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sched/fair: Do not even the number of busy CPUs via asym_packing

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Now that find_busiest_group() triggers load balancing between a fully_
busy SMT2 core and an idle non-SMT core, it is no longer needed to force
balancing via asym_packing. Use asym_packing only as intended: when there
is high-priority CPU that is idle.

After this change, the same logic apply to SMT and non-SMT local groups.
It makes less sense having a separate function to deal specifically with
SMT. Fold the logic in asym_smt_can_pull_tasks() into sched_asym().

Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Zhang Rui <rui.zhang@intel.com>
Link: https://lore.kernel.org/r/20230406203148.19182-8-ricardo.neri-calderon@linux.intel.com
This commit is contained in:
Ricardo Neri 2023-04-06 13:31:43 -07:00 committed by Peter Zijlstra
parent 43726bdedd
commit c9ca07886a
1 changed files with 21 additions and 65 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

86
kernel/sched/fair.c
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@ -9350,74 +9350,26 @@ static bool sched_use_asym_prio(struct sched_domain *sd, int cpu)
}
/**
* asym_smt_can_pull_tasks - Check whether the load balancing CPU can pull tasks
* @dst_cpu: Destination CPU of the load balancing
* sched_asym - Check if the destination CPU can do asym_packing load balance
* @env: The load balancing environment
* @sds: Load-balancing data with statistics of the local group
* @sgs: Load-balancing statistics of the candidate busiest group
* @sg: The candidate busiest group
* @group: The candidate busiest group
*
* Check the state of the SMT siblings of both @sds::local and @sg and decide
* if @dst_cpu can pull tasks.
* @env::dst_cpu can do asym_packing if it has higher priority than the
* preferred CPU of @group.
*
* This function must be called only if all the SMT siblings of @dst_cpu are
* idle, if any.
* SMT is a special case. If we are balancing load between cores, @env::dst_cpu
* can do asym_packing balance only if all its SMT siblings are idle. Also, it
* can only do it if @group is an SMT group and has exactly on busy CPU. Larger
* imbalances in the number of CPUS are dealt with in find_busiest_group().
*
* If @dst_cpu does not have SMT siblings, it can pull tasks if two or more of
* the SMT siblings of @sg are busy. If only one CPU in @sg is busy, pull tasks
* only if @dst_cpu has higher priority.
* If we are balancing load within an SMT core, or at DIE domain level, always
* proceed.
*
* When dealing with SMT cores, only use priorities if the SMT core has exactly
* one busy sibling. find_busiest_group() will handle bigger imbalances in the
* number of busy CPUs.
*
* Return: true if @dst_cpu can pull tasks, false otherwise.
* Return: true if @env::dst_cpu can do with asym_packing load balance. False
* otherwise.
*/
static bool asym_smt_can_pull_tasks(int dst_cpu, struct sd_lb_stats *sds,
struct sg_lb_stats *sgs,
struct sched_group *sg)
{
#ifdef CONFIG_SCHED_SMT
bool local_is_smt;
int sg_busy_cpus;
local_is_smt = sds->local->flags & SD_SHARE_CPUCAPACITY;
sg_busy_cpus = sgs->group_weight - sgs->idle_cpus;
if (!local_is_smt) {
/*
* If we are here, @dst_cpu is idle and does not have SMT
* siblings. Pull tasks if candidate group has two or more
* busy CPUs.
*/
if (sg_busy_cpus >= 2) /* implies sg_is_smt */
return true;
/*
* @dst_cpu does not have SMT siblings. @sg may have SMT
* siblings and only one is busy. In such case, @dst_cpu
* can help if it has higher priority and is idle (i.e.,
* it has no running tasks).
*/
return sched_asym_prefer(dst_cpu, sg->asym_prefer_cpu);
}
/*
* If we are here @dst_cpu has SMT siblings and are also idle.
*
* CPU priorities does not make sense for SMT cores with more than one
* busy sibling.
*/
if (group->flags & SD_SHARE_CPUCAPACITY && sg_busy_cpus != 1)
return false;
return sched_asym_prefer(dst_cpu, sg->asym_prefer_cpu);
#else
/* Always return false so that callers deal with non-SMT cases. */
return false;
#endif
}
static inline bool
sched_asym(struct lb_env *env, struct sd_lb_stats *sds, struct sg_lb_stats *sgs,
struct sched_group *group)
@ -9426,10 +9378,14 @@ sched_asym(struct lb_env *env, struct sd_lb_stats *sds, struct sg_lb_stats *sgs
if (!sched_use_asym_prio(env->sd, env->dst_cpu))
return false;
/* Only do SMT checks if either local or candidate have SMT siblings. */
if ((sds->local->flags & SD_SHARE_CPUCAPACITY) ||
(group->flags & SD_SHARE_CPUCAPACITY))
return asym_smt_can_pull_tasks(env->dst_cpu, sds, sgs, group);
/*
* CPU priorities does not make sense for SMT cores with more than one
* busy sibling.
*/
if (group->flags & SD_SHARE_CPUCAPACITY) {
if (sgs->group_weight - sgs->idle_cpus != 1)
return false;
}
return sched_asym_prefer(env->dst_cpu, group->asym_prefer_cpu);
}