Re: [PATCH v2 1/7] sched/fair: Fix zero_vruntime tracking

[Dietmar Eggemann]

On 23.02.26 15:15, Peter Zijlstra wrote:
> On Mon, Feb 23, 2026 at 02:09:52PM +0100, Dietmar Eggemann wrote:
>> On 19.02.26 08:58, Peter Zijlstra wrote:
>>> It turns out that zero_vruntime tracking is broken when there is but a single
>>> task running. Current update paths are through __{en,de}queue_entity(), and
>>> when there is but a single task, pick_next_task() will always return that one
>>> task, and put_prev_set_next_task() will end up in neither function.
>>
>> Tried hard but I don't get the last clause.
> 
> When prev==next, then put_prev_set_next_task() bails out and we'll never
> hit __enqueue_entity()/__dequeue_entity().

Ah, I see. But IMHO put_prev_set_next_task() is never called for the
testcase below (CPU hog is prev and next in put_prev_set_next_task())

But (prev != p) in pick_next_task_fair() is avoided ?

pick_next_task_fair()

    if (prev != p) {

        while (!is_same_group())
            put_prev_entity()
            set_next_entity()

        put_prev_entity()
        set_next_entity()
    }

[...]

>>> Test case:
>>>   # taskset -c -p 1 $$
>>>   # taskset -c 2 bash -c 'while :; do :; done&'
>>>   # cat /sys/kernel/debug/sched/debug | awk '/^cpu#/ {P=0} /^cpu#2,/ {P=1} {if (P) print $0}' | grep -e zero_vruntime -e "^>"
>>                                                                     ^
>>                                                                     |
>>                                                                Works
>> only w/o this comma for me.
> 
> Hmm, weird, for me:
> 
> cat /sys/kernel/debug/sched/debug | grep ^cpu#2
> cpu#2, 2500.000 MHz
> cpu#20, 2500.000 MHz
> cpu#21, 2500.000 MHz
> cpu#22, 2500.000 MHz
> cpu#23, 2500.000 MHz
> cpu#24, 2500.000 MHz
> cpu#25, 2500.000 MHz
> cpu#26, 2500.000 MHz
> cpu#27, 2500.000 MHz
> cpu#28, 2500.000 MHz
> cpu#29, 2500.000 MHz
> 
> And that ',' was added because otherwise it would match the full 20
> range of CPUs too, which was not intended ;-)

Ah, the ', X Mhz' thing is X86 specific.

   awk '/^cpu#/ {P=0} /^cpu#2(,|$)/ {P=1} {if (P) print $0}'

works on Arm64 as well.