep the
real stuff in orig_power and use that to compute the level above.

Except you don't use it in the load-balancer proper, so normalization is
still hosed.

Its a creative solution, but I'd rather see cpu_power returned to a
straight sum of actual power to normalize the inter-cpu runqueue weights
and do the placement decision using something else.

> Signed-off-by: Andreas Herrmann <andreas.herrmann3@amd.com>
> ---
> include/linux/sched.h | 8 +++++++-
> kernel/sched.c | 36 ++++++++++++++++++++++++------------
> 2 files changed, 31 insertions(+), 13 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index c53bdd8..d230717 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -890,7 +890,13 @@ struct sched_group {
> * (see include/linux/reciprocal_div.h)
> */
> u32 reciprocal_cpu_power;
> -
> + /*
> + * Backup of original power for this group.
> + * It is used to pass correct power information to upper
> + * domain level groups in case __cpu_power is limited for
> + * performance reasons.
> + */
> + unsigned int orig_power;
> /*
> * The CPUs this group covers.
> *
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 7a0d710..464b6ba 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -8376,6 +8376,7 @@ static void init_numa_sched_groups_power(struct sched_group *group_head)
>
> sg_inc_cpu_power(sg, sd->groups->__cpu_power);
> }
> + sg->orig_power = sg->__cpu_power;
> sg = sg->next;
> } while (sg != group_head);
> }
> @@ -8514,18 +8515,9 @@ static void init_sched_groups_power(int cpu, struct sched_domain *sd)
> child = sd->child;
>
> sd->groups->__cpu_power = 0;
> -
> - /*
> - * For perf policy, if the groups in child domain share resources
> - * (for example cores sharing some portions of the cache hierarchy
> - * or SMT), then set this domain groups cpu_power such that each group
> - * can handle only one task, when there are other idle groups in the
> - * same sched domain.
> - */
> - if (!child || (!(sd->flags & SD_POWERSAVINGS_BALANCE) &&
> - (child->flags &
> - (SD_SHARE_CPUPOWER | SD_SHARE_PKG_RESOURCES)))) {
> + if (!child) {
> sg_inc_cpu_power(sd->groups, SCHED_LOAD_SCALE);
> + sd->groups->orig_power = sd->groups->__cpu_power;
> return;
> }
>
> @@ -8534,9 +8526,29 @@ static void init_sched_groups_power(int cpu, struct sched_domain *sd)
> */
> group = child->groups;
> do {
> - sg_inc_cpu_power(sd->groups, group->__cpu_power);
> + sg_inc_cpu_power(sd->groups, group->orig_power);
> group = group->next;
> } while (group != child->groups);
> + sd->groups->orig_power = sd->groups->__cpu_power;
> +
> + /*
> + * For perf policy, if the groups in child domain share resources
> + * (for example cores sharing some portions of the cache hierarchy
> + * or SMT), then set this domain groups cpu_power such that each group
> + * can handle only one task, when there are other idle groups in the
> + * same sched domain.
> + * Note: Unmodified power information is kept in orig_power and
> + * can be used in higher domain levels to calculate
> + * and reflect the correct capacity of a sched_group.
> + * This is required for power_savings scheduling.
> + */
> + if (!(sd->flags & SD_POWERSAVINGS_BALANCE) &&
> + ((child->flags &
> + (SD_SHARE_CPUPOWER | SD_SHARE_PKG_RESOURCES)))) {
> + sd->groups->__cpu_power = 0;
> + sg_inc_cpu_power(sd->groups, SCHED_LOAD_SCALE);
> + }
> +
> }
>
> /*


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