Re: [PATCH v5 1/1] sched/deadline: Fix dl_server runtime calculation formula

[Peter Zijlstra]

On Tue, Jul 01, 2025 at 08:28:12PM -0700, John Stultz wrote:
> On Tue, Jul 1, 2025 at 7:14???PM Kuyo Chang <kuyo.chang@xxxxxxxxxxxx> wrote:
> >
> > In our testing with 6.12 based kernel on a big.LITTLE system, we were
> > seeing instances of RT tasks being blocked from running on the LITTLE
> > cpus for multiple seconds of time, apparently by the dl_server. This
> > far exceeds the default configured 50ms per second runtime.
> >
> > This is due to the fair dl_server runtime calculation being scaled
> > for frequency & capacity of the cpu.
> >
> > Consider the following case under a Big.LITTLE architecture:
> > Assume the runtime is: 50,000,000 ns, and Frequency/capacity
> > scale-invariance defined as below:
> > Frequency scale-invariance: 100
> > Capacity scale-invariance: 50
> > First by Frequency scale-invariance,
> > the runtime is scaled to 50,000,000 * 100 >> 10 = 4,882,812
> > Then by capacity scale-invariance,
> > it is further scaled to 4,882,812 * 50 >> 10 = 238,418.
> > So it will scaled to 238,418 ns.
> >
> > This smaller "accounted runtime" value is what ends up being
> > subtracted against the fair-server's runtime for the current period.
> > Thus after 50ms of real time, we've only accounted ~238us against the
> > fair servers runtime. This 209:1 ratio in this example means that on
> > the smaller cpu the fair server is allowed to continue running,
> > blocking RT tasks, for over 10 seconds before it exhausts its supposed
> > 50ms of runtime.  And on other hardware configurations it can be even
> > worse.
> >
> > For the fair deadline_server, to prevent realtime tasks from being
> > unexpectedly delayed, we really do want to use fixed time, and not
> > scaled time for smaller capacity/frequency cpus. So remove the scaling
> > from the fair server's accounting to fix this.
> >
> > Signed-off-by: kuyo chang <kuyo.chang@xxxxxxxxxxxx>
> > Acked-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
> > Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> > Suggested-by: John Stultz <jstultz@xxxxxxxxxx>
> > Tested-by: John Stultz <jstultz@xxxxxxxxxx>
> 
> Thanks so much again for the multiple iterations here.
> 
> I've re-tested this and it still looks good.
> 
> For what its worth:
> Acked-by: John Stultz <jstultz@xxxxxxxxxx>
> 
> Juril/Peter: if/when you take this, could you add a:
> Fixes: a110a81c52a9 ("sched/deadline: Deferrable dl server")

Nice, b4 picked it up and DTRT.

Thanks!