linux-kernel - Re: [PATCH 2/2] sched/eevdf: Use sched_attr::sched

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Message-ID: <20231210224724.eteuahkempeeojfz@airbuntu>
Date:   Sun, 10 Dec 2023 22:47:24 +0000
From:   Qais Yousef <qyousef@...alina.io>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, linux-kernel@...r.kernel.org,
        vincent.guittot@...aro.org, juri.lelli@...hat.com,
        dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
        mgorman@...e.de, bristot@...hat.com, corbet@....net,
        chris.hyser@...cle.com, patrick.bellasi@...bug.net, pjt@...gle.com,
        pavel@....cz, qperret@...gle.com, tim.c.chen@...ux.intel.com,
        joshdon@...gle.com, timj@....org, kprateek.nayak@....com,
        yu.c.chen@...el.com, youssefesmat@...omium.org,
        joel@...lfernandes.org, efault@....de, tglx@...utronix.de,
        daniel.m.jordan@...cle.com
Subject: Re: [PATCH 2/2] sched/eevdf: Use sched_attr::sched_runtime to set
 request/slice suggestion

On 09/20/23 00:37, Peter Zijlstra wrote:
> On Tue, Sep 19, 2023 at 11:07:08PM +0100, Qais Yousef wrote:
> > On 09/15/23 14:43, peterz@...radead.org wrote:
> > > Allow applications to directly set a suggested request/slice length using
> > > sched_attr::sched_runtime.
> > 
> > I'm probably as eternally confused as ever, but is this going to be the latency
> > hint too? I find it hard to correlate runtime to latency if it is.
> 
> Yes. Think of it as if a task has to save up for it's slice. Shorter
> slice means a shorter time to save up for the time, means it can run
> sooner. Longer slice, you get to save up longer.
> 
> Some people really want longer slices to reduce cache trashing or
> held-lock-preemption like things. Oracle, Facebook, or virt thingies.
> 
> Other people just want very short activations but wants them quickly.

I did check with several folks around here in the Android world, and none of us
can see how in practice we can use this interface.

It is helpful for those who have a specific system and workload they want to
tune them together. But as a generic app developer interface it will be
impossible to use.

Is that sched-qos thingy worth trying to pursue as an alternative for app
developers? I think from their perspective they just can practically say I care
about running ASAP or not; so a boolean flag to trigger the desire for short
wake up latency. How to implement that, that'd be my pain. But do you see an
issue in principle in trying to go down that route and see how far I (we if
anyone else is interested) can get?

I think the two can co-exist each serving a different purpose.

Or is there something about this interface that makes it usable in this manner
but I couldn't get it?

Thanks!

--
Qais Yousef