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

[Andrea Righi <arighi@nvidia.com>]

Hi Peter,

On Tue, May 26, 2026 at 02:07:53PM +0200, Juri Lelli wrote:
> 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>

If this makes sense to you, could you add it to your queue:sched/core?

Otherwise it's possible to trigger the issues above by changing dl_server
bandwidth for offline CPUs.

Thanks,
-Andrea