linux-kernel - Re: [CFS Bandwidth Control v4 3/7] sched: throttle cfs_rq entities which exceed their local quota

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Message-ID: <AANLkTi=n6vifGft=vTABdYo3-g4-mFs+rWNWO-TnHsE3@mail.gmail.com>
Date:	Sun, 27 Feb 2011 22:38:29 -0800
From:	Paul Turner <pjt@...gle.com>
To:	bharata@...ux.vnet.ibm.com
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	linux-kernel@...r.kernel.org,
	Dhaval Giani <dhaval@...ux.vnet.ibm.com>,
	Balbir Singh <balbir@...ux.vnet.ibm.com>,
	Vaidyanathan Srinivasan <svaidy@...ux.vnet.ibm.com>,
	Gautham R Shenoy <ego@...ibm.com>,
	Srivatsa Vaddagiri <vatsa@...ibm.com>,
	Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
	Ingo Molnar <mingo@...e.hu>,
	Pavel Emelyanov <xemul@...nvz.org>,
	Herbert Poetzl <herbert@...hfloor.at>,
	Avi Kivity <avi@...hat.com>,
	Chris Friesen <cfriesen@...tel.com>,
	Nikhil Rao <ncrao@...gle.com>
Subject: Re: [CFS Bandwidth Control v4 3/7] sched: throttle cfs_rq entities
 which exceed their local quota

On Sun, Feb 27, 2011 at 7:50 PM, Bharata B Rao
<bharata@...ux.vnet.ibm.com> wrote:
> On Fri, Feb 25, 2011 at 12:51:01PM -0800, Paul Turner wrote:
>> On Fri, Feb 25, 2011 at 5:58 AM, Bharata B Rao
>> <bharata@...ux.vnet.ibm.com> wrote:
>> > On Thu, Feb 24, 2011 at 07:10:58PM -0800, Paul Turner wrote:
>> >> On Wed, Feb 23, 2011 at 5:32 AM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
>> >> > On Tue, 2011-02-15 at 19:18 -0800, Paul Turner wrote:
>> >>
>> >> >> +     update_cfs_load(cfs_rq, 0);
>> >> >> +
>> >> >> +     /* prevent previous buddy nominations from re-picking this se */
>> >> >> +     clear_buddies(cfs_rq_of(se), se);
>> >> >> +
>> >> >> +     /*
>> >> >> +      * It's possible for the current task to block and re-wake before task
>> >> >> +      * switch, leading to a throttle within enqueue_task->update_curr()
>> >> >> +      * versus an an entity that has not technically been enqueued yet.
>> >> >
>> >> > I'm not quite seeing how this would happen.. care to expand on this?
>> >> >
>> >>
>> >> I'm not sure the example Bharata gave is correct -- I'm going to treat
>> >> that discussion separately as it's not the intent here.
>> >
>> > Just for the record, my examples were not given for the above question from
>> > Peter.
>> >
>> > I answered two questions and I am tempted to stand by those until proven
>> > wrong :)
>>
>> This is important to get right, I'm happy to elaborate.
>>
>> >
>> > 1. Why do we have cfs_rq_throtted() check in dequeue_task_fair() ?
>>
>> The check is primarily needed because we could become throttled as
>> part of a regular dequeue.  At which point we bail because the parent
>> dequeue is actually complete.
>>
>> (Were it necessitated by load balance we could actually not do this
>> and just perform a hierarchal check within load_balance_fair)
>>
>> > ( => How could we be running if our parent was throttled ?)
>> >
>>
>> The only way we can be running if our parent was throttled is if /we/
>> triggered that throttle and have been marked for re-schedule.
>>
>> > Consider the following hierarchy.
>> >
>> > Root Group
>> >   |
>> >   |
>> > Group 1 (Bandwidth constrained group)
>> >   |
>> >   |
>> > Group 2 (Infinite runtime group)
>> >
>> > Assume both the groups have tasks in them.
>> >
>> > When Group 1 is throttled, its cfs_rq is marked throttled, and is removed from
>> > Root group's runqueue. But leaf tasks in Group 2 continue to be enqueued in
>> > Group 1's runqueue.
>> >
>>
>> Yes, the hierarchy state is maintained in isolation.
>>
>> > Load balancer kicks in on CPU A and figures out that it can pull a few tasks
>> > from CPU B (busiest_cpu). It iterates through all the task groups
>> > (load_balance_fair) and considers Group 2 also. It tries to pull a task from
>> > CPU B's cfs_rq for Group 2. I don't see anything that would prevent the
>> > load balancer from bailing out here.
>>
>> Per above, the descendants of a throttled group are also identified
>> (and appropriately skipped) using h_load.
>
> This bit is still unclear to me. We do nothing in tg_load_down() to treat
> throttled cfs_rqs differently when calculating h_load.

>From above:

"I agree.  We ensure this does not happen by making the h_load zero.
Something I thought I was doing but apparently not, will fix in
repost."

>Nor do we do
> anything in load_balance_fair() to explicitly identify descendents of
> throttled group using h_load AFAICS. All we have is
> cfs_rq_throttled() check, which I think should be converted to entity_on_rq()
> to check for the throttled hierarchy and discard pulling from throttled
> hierarchies.
>
> Regards,
> Bharata.
>
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