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

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Message-ID: <20230919223704.GG424@noisy.programming.kicks-ass.net>
Date:   Wed, 20 Sep 2023 00:37:04 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Qais Yousef <qyousef@...alina.io>
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 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.