linux-kernel - Re: [RFC][PATCH 4/4] sched, numa: Ignore pinned tasks

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Date:	Mon, 18 May 2015 10:13:35 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
CC:	mingo@...nel.org, dedekind1@...il.com,
	linux-kernel@...r.kernel.org, mgorman@...e.de, rostedt@...dmis.org,
	juri.lelli@....com
Subject: Re: [RFC][PATCH 4/4] sched, numa: Ignore pinned tasks

On 05/18/2015 09:06 AM, Peter Zijlstra wrote:
> On Mon, 2015-05-18 at 18:30 +0530, Srikar Dronamraju wrote:
>>>  
>>>  static void account_numa_dequeue(struct rq *rq, struct task_struct *p)
>>>  {
>>> +	if (p->nr_cpus_allowed == 1) {
>>> +		rq->nr_pinned_running--;
>>> +		WARN_ON_ONCE(p->numa_preferred_nid != -1);
>>> +	}
>>>  	rq->nr_numa_running -= (p->numa_preferred_nid != -1);
>>>  	rq->nr_preferred_running -= (p->numa_preferred_nid == task_node(p));
>>>  }
>>
>>
>> Shouldnt we reset p->numa_preferred_nid when we are setting the allowed
>> cpus in set_cpus_allowed_common()? 
>>
>> Otherwise if an process is set a preferred node based on its numa faults
>> but then is pinned to a different cpu, then we can see this warning.:w!
> 
> We should never get preferred_nid set when nr_cpus_allowed == 1, see the
> hunk that changes task_tick_numa.
> 
> So we set preferred = -1 on pinning, do not partake in numa balancing
> while this is so, therefore it should still be so when we dequeue,
> right?

It could be pinned after it has been running for a while,
with taskset -c <cpu> -p <pid>

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