Re: [PATCH 2/2] sched/fair: Randomize equally shallow slow-path candidates

[Christian Loehle]

On 9/16/26 20:43, Shubhang wrote:
> Hi Christian,
> 
> On Wed, 16 Sep 2026, Christian Loehle wrote:
> 
>> +            } else if (!idle || idle->exit_latency == min_exit_latency) {
>> +                nr_candidates++;
>> +                if (nr_candidates == 1 ||
>> +                    !reciprocal_scale(sched_rng(), nr_candidates))
>> +                    shallowest_idle_cpu = i;
> 
> available_idle_cpu(i) ensures that this is an idle CPU, but !idle means that no active cpuidle state, meaning no exit latency is available for comparison.
> 
> [PATCH 1/2] treats such a CPU as a fallback i.e. it is selected only when no idle candidate has been found yet. Here it becomes an equal reservoir candidate, even after selecting a CPU with the minimum known exit latency. That is, it is added to the random selection pool and can replace shallowest_idle_cpu.
> 
> Is that intentional ? If not, should reservoir sampling be limited to
> candidates with `idle->exit_latency == min_exit_latency`, while retaining the first !idle CPU only as the fallback?

Hi Shubhang,

Thanks for taking a look. Including !idle candidates was intentional,
although you're right that this changes their treatment.

I think there's a case for giving NULL a zero ranking. With a working
cpuidle driver, NULL can mean the CPU is preparing for entry or finishing
after exit, making it a good low-latency candidate.
Architecture fallbacks are also reasonable candidates AFAICS.
In any case, I'd prefer to address that separately (perhaps through a
helper in the idle/cpuidle code rather than embedding those assumptions
in fair.c and will drop !idle CPUs competing with min_exit_latency CPUs,
the patch doesn't need it.
Thanks!
Christian