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

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Date:   Wed, 9 Nov 2022 15:18:52 +0000
From:   Lukasz Luba <lukasz.luba@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Jian-Min Liu <jian-min.liu@...iatek.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...nel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Vincent Donnefort <vdonnefort@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Patrick Bellasi <patrick.bellasi@...bug.net>,
        Abhijeet Dharmapurikar <adharmap@...cinc.com>,
        Qais Yousef <qais.yousef@....com>,
        linux-kernel@...r.kernel.org,
        Jonathan JMChen <jonathan.jmchen@...iatek.com>,
        Kajetan Puchalski <kajetan.puchalski@....com>
Subject: Re: [RFC PATCH 0/1] sched/pelt: Change PELT halflife at runtime

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