linux-kernel - Re: [PATCH v2] sched/eevdf: Prevent vlag from going out of bounds when reweight

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Date: Mon, 22 Apr 2024 13:17:44 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Xuewen Yan <xuewen.yan94@...il.com>
Cc: Xuewen Yan <xuewen.yan@...soc.com>, mingo@...hat.com,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, bristot@...hat.com, vschneid@...hat.com,
	yu.c.chen@...el.com, ke.wang@...soc.com, di.shen@...soc.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched/eevdf: Prevent vlag from going out of bounds
 when reweight_eevdf

On Mon, Apr 22, 2024 at 07:07:25PM +0800, Xuewen Yan wrote:
> On Mon, Apr 22, 2024 at 5:42 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Mon, Apr 22, 2024 at 04:33:37PM +0800, Xuewen Yan wrote:
> >
> > > On the Android system, the nice value of a task will change very
> > > frequently. The limit can also be exceeded.
> > > Maybe the !on_rq case is still necessary.
> > > So I'm planning to propose another patch for !on_rq case later after
> > > careful testing locally.
> >
> > So the scaling is: vlag = vlag * old_Weight / weight
> >
> > But given that integer devision is truncating, you could expect repeated
> > application of such scaling would eventually decrease the vlag instead
> > of grow it.
> >
> > Is there perhaps an invocation of reweight_task() missing? Looking at
> 
> Is it necessary to add reweight_task in the prio_changed_fair()?

I think that's the wrong place. Note how __setscheduler_params() already
has set_load_weight(). And all other callers of ->prio_changed() already
seem to do set_load_weight() as well.

But that idle policy thing there still looks wrong, that sets the weight
very low but doesn't re-adjust anything.