Re: [PATCH] sched/deadline: Fix DL server initialization for initially offline CPUs

[Hui Su]

>Hi Hui Su,
>
>On 11/08/26 18:06, Hui Su wrote:
>> Commit 9f239df55546 ("sched/deadline: Initialize dl_servers after SMP")
>> moved DL server initialization to sched_init_smp(), assuming that all
>> CPUs are online after SMP initialization.
>>
>> That assumption does not hold when CPUs are intentionally left offline
>> at boot, for example with maxcpus=. sched_init_dl_servers() only walks
>> online CPUs, so the fair and sched_ext DL servers of an initially
>> offline CPU never get their default DL parameters configured. Bringing
>> the CPU online later does not run this initialization again, leaving the
>> server parameters at zero.
>>
>> For example, boot a two-vCPU QEMU guest with maxcpus=1 and then bring
>> CPU1 online:
>>
>>   # echo 1 > /sys/devices/system/cpu/cpu1/online
>>   # cat /sys/kernel/debug/sched/fair_server/cpu1/runtime
>>   0
>>   # cat /sys/kernel/debug/sched/fair_server/cpu1/period
>>   0
>>
>> With a zero runtime the fair server cannot be activated, so fair tasks
>> on the late-online CPU can be starved when higher-priority tasks
>> monopolize the CPU, potentially causing user-space workloads to stall.
>>
>> Initialize DL servers for all possible CPUs instead. For CPUs that are
>> already online, preserve the existing initialization behavior. For an
>> initially offline CPU, initialize the server parameters and keep its
>> bandwidth reservation local to the runqueue, but do not account it to
>> the root domain until the CPU becomes active. A subsequent root-domain
>> rebuild then publishes the reservation.
>>
>> Avoid updating the runqueue clock and setting up the current CBS period
>> for offline CPUs. The configured runtime and period are initialized at
>> boot, while the dynamic runtime and absolute deadline are initialized
>> through the normal CBS wakeup path when the server is first started.
>>
>> After the fix, the same test reports:
>>
>>   # cat /sys/kernel/debug/sched/fair_server/cpu1/runtime
>>   50000000
>>   # cat /sys/kernel/debug/sched/fair_server/cpu1/period
>>   1000000000
>>
>> Fixes: 9f239df55546 ("sched/deadline: Initialize dl_servers after SMP")
>> Cc: stable@xxxxxxxxxxxxxxx
>> Signed-off-by: Hui Su <sh_def@xxxxxxx>
>> ---
>
>Thanks for the fix, makes sense to me.
>
>Acked-by: Juri Lelli <juri.lelli@xxxxxxxxxx>
>
>BTW, while reviewing, sashiko noted a related pre-existing issue in the
>init=false path of dl_server_apply_params(): __dl_sub() and __dl_add() are
>called unconditionally with cpus that could be 0 during CPU hot-unplug, when
>the CPU is no longer active but still online. sched_server_write_common() only
>checks cpu_online(), so a debugfs write during that window could hit a
>divide-by-zero.
>
>Could you take a look and consider posting an additional patch to address it?
>---

Thanks for pointing this out. I will take a look at the issue and post a
separate patch to fix it.

Thanks.