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

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: [email protected]
> Signed-off-by: Hui Su <[email protected]>
> ---

Thanks for the fix, makes sense to me.

Acked-by: Juri Lelli <[email protected]>

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,
Juri