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