Re: [RFC PATCH v2 00/17] Core scheduling v2

["Li, Aubrey" <aubrey.li@linux.intel.com>]

On 2019/5/18 8:58, Li, Aubrey wrote:
> On 2019/4/30 12:42, Ingo Molnar wrote:
>>
>>>> What's interesting is how in the over-saturated case (the last three
>>>> rows: 128, 256 and 512 total threads) coresched-SMT leaves 20-30% CPU
>>>> performance on the floor according to the load figures.
>>>
> 
> Sorry for a delay, I got a chance to obtain some profiling results. Here
> is the story on my side. I still used the previous testing 128/128 case
> (256 threads totally), and focus on CPU53(randomly pickup) only.
> 
> Firstly, mpstat reports cpu utilization,
> - baseline is 100%,
> - coresched-SMT is 87.51%
> 
> Then I traced sched_switch trace point, in 100s sampling period,
> - baseline context switch 14083 times, next task idle 0 times
> - coresched-SMT context switch 15101 times, next task idle 880 times
> 
> So I guess pick_next_task() is mostly the interesting place, then I
> dig into the trace log on coresched-SMT case:
> - CPU53 selected idle task 767 times (matched with the data of sched_switch)
> 
> There are 3 branches of CPU53 selecting idle task in pick_next_task():
> - pick pre selected 765 times
> - unconstrained pick 1 times
> - picked: swapper/53/0 1 times
> 
> Where CPU53's "pick pre selected idle task" from? I guess its from its
> brother CPU1, so I checked CPU1's trace log and found:
> - CPU1 helped its sibling CPU53 select idle task 800 times
> 
> So for CPU53, the most interesting part occurs in pick_task(), that is:
> -The sibling CPU1 helped to select idle task in pick_task()
> 
> Forgive me to paste this routine() here:
> =====================================================
> +// XXX fairness/fwd progress conditions
> +static struct task_struct *
> +pick_task(struct rq *rq, const struct sched_class *class, struct task_struct *max)
> +{
> +	struct task_struct *class_pick, *cookie_pick;
> +	unsigned long cookie = 0UL;
> +
> +	/*
> +	 * We must not rely on rq->core->core_cookie here, because we fail to reset
> +	 * rq->core->core_cookie on new picks, such that we can detect if we need
> +	 * to do single vs multi rq task selection.
> +	 */
> +
> +	if (max && max->core_cookie) {
> +		WARN_ON_ONCE(rq->core->core_cookie != max->core_cookie);
> +		cookie = max->core_cookie;
> +	}
> +
> +	class_pick = class->pick_task(rq);
> +	if (!cookie)
> +		return class_pick;
> +
> +	cookie_pick = sched_core_find(rq, cookie);
> +	if (!class_pick)
> +		return cookie_pick;
> +
> +	/*
> +	 * If class > max && class > cookie, it is the highest priority task on
> +	 * the core (so far) and it must be selected, otherwise we must go with
> +	 * the cookie pick in order to satisfy the constraint.
> +	 */
> +	if (cpu_prio_less(cookie_pick, class_pick) && core_prio_less(max, class_pick))
> +		return class_pick;
> +
> +	return cookie_pick;
> +}
> =================================================================
> 
> And the most related log of the case:
> =================================================================
> <...>-21553 [001] dN.. 87341.514992: __schedule: cpu(1): selected: gemmbench/21294 ffff888823df8900
> <...>-21553 [001] dN.. 87341.514992: __schedule: max: gemmbench/21294 ffff888823df8900
> <...>-21553 [001] dN.. 87341.514995: __schedule: (swapper/53/0;140,0,0) ?< (sysbench/21503;140,457178607302,0)
> <...>-21553 [001] dN.. 87341.514996: __schedule: (gemmbench/21294;119,219715519947,0) ?< (sysbench/21503;119,457178607302,0)
> <...>-21553 [001] dN.. 87341.514996: __schedule: cpu(53): selected: swapper/53/0 0
> 
> It said,
> - CPU1 selected gemmbench for itself
> - and gemmbench was assigned to max of this core
> - then CPU1 helped CPU53 to pick_task()
> -- CPU1 used class->pick_task(), selected sysbench for CPU53
> -- CPU1 used cookie_pick, selected swapper(idle task) for CPU53
> -- the class_pick(sysbench) unfortunately didn't pass the priority check
> - idle task picked up at the end(sadly).
> 
> So, I think if we want to improve CPU utilization under this scenario,
> the straightforward tweak is picking up class_pick if cookie_pick is idle.

Another quick thought is, in CPU53's own path of pick_next_task, give up
pre selected(by CPU1) if pre selected is idle?


> But I know, this is a violation of the design philosophy(avoid L1TF) of
> this proposal.
> 
> Does it make sense to add a knob to switch security/performance?
> Welcome any comments!
> 
> Thanks,
> -Aubrey
>