linux-kernel - Re: [PATCH v3 3/7] sched: throttle cfs_rq entities which exceed their local quota

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Date:	Wed, 13 Oct 2010 12:17:57 +0530
From:	Bharata B Rao <bharata@...ux.vnet.ibm.com>
To:	Paul Turner <pjt@...gle.com>
Cc:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>,
	linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval.giani@...il.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Herbert Poetzl <herbert@...hfloor.at>,
	Avi Kivity <avi@...hat.com>,
	Chris Friesen <cfriesen@...tel.com>,
	Paul Menage <menage@...gle.com>,
	Mike Waychison <mikew@...gle.com>,
	Nikhil Rao <ncrao@...gle.com>
Subject: Re: [PATCH v3 3/7] sched: throttle cfs_rq entities which exceed
	their local quota

On Tue, Oct 12, 2010 at 11:44:29PM -0700, Paul Turner wrote:
> On Tue, Oct 12, 2010 at 11:34 PM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@...fujitsu.com> wrote:
> > On Tue, 12 Oct 2010 13:22:02 +0530
> > Bharata B Rao <bharata@...ux.vnet.ibm.com> wrote:
> >
> >> sched: throttle cfs_rq entities which exceed their local quota
> >>
> >> From: Paul Turner <pjt@...gle.com>
> >>
> >> In account_cfs_rq_quota() (via update_curr()) we track consumption versus a
> >> cfs_rq's local quota and whether there is global quota available to continue
> >> enabling it in the event we run out.
> >>
> >> This patch adds the required support for the latter case, throttling entities
> >> until quota is available to run.  Throttling dequeues the entity in question
> >> and sends a reschedule to the owning cpu so that it can be evicted.
> >>
> >> The following restrictions apply to a throttled cfs_rq:
> >> - It is dequeued from sched_entity hierarchy and restricted from being
> >>   re-enqueued.  This means that new/waking children of this entity will be
> >>   queued up to it, but not past it.
> >> - It does not contribute to weight calculations in tg_shares_up
> >> - In the case that the cfs_rq of the cpu we are trying to pull from is throttled
> >>   it is  is ignored by the loadbalancer in __load_balance_fair() and
> >>   move_one_task_fair().
> >>
> >> Signed-off-by: Paul Turner <pjt@...gle.com>
> >> Signed-off-by: Nikhil Rao <ncrao@...gle.com>
> >> Signed-off-by: Bharata B Rao <bharata@...ux.vnet.ibm.com>
> >> ---
> >>  kernel/sched.c      |   12 ++++++++
> >>  kernel/sched_fair.c |   70 ++++++++++++++++++++++++++++++++++++++++++++++++----
> >>  2 files changed, 76 insertions(+), 6 deletions(-)
> >>
> >> --- a/kernel/sched.c
> >> +++ b/kernel/sched.c
> >> @@ -387,6 +387,7 @@ struct cfs_rq {
> >>  #endif
> >>  #ifdef CONFIG_CFS_BANDWIDTH
> >>       u64 quota_assigned, quota_used;
> >> +     int throttled;
> >>  #endif
> >>  #endif
> >>  };
> >> @@ -1668,6 +1669,8 @@ static void update_group_shares_cpu(stru
> >>       }
> >>  }
> >>
> >> +static inline int cfs_rq_throttled(struct cfs_rq *cfs_rq);
> >> +
> >
> > I just curious that static-inline forward declaration is inlined ?
> >
> 
> Hm.   This function is tiny, I should just move it up, thanks.
> 
> >>  /*
> >>   * Re-compute the task group their per cpu shares over the given domain.
> >>   * This needs to be done in a bottom-up fashion because the rq weight of a
> >> @@ -1688,7 +1691,14 @@ static int tg_shares_up(struct task_grou
> >>       usd_rq_weight = per_cpu_ptr(update_shares_data, smp_processor_id());
> >>
> >>       for_each_cpu(i, sched_domain_span(sd)) {
> >> -             weight = tg->cfs_rq[i]->load.weight;
> >> +             /*
> >> +              * bandwidth throttled entities cannot contribute to load
> >> +              * balance
> >> +              */
> >> +             if (!cfs_rq_throttled(tg->cfs_rq[i]))
> >> +                     weight = tg->cfs_rq[i]->load.weight;
> >> +             else
> >> +                     weight = 0;
> >
> > cpu.share and bandwidth control can't be used simultaneously or...
> > is this fair ? I'm not familiar with scheduler but this allows boost this tg.
> > Could you add a brief documentaion of a spec/feature. in the next post ?
> >
> 
> Bandwidth control is orthogonal to shares, shares continue controls
> distribution of bandwidth when within quota.  Bandwidth control only
> has 'perceivable' effect when you exceed your reservation within a
> quota period.

So if a group gets throttled since its approaching its limit, it might
not be possible to see perfect fairness b/n groups since bandwidth control
kind of takes priority.

Regards,
Bharata.
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