Re: [PATCH] sched_ext: Print deprecation warnings only once

[Breno Leitao <leitao@debian.org>]

On Tue, Jun 23, 2026 at 05:53:33PM +0200, Andrea Righi wrote:
> Hi Breno and Christian,
> 
> On Tue, Jun 23, 2026 at 04:45:55PM +0100, Christian Loehle wrote:
> > On 6/23/26 15:38, Breno Leitao wrote:
> > > The deprecation notices for direct p->scx.slice/dsq_vtime writes and for
> > > ops->cpu_acquire/release() use plain pr_warn(), so they repeat on every
> > > scheduler (re)load and flood the kernel log.
> > > > The slice/dsq_vtime notice is emitted from the BPF verifier's
> > > btf_struct_access callback, which runs once per write access and is
> > > re-evaluated as the verifier explores paths, so loading a single
> > > scheduler can print it several times. The cpu_acquire/release notice is
> > > printed on every scheduler enable.
> > > 
> > > Switch both to pr_warn_once() so each deprecation is reported a single
> > > time, and add the missing newline to the slice/dsq_vtime message.
> > 
> > I guess this should be warned for once per BPF scheduler like
> > sch->warned_deprecated_rq
> 
> Yeah, I agree. The thing is that we can load multiple different BPF schedulers,
> and we'd want to get these messages for each of them. So ideally we need
> something like sch->warned_deprecated_rq.

Agreed, pr_warn_once() is too coarse - it mutes the notice for every
scheduler loaded after the first one since boot.

For ops->cpu_acquire/release() this is easy: the notice comes from
validate_ops(), which has the scx_sched, so I'll gate it on a
per-scheduler warned_deprecated_cpu_rel flag, just like
warned_deprecated_rq.

The slice/dsq_vtime notice is the one I wanted to ask about. It's
emitted from bpf_scx_btf_struct_access(), the .btf_struct_access
verifier callback.

That runs at program load/verification time, before the struct_ops is
registered and the scx_sched is allocated (that only happens later in
scx_enable()). Any idea on what to do in this case?

Thanks,
--breno