[Devel] Re: [PATCH v3 4/6] add a new scheduler hook for context switch

[Peter Zijlstra]

On Wed, 2012-05-30 at 13:48 +0400, Glauber Costa wrote:
> To be able to count the number of switches per-cgroup, and merging
> Paul's hint that it would be better to do it in fair.c and rt.c,
> I am introducing a new write-side walk through a new scheduler hook,
> called at every context switch away from a task (prev).
> 
> Read-side is greatly simplified, and as I'll show, the performance impact
> does not seem huge. First, aside from the function call, this walk is
> O(depth), which is not likely to be huge (if it is, the performance
> impact is indeed bad - but one can argue this is a good punishment)
> 
> Also, this walk is likely to be cache-hot, since it is at most the very
> same loop done by put_prev_task, except it loops depth - 1 instead of depth
> times. This is specially important not to hurt tasks in the root cgroup,
> that will pay just a branch.

/me cries a little.. I was hoping to fix put_prev_task.. see:

  https://lkml.org/lkml/2012/2/16/487

(I've actually got a 4 patch split out of that if anybody cares)

Its just one of those things stuck behind the -ENOTIME tree :/

The plan is to 'merge' put_prev_task and pick_next_task into one and
avoid a lot of the up-down walking.

You just added a constraint for always having to walk the entire thing
up -- cgroups is too damn expensive already, we should be trimming this
nonsense not bloating it.

> However, put_prev_task is called many times from multiple places,
> and the logic to differentiate a context switch from another kind of
> put would make a mess out of it.

I'm hoping the fold of put_prev in pick_next as per that patch I
referenced could help some, but the cross class switch makes that messy
still :/

Reducing the indirect calls is good, adding them is bad.. which makes me
the worst offender I'm afraid.

> On a 4-way x86_64, hackbench -pipe 1 process 4000 shows the following results:
> - units are seconds to complete the whole benchmark
> - percentual stdev for easier assesment
> 
> Task sitting in the root cgroup:
> Without patchset: 4.857700 (0.69 %)
> With    patchset: 4.863700 (0.63 %)
> Difference      : 0.12 %

Just increase the repeat count :-)

$ perf stat -e cycles --repeat 100 perf bench sched messaging -p -g 100

    48,826,146,710 cycles  #    2.470 GHz  ( +-  0.17% )
       2.149005270 seconds time elapsed    ( +-  0.12% )

Anyway, a few nits on the below patch..



> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 4c1d7e9..db4f2c3 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1894,6 +1894,14 @@ fire_sched_out_preempt_notifiers(struct task_struct *curr,
>  
>  #endif /* CONFIG_PREEMPT_NOTIFIERS */
>  
> +static void 

inline

> sched_class_context_switch(struct rq *rq, struct task_struct *prev)
> +{
> +#if defined(CONFIG_FAIR_GROUP_SCHED) || defined(CONFIG_RT_GROUP_SCHED)
> +	if (prev->sched_class->context_switch)
> +		prev->sched_class->context_switch(rq, prev);
> +#endif
> +}
> +

> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 940e6d1..c26fe38 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -2993,6 +2993,20 @@ static struct task_struct *pick_next_task_fair(struct rq *rq)
>  	return p;
>  }
>  
> +static void context_switch_fair(struct rq *rq, struct task_struct *p)
> +{
> +#ifdef CONFIG_FAIR_GROUP_SCHED
> +	struct cfs_rq *cfs_rq;
> +	struct sched_entity *se = &p->se;
> +
> +	while (se->parent) {
> +		se = se->parent;
> +		cfs_rq = group_cfs_rq(se);
> +		cfs_rq->nr_switches++;
> +	}
> +#endif
> +}
> +

Put the whole function inside an existing #ifdef block of the right
kind.

>  /*
>   * Account for a descheduled task:
>   */
> @@ -5255,6 +5269,7 @@ const struct sched_class fair_sched_class = {
>  	.check_preempt_curr	= check_preempt_wakeup,
>  
>  	.pick_next_task		= pick_next_task_fair,
> +	.context_switch		= context_switch_fair,

Put the #ifdeffery here, so that the method is NULL when !FAIR_GROUP,
saves an indirect nop call for some weird .configs.

>  	.put_prev_task		= put_prev_task_fair,
>  
>  #ifdef CONFIG_SMP

idem for sched/rt.c