Hello, Tvrtko. On Mon, Oct 19, 2026 at 11:02:08AM +0100, Tvrtko Ursulin wrote: > > apply_workqueue_attrs() is unrestricted, so a caller can hand a WQ_RT wq the > > attrs from alloc_workqueue_attrs() and turn it into a normal non-strict wq > > while the flag stays set. Maybe apply_wqattrs_prepare() should force prio > > and affinity for WQ_RT instead of restricting only the sysfs side? > > I am not entirely sure about this one. It could be done in > apply_wqattrs_prepare(), but it would end up redundant on the sysfs > path. Because currently there are two paths: ... > Am I missing something?
The missing one is apply_workqueue_attrs() called directly with attrs from alloc_workqueue_attrs(), which is what padata_setup_cpumasks() does for its own wq. Forcing prio and affinity in apply_wqattrs_prepare() covers all the paths in one place. Being redundant on the sysfs path is fine. > > alloc_ordered_workqueue() with WQ_RT passes this and ends up with a single > > pool spanning all CPUs because ordered wqs use dfl_pwq everywhere, while the > > doc and sysfs say strict per-CPU. Should __WQ_ORDERED be rejected too? The > > nested ifs can also be a single condition. > > Not sure - Tejun what is your guidance here? Is this purely an > implementation challenge because conceptually it should work, no? Yeah, the priority part would work. It's just that an ordered wq has a single pwq for all CPUs, so the per-CPU affinity can't apply. Let's reject __WQ_ORDERED for now and allow it when there's a user. > P.S. The nested ifs could be a single condition but I wanted to mimic > the existing style so it looks nicer: ... > I can change if desired. Your call. A single condition would read fine too. > > The three stores check WQ_RT and return -EINVAL while nice goes read-only > > through is_visible below. Can all four go through > > wq_sysfs_unbound_group_visible() returning 0444 for WQ_RT? That drops the > > three store checks. ... > Could do that if it would be acceptable for root to be still able to > write to them - Tejun? Root can't write to them. sysfs sets KERNFS_ROOT_EXTRA_OPEN_PERM_CHECK, so kernfs_fop_open() fails a write open of a file without a write bit even with CAP_DAC_OVERRIDE. Can you make nice, affinity_scope and affinity_strict read-only through is_visible? cpumask can stay writable as narrowing where the RT workers run doesn't break anything. > > Can we keep printing nice for non-RT pools? prio=120 is the internal > > encoding, sysfs and the pool dumps print nice, and the example output in > > workqueue.rst would go stale. prog['RT_PRIORITY'] would match the rest of > > the file too. > > We probably could if we wanted to. Tejun? > > Complication would be having to hardcode some priority conversion > helpers into the script. Yeah, let's keep nice. It's just prio - DEFAULT_PRIO and hardcoding 120 in the script is fine. > > One more thing which isn't in the diff. The rescuer of a WQ_RT | > > WQ_MEM_RECLAIM wq, which is what panthor-drm-rt is, still runs at nice -20, > > so under memory pressure the RT wq's work items run as CFS. Should > > rescuer_thread() use sched_set_fifo_low() for WQ_RT? > > I did not think we should care much about latency when under severe > memory pressure. It should be enough to guarantee forward progress and > the latency restores back to normal once memory pressure stabilises. Agreed. Thanks. -- tejun
