Re: [PATCH] sched/deadline: Reject debugfs dl_server writes for offline CPUs

[Juri Lelli <juri.lelli@redhat.com>]

Hi Andrea,

On 26/05/26 12:05, Andrea Righi wrote:
> Writing runtime or period via the per-CPU dl_server debugfs files
> (/sys/kernel/debug/sched/{fair,ext}_server/cpu*/{runtime,period}) on an
> offline CPU can trigger two distinct kernel issues:
> 
> 1) Divide-by-zero in dl_server_apply_params():
> 
>   Oops: divide error: 0000 [#1] SMP NOPTI
>   RIP: 0010:dl_server_apply_params+0x239/0x3a0
>   Call Trace:
>    sched_server_write_common.isra.0+0x21a/0x3c0
>    full_proxy_write+0x78/0xd0
>    vfs_write+0xe7/0x6e0
> 
>   Both __dl_sub() and __dl_add() divide by cpus internally, which can be
>   0 once the CPU has been removed from any active root-domain span (this
>   has been latent since the debugfs interface was introduced).
> 
> 2) WARN_ON_ONCE in dl_server_start():
> 
>   WARNING: kernel/sched/deadline.c:1805 at dl_server_start+0x232/0x270
> 
>   Commit ee6e44dfe6e5 ("sched/deadline: Stop dl_server before CPU goes
>   offline") added this check to catch enqueueing the server on an
>   offline rq.
> 
> There's no meaningful semantics for re-configuring the per-CPU dl_server
> bandwidth while the CPU is offline, so simply reject the write with
> -EBUSY so userspace gets a clear error.
> 
> Reported-by: Sashiko <sashiko-bot@kernel.org>
> Closes: https://lore.kernel.org/all/20260526092228.3B6891F00A3A@smtp.kernel.org/
> Fixes: d741f297bcea ("sched/fair: Fair server interface")
> Signed-off-by: Andrea Righi <arighi@nvidia.com>
> ---
>  kernel/sched/debug.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/kernel/sched/debug.c b/kernel/sched/debug.c
> index ed3a0d65da0ca..e57ad8c78a60e 100644
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -415,6 +415,9 @@ static ssize_t sched_server_write_common(struct file *filp, const char __user *u
>  			return  -EINVAL;
>  		}
>  
> +		if (!cpu_online(cpu_of(rq)))
> +			return -EBUSY;
> +
>  		update_rq_clock(rq);
>  		dl_server_stop(dl_se);
>  		retval = dl_server_apply_params(dl_se, runtime, period, 0);

I was looking at Sashiko findings and wondered what to do about this as
well. I think what you are proposing should be fine, unless for some
reason one wants to tweak dl-server parameters before swithcing a CPU
on. but since hotplug it's a disruptive operation already, I would say
imposing to make such a change after CPU is online should be ok (and
simpler to get right from a bandwidth accounting pov).

Reviewed-by: Juri Lelli <juri.lelli@redhat.com>

Thanks,
Juri