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SubjectRe: [RFC PATCH v2 00/17] Core scheduling v2
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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.
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

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