Re: [PATCH 1/1] sched/deadline: Fix fair_server runtime calculation formula

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Jun 17, 2025 at 02:33:15PM +0200, Juri Lelli wrote:
> On 17/06/25 10:55, Peter Zijlstra wrote:
> > On Sat, Jun 14, 2025 at 10:04:55AM +0800, Kuyo Chang wrote:
> > > From: kuyo chang <kuyo.chang@mediatek.com>
> > > 
> > > [Symptom]
> > > The calculation formula for fair_server runtime is based on
> > > Frequency/CPU scale-invariance.
> > > This will cause excessive RT latency (expect absolute time).
> > > 
> > > [Analysis]
> > > Consider the following case under a Big.LITTLE architecture:
> > > 
> > > Assume the runtime is : 50,000,000 ns, and FIE/CIE as below
> > > FIE: 100
> > > CIE:50
> > > First by FIE, the runtime is scaled to 50,000,000 * 100 >> 10 = 4,882,812
> > > Then by CIE, it is further scaled to 4,882,812 * 50 >> 10 = 238,418.
> > 
> > What's this FIE/CIE stuff? Is that some ARM lingo?
> > 
> > 
> > > diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> > > index ad45a8fea245..8bfa846cf0dc 100644
> > > --- a/kernel/sched/deadline.c
> > > +++ b/kernel/sched/deadline.c
> > > @@ -1504,7 +1504,10 @@ static void update_curr_dl_se(struct rq *rq, struct sched_dl_entity *dl_se, s64
> > >  	if (dl_entity_is_special(dl_se))
> > >  		return;
> > >  
> > > -	scaled_delta_exec = dl_scaled_delta_exec(rq, dl_se, delta_exec);
> > > +	if (dl_se == &rq->fair_server)
> > > +		scaled_delta_exec = delta_exec;
> > > +	else
> > > +		scaled_delta_exec = dl_scaled_delta_exec(rq, dl_se, delta_exec);
> > 
> > Juri, the point it a bit moot atm, but is this something specific to the
> > fair_server in particular, or all servers?
> 
> I believe for other servers (i.e., rt-server work from Yuri and Luca) it
> might be useful to have it configurable somehow. I actually had a recent
> discussion about this concerning single task entities (traditional
> deadline servers) for which as well there might be cases where one might
> want not to scale considering frequency and capacity.
> 
> > Because if this is something all servers require then the above is
> > ofcourse wrong.
> 
> To me it looks like we want this (no scaling) for fair_server (and
> possibly scx_server?) as for them we are only looking into a 'fixed
> time' type of isolation. Full fledged servers (hierarchical scheduling)
> maybe have it configurable, or enabled by default as a start (as we have
> it today).

Right. Then we should write the above like:

	scaled_delta_exec = delta_exec;
	if (!dl_se->dl_server)
		scaled_delta_exec = dl_scaled_delta_exec(rq, dl_se, delta_exec);

and let any later server users add bits on if they want more options.