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charset=us-ascii Content-Disposition: inline In-Reply-To: <20250617085558.GN1613376@noisy.programming.kicks-ass.net> 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 > > > > [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). Best, Juri