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    SubjectRe: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own cacheline
    On Thu, Dec 03, 2015 at 09:56:02AM -0800, bsegall@google.com wrote:
    > Peter Zijlstra <peterz@infradead.org> writes:

    > > @@ -7402,11 +7405,12 @@ void __init sched_init(void)
    > > #endif /* CONFIG_RT_GROUP_SCHED */
    > >
    > > #ifdef CONFIG_CGROUP_SCHED
    > > + task_group_cache = KMEM_CACHE(task_group, 0);
    > > +
    > > list_add(&root_task_group.list, &task_groups);
    > > INIT_LIST_HEAD(&root_task_group.children);
    > > INIT_LIST_HEAD(&root_task_group.siblings);
    > > autogroup_init(&init_task);
    > > -
    > > #endif /* CONFIG_CGROUP_SCHED */
    > >
    > > for_each_possible_cpu(i) {
    > > --- a/kernel/sched/sched.h
    > > +++ b/kernel/sched/sched.h
    > > @@ -248,7 +248,12 @@ struct task_group {
    > > unsigned long shares;
    > >
    > > #ifdef CONFIG_SMP
    > > - atomic_long_t load_avg;
    > > + /*
    > > + * load_avg can be heavily contended at clock tick time, so put
    > > + * it in its own cacheline separated from the fields above which
    > > + * will also be accessed at each tick.
    > > + */
    > > + atomic_long_t load_avg ____cacheline_aligned;
    > > #endif
    > > #endif
    > >
    >
    > This loses the cacheline-alignment for task_group, is that ok?

    I'm a bit dense (its late) can you spell that out? Did you mean me
    killing SLAB_HWCACHE_ALIGN? That should not matter because:

    #define KMEM_CACHE(__struct, __flags) kmem_cache_create(#__struct,\
    sizeof(struct __struct), __alignof__(struct __struct),\
    (__flags), NULL)

    picks up the alignment explicitly.

    And struct task_group having one cacheline aligned member, means that
    the alignment of the composite object (the struct proper) must be an
    integer multiple of this (typically 1).




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