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

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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.
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[PATCH v2 0/3] sched/fair: Reduce contention on tg's load_avg Waiman Long <Waiman.Long@hpe.com> - 2015-12-02 19:50 +0100
  [PATCH v2 1/3] sched/fair: Avoid redundant idle_cpu() call in update_sg_lb_stats() Waiman Long <Waiman.Long@hpe.com> - 2015-12-02 19:50 +0100
  [PATCH v2 3/3] sched/fair: Disable tg load_avg update for root_task_group Waiman Long <Waiman.Long@hpe.com> - 2015-12-02 19:50 +0100
    Re: [PATCH v2 3/3] sched/fair: Disable tg load_avg update for root_task_group bsegall@google.com - 2015-12-02 21:00 +0100
    [tip:sched/core] sched/fair:   Disable the task group load_avg update for the root_task_group tip-bot for Waiman Long <tipbot@zytor.com> - 2015-12-04 13:00 +0100
  [PATCH v2 2/3] sched/fair: Move hot load_avg into its own cacheline Waiman Long <Waiman.Long@hpe.com> - 2015-12-02 19:50 +0100
    Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own cacheline bsegall@google.com - 2015-12-02 21:10 +0100
      Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own cacheline Waiman Long <waiman.long@hpe.com> - 2015-12-03 20:30 +0100
        Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own cacheline bsegall@google.com - 2015-12-03 20:50 +0100
    Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own  cacheline Mike Galbraith <umgwanakikbuti@gmail.com> - 2015-12-03 05:40 +0100
      Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own cacheline Waiman Long <waiman.long@hpe.com> - 2015-12-03 20:40 +0100
        Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own  cacheline Mike Galbraith <umgwanakikbuti@gmail.com> - 2015-12-04 03:10 +0100
          Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own cacheline Waiman Long <waiman.long@hpe.com> - 2015-12-04 21:20 +0100
    Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own  cacheline Peter Zijlstra <peterz@infradead.org> - 2015-12-03 12:00 +0100
      Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own cacheline Waiman Long <waiman.long@hpe.com> - 2015-12-03 20:40 +0100
    Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own  cacheline Peter Zijlstra <peterz@infradead.org> - 2015-12-03 12:20 +0100
      Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own cacheline bsegall@google.com - 2015-12-03 19:10 +0100
        Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own  cacheline Peter Zijlstra <peterz@infradead.org> - 2015-12-03 19:20 +0100
          Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own cacheline bsegall@google.com - 2015-12-03 19:30 +0100
      Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own cacheline Waiman Long <waiman.long@hpe.com> - 2015-12-03 21:00 +0100
        Re: [PATCH v2 2/3] sched/fair: Move hot load_avg into its own  cacheline Peter Zijlstra <peterz@infradead.org> - 2015-12-03 21:10 +0100
    [tip:sched/core] sched/fair: Move the cache-hot 'load_avg'   variable into its own cacheline tip-bot for Waiman Long <tipbot@zytor.com> - 2015-12-04 13:00 +0100

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