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

[Juri Lelli]

Hello,

On 14/06/25 10:04, Kuyo Chang wrote:
> From: kuyo chang <kuyo.chang@xxxxxxxxxxxx>
> 
> [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.
> 
> So it will scaled to 238,418 ns.
> 
> [Solution]
> The runtime for fair_server should be absolute time
> asis RT bandwidth control.
> Fix the runtime calculation formula for the fair_server.
> 
> Signed-off-by: kuyo chang <kuyo.chang@xxxxxxxxxxxx>
> ---

Right, I would agree we don't actually want to scale fair_server runtime
by frequency/capacity. Your change looks good to me.

Acked-by: Juri Lelli <juri.lelli@xxxxxxxxxx>

Thanks!
Juri