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    Subject[tip: sched/core] sched/fair: Do not even the number of busy CPUs via asym_packing
    The following commit has been merged into the sched/core branch of tip:

    Commit-ID: c9ca07886aaa40225a29e5c1e46ac31d2e14f53a
    Gitweb: https://git.kernel.org/tip/c9ca07886aaa40225a29e5c1e46ac31d2e14f53a
    Author: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
    AuthorDate: Thu, 06 Apr 2023 13:31:43 -07:00
    Committer: Peter Zijlstra <peterz@infradead.org>
    CommitterDate: Mon, 08 May 2023 10:58:36 +02:00

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

    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
    ---
    kernel/sched/fair.c | 86 ++++++++++----------------------------------
    1 file changed, 21 insertions(+), 65 deletions(-)

    diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
    index 3bb8934..48b6f0c 100644
    --- a/kernel/sched/fair.c
    +++ b/kernel/sched/fair.c
    @@ -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);
    }
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