linux-kernel - Re: [RFC][PATCH 13/16] sched: Add core wide task selection and scheduling.

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Date:   Tue, 9 Apr 2019 11:09:45 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Aaron Lu <aaron.lu@...ux.alibaba.com>,
        Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, tglx@...utronix.de, pjt@...gle.com,
        torvalds@...ux-foundation.org, linux-kernel@...r.kernel.org,
        subhra.mazumdar@...cle.com, fweisbec@...il.com,
        keescook@...omium.org, kerrnel@...gle.com,
        Aubrey Li <aubrey.intel@...il.com>,
        Julien Desfossez <jdesfossez@...italocean.com>
Subject: Re: [RFC][PATCH 13/16] sched: Add core wide task selection and
 scheduling.

On 4/5/19 7:55 AM, Aaron Lu wrote:
> On Tue, Apr 02, 2019 at 10:28:12AM +0200, Peter Zijlstra wrote:
>> Another approach would be something like the below:
>>
>>
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -87,7 +87,7 @@ static inline int __task_prio(struct tas
>>   */
>>  
>>  /* real prio, less is less */
>> -static inline bool __prio_less(struct task_struct *a, struct task_struct *b, bool runtime)
>> +static inline bool __prio_less(struct task_struct *a, struct task_struct *b, u64 vruntime)
>>  {
>>  	int pa = __task_prio(a), pb = __task_prio(b);
>>  
>> @@ -104,21 +104,25 @@ static inline bool __prio_less(struct ta
>>  	if (pa == -1) /* dl_prio() doesn't work because of stop_class above */
>>  		return !dl_time_before(a->dl.deadline, b->dl.deadline);
>>  
>> -	if (pa == MAX_RT_PRIO + MAX_NICE && runtime) /* fair */
>> -		return !((s64)(a->se.vruntime - b->se.vruntime) < 0);
>> +	if (pa == MAX_RT_PRIO + MAX_NICE) /* fair */
>> +		return !((s64)(a->se.vruntime - vruntime) < 0);
> 		                                         ~~~
> I think <= should be used here, so that two tasks with the same vruntime
> will return false. Or we could bounce two tasks having different tags
> with one set to max in the first round and the other set to max in the
> next round. CPU would stuck in __schedule() with irq disabled.
> 
>>  
>>  	return false;
>>  }
>>  
>>  static inline bool cpu_prio_less(struct task_struct *a, struct task_struct *b)
>>  {
>> -	return __prio_less(a, b, true);
>> +	return __prio_less(a, b, b->se.vruntime);
>>  }
>>  
>>  static inline bool core_prio_less(struct task_struct *a, struct task_struct *b)
>>  {
>> -	/* cannot compare vruntime across CPUs */
>> -	return __prio_less(a, b, false);
>> +	u64 vruntime = b->se.vruntime;
>> +
>> +	vruntime -= task_rq(b)->cfs.min_vruntime;
>> +	vruntime += task_rq(a)->cfs.min_vruntime
> 
> After some testing, I figured task_cfs_rq() should be used instead of
> task_rq(:-)
> 
> With the two changes(and some other minor ones that still need more time
> to sort out), I'm now able to start doing 2 full CPU kbuilds in 2 tagged
> cgroups. Previouslly, the system would hang pretty soon after I started
> kbuild in any tagged cgroup(presumbly, CPUs stucked in __schedule() with
> irqs disabled).
> 
> And there is no warning appeared due to two tasks having different tags
> get scheduled on the same CPU.
> 
> Thanks,
> Aaron
> 

Peter,

Now that we have accumulated quite a number of different fixes to your orginal
posted patches.  Would you like to post a v2 of the core scheduler with the fixes?

Thanks.

Tim