Re: [PATCH 1/6] sched: migration changes for core scheduling

[Li, Aubrey]

On 2021/3/22 15:48, Peter Zijlstra wrote:
> On Sun, Mar 21, 2021 at 09:34:00PM +0800, Li, Aubrey wrote:
>> Hi Peter,
>>
>> On 2021/3/20 23:34, Peter Zijlstra wrote:
>>> On Fri, Mar 19, 2021 at 04:32:48PM -0400, Joel Fernandes (Google) wrote:
>>>> @@ -7530,8 +7543,9 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>>>>  	 * We do not migrate tasks that are:
>>>>  	 * 1) throttled_lb_pair, or
>>>>  	 * 2) cannot be migrated to this CPU due to cpus_ptr, or
>>>> -	 * 3) running (obviously), or
>>>> -	 * 4) are cache-hot on their current CPU.
>>>> +	 * 3) task's cookie does not match with this CPU's core cookie
>>>> +	 * 4) running (obviously), or
>>>> +	 * 5) are cache-hot on their current CPU.
>>>>  	 */
>>>>  	if (throttled_lb_pair(task_group(p), env->src_cpu, env->dst_cpu))
>>>>  		return 0;
>>>> @@ -7566,6 +7580,13 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>>>>  		return 0;
>>>>  	}
>>>>  
>>>> +	/*
>>>> +	 * Don't migrate task if the task's cookie does not match
>>>> +	 * with the destination CPU's core cookie.
>>>> +	 */
>>>> +	if (!sched_core_cookie_match(cpu_rq(env->dst_cpu), p))
>>>> +		return 0;
>>>> +
>>>>  	/* Record that we found atleast one task that could run on dst_cpu */
>>>>  	env->flags &= ~LBF_ALL_PINNED;
>>>>  
>>>
>>> This one is too strong.. persistent imbalance should be able to override
>>> it.
>>>
>>
>> IIRC, this change can avoid the following scenario:
>>
>> One sysbench cpu thread(cookieA) and sysbench mysql thread(cookieB) running
>> on the two siblings of core_1, the other sysbench cpu thread(cookieA) and
>> sysbench mysql thread(cookieB) running on the two siblings of core2, which
>> causes 50% force idle.
>>
>> This is not an imbalance case.
> 
> But suppose there is an imbalance; then this cookie crud can forever
> stall balance.
> 
> Imagine this cpu running a while(1); with a uniqie cookie on, then it
> will _never_ accept other tasks == BAD.
> 

How about putting the following check in sched_core_cookie_match()?

+	/*
+	 * Ignore cookie match if there is a big imbalance between the src rq
+	 * and dst rq.
+	 */
+	if ((src_rq->cfs.h_nr_running - rq->cfs.h_nr_running) > 1)
+		return true;

This change has significant impact of my sysbench cpu+mysql colocation.

- with this change,
  sysbench cpu tput = 2796 events/s, sysbench mysql = 1315 events/s

- without it, 
  sysbench cpu tput= 3513 events/s, sysbench mysql = 646 events.

Do you have any suggestions before we drop it?

Thanks,
-Aubrey