linux-kernel - Re: [patch 12/15] sched: maintain throttled rqs as a list

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Date:	Sun, 24 Apr 2011 14:23:38 -0700
From:	Paul Turner <pjt@...gle.com>
To:	Hidetoshi Seto <seto.hidetoshi@...fujitsu.com>
Cc:	linux-kernel@...r.kernel.org,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Bharata B Rao <bharata@...ux.vnet.ibm.com>,
	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>, Pavel Emelyanov <xemul@...nvz.org>
Subject: Re: [patch 12/15] sched: maintain throttled rqs as a list

On Thu, Apr 21, 2011 at 7:50 PM, Hidetoshi Seto
<seto.hidetoshi@...fujitsu.com> wrote:
> Hi Paul,
>
> (2011/03/23 12:03), Paul Turner wrote:
>> @@ -1434,7 +1435,11 @@ static void throttle_cfs_rq(struct cfs_r
>>       if (!se)
>>               rq->nr_running += task_delta;
>>
>> +     raw_spin_lock(&cfs_b->lock);
>>       cfs_rq->throttled = 1;
>> +     list_add_tail_rcu(&cfs_rq->throttled_list, &cfs_b->throttled_cfs_rq);
>> +     raw_spin_unlock(&cfs_b->lock);
>> +
>>       cfs_rq->throttled_timestamp = rq_of(cfs_rq)->clock;
>>  }
>>
>
> Though I'm looking forward to see your updates soon, just FYI,
> I found that somehow throttle_cfs_rq() was called with already
> throttled cfs_rq.
>
> So it breaks list throttled_cfs_rq by repeating list_add_tail_rcu()
> and results in lockups like that Bharata and Xiao already reported.
>
> There should be better fix, but following first aid to put_prev_entity()
> worked for me:
>
> -       if (!within_bandwidth(cfs_rq))
> +       if (!within_bandwidth(cfs_rq) && !cfs_rq->throttled)
>                throttle_cfs_rq(cfs_rq);
>        else
>                return_cfs_rq_quota(cfs_rq);
>
> I believe you can do better next time.
>

Aha!

Good catch -- Thanks Hidetoshi!  I've been looking for that one.

I'm figuring out exactly how this came about and I will repost.

Thanks!

>
> Thanks,
> H.Seto
>
>
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