Re: [RFC PATCH 0/1] sched/pelt: Change PELT halflife at runtime

[Lukasz Luba]

Hi Peter,

On 11/7/22 13:41, Peter Zijlstra wrote:
> On Thu, Sep 29, 2022 at 03:41:47PM +0100, Kajetan Puchalski wrote:
>
> > Based on all the tests we've seen, jankbench or otherwise, the
> > improvement can mainly be attributed to the faster ramp up of frequency
> > caused by the shorter PELT window while using schedutil.
>
> Would something terrible like the below help some?
>
> If not, I suppose it could be modified to take the current state as
> history. But basically it runs a faster pelt sum along side the regular
> signal just for ramping up the frequency.

[snip]

> +
> +			rcu_read_lock();
> +			curr = rcu_dereference(rq->curr);
> +			if (likely(curr->sched_class == &fair_sched_class)) {
> +				u64 runtime = curr->se.sum_exec_runtime - curr->se.exec_start;
> +				util = max_t(unsigned long, util,
> +					     faster_est_approx(runtime * 2));

That's a really nice hack :)

I wonder why we end up in such situation on Android. Maybe there is
a different solution?
Maybe shorter tick (then also align PELT Half-Life)?

The problem is mostly in those high-FPS phones. You know, we now have
phones with 144Hz displays and even games > 100FPS (which wasn't the
case a few years ago when we invested a lot of effort into this
PELT+EAS). We also have a lot faster CPUs (~2x in 3-4 years).

IMO those games (and OS mechanisms assisting them) would have different
needs probably (if they do this 'soft-real-time simulations' with such
high granularity ~120/s -> every ~8ms).

IMO one old setting might not fit well into this: 4ms tick (which is the
Android use case), which then implies scheduler min granularity, which
we also align with the y^32 PELT.
Is this a correct chain of thinking?

Would it make sense to ask Android phone vendors to experiment with
1ms tick (w/ aligned PELT HL)? With that change, there might be some
power spikes issues, though.

Regards,
Lukasz