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

Peter Zijlstra
2015-12-03T18:17:21Z

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).