Re: [patch 00/17] CFS Bandwidth Control v7.1

[Jason Baron <jbaron@redhat.com>]

On Thu, Jul 07, 2011 at 07:59:48PM +0200, Peter Zijlstra wrote:
> On Thu, 2011-07-07 at 16:38 +0200, Peter Zijlstra wrote:
> > On Thu, 2011-07-07 at 13:23 +0200, Ingo Molnar wrote:
> > > 
> > > The +1.5% increase in vanilla kernel context switching performance is 
> > > unfortunate - where does that overhead come from?
> > 
> > Looking at the asm output, I think its partly because things like:
> > 
> > @@ -602,6 +618,8 @@ static void update_curr(struct cfs_rq *c
> >                 cpuacct_charge(curtask, delta_exec);
> >                 account_group_exec_runtime(curtask, delta_exec);
> >         }
> > +
> > +       account_cfs_rq_runtime(cfs_rq, delta_exec);
> >  }
> > 
> > 
> > +static void account_cfs_rq_runtime(struct cfs_rq *cfs_rq,
> > +               unsigned long delta_exec)
> > +{
> > +       if (!cfs_rq->runtime_enabled)
> > +               return;
> > +
> > +       cfs_rq->runtime_remaining -= delta_exec;
> > +       if (cfs_rq->runtime_remaining > 0)
> > +               return;
> > +
> > +       assign_cfs_rq_runtime(cfs_rq);
> > +}
> > 
> > generate a call, only to then take the first branch out, marking that
> > function __always_inline would cure the call problem. Going beyond that
> > would be using static_branch() to track if there is any bandwidth
> > tracking required at all.
> 
> Right, so that cfs_rq->runtime_enabled is almost a guaranteed cacheline
> miss as well, its at the tail end of cfs_rq, then again, the smp-load
> update will want to touch that same cacheline so its not a complete
> waste of time.
> 
> The other big addition to all the fast paths are the various throttled
> checks, those do miss a complete new cacheline.. adding a
> static_branch() to that might make sense.
> 
> compile tested only..
> 

I'm curious to see how the asm look like here for the static
branches in this case, can you post it?

thanks,

-Jason


> ---
> Index: linux-2.6/kernel/sched.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched.c
> +++ linux-2.6/kernel/sched.c
> @@ -71,6 +71,7 @@
>  #include <linux/ctype.h>
>  #include <linux/ftrace.h>
>  #include <linux/slab.h>
> +#include <linux/jump_label.h>
>  
>  #include <asm/tlb.h>
>  #include <asm/irq_regs.h>
> @@ -297,6 +298,7 @@ struct task_group {
>  	struct autogroup *autogroup;
>  #endif
>  
> +	int runtime_enabled;
>  	struct cfs_bandwidth cfs_bandwidth;
>  };
>  
> @@ -410,6 +412,8 @@ struct cfs_rq {
>  };
>  
>  #ifdef CONFIG_CFS_BANDWIDTH
> +static struct jump_label_key cfs_bandwidth_enabled;
> +
>  static inline struct cfs_bandwidth *tg_cfs_bandwidth(struct task_group *tg)
>  {
>  	return &tg->cfs_bandwidth;
> @@ -9075,6 +9079,15 @@ static int tg_set_cfs_bandwidth(struct t
>  			unthrottle_cfs_rq(cfs_rq);
>  		raw_spin_unlock_irq(&rq->lock);
>  	}
> +
> +	if (runtime_enabled && !tg->runtime_enabled)
> +		jump_label_inc(&cfs_bandwidth_enabled);
> +
> +	if (!runtime_enabled && tg->runtime_enabled)
> +		jump_label_dec(&cfs_bandwidth_enabled);
> +
> +	tg->runtime_enabled = runtime_enabled;
> +
>  out_unlock:
>  	mutex_unlock(&cfs_constraints_mutex);
>  
> Index: linux-2.6/kernel/sched_fair.c
> ===================================================================
> --- linux-2.6.orig/kernel/sched_fair.c
> +++ linux-2.6/kernel/sched_fair.c
> @@ -1410,10 +1410,10 @@ static void expire_cfs_rq_runtime(struct
>  	}
>  }
>  
> -static void account_cfs_rq_runtime(struct cfs_rq *cfs_rq,
> +static __always_inline void account_cfs_rq_runtime(struct cfs_rq *cfs_rq,
>  		unsigned long delta_exec)
>  {
> -	if (!cfs_rq->runtime_enabled)
> +	if (!static_branch(&cfs_bandwidth_enabled) || !cfs_rq->runtime_enabled)
>  		return;
>  
>  	/* dock delta_exec before expiring quota (as it could span periods) */
> @@ -1433,13 +1433,13 @@ static void account_cfs_rq_runtime(struc
>  
>  static inline int cfs_rq_throttled(struct cfs_rq *cfs_rq)
>  {
> -	return cfs_rq->throttled;
> +	return static_branch(&cfs_bandwidth_enabled) && cfs_rq->throttled;
>  }
>  
>  /* check whether cfs_rq, or any parent, is throttled */
>  static inline int throttled_hierarchy(struct cfs_rq *cfs_rq)
>  {
> -	return cfs_rq->throttle_count;
> +	return static_branch(&cfs_bandwidth_enabled) && cfs_rq->throttle_count;
>  }
>  
>  /*
> 
> --
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