Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own cacheline

bsegall
2015-12-03T18:23:06Z

Peter Zijlstra <peterz@infradead.org> writes:

> 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).

Ah, yeah, I forgot about this, my fault.