Re: [PATCH for-4.21 00/10] x86/HPET: broadcast IRQ and other improvements

[Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>]

On 10/16/25 9:30 AM, Jan Beulich wrote:

> While 1db7829e5657 ("x86/hpet: do local APIC EOI after interrupt processing")
> helped quite a bit, nested interrupts could still occur. First and foremost
> as a result from IRQ migration (where we don't have any control over the
> vectors chosen). Hence besides reducing the number of IRQs that can be raised
> (first two patches) and possibly the number of invocations of
> handle_hpet_broadcast() from the IRQ handler (optional patch 4), the main
> goal here is to eliminate the potential for nested IRQs (patch 3). These
> patches are imo 4.21 candidates (with patch 4 being questionable altogether;
> see there).
>     
I am not sure that patch 4 is very useful at the current stage.
The first three patches look useful enough for now, so:
 Releaase-Acked-by: Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>

~ Oleksii

> Patches 5 and onwards likely aren't important enough anymore at
> this point of the release cycle, even if those with a Fixes: tag would likely
> end up being backported later on.
>
> The one related thing I haven't been able to find a viable solution for is
> the elimination of the cpumask_t local variable in handle_hpet_broadcast().
> That'll get in the way of possible future increases of the NR_CPUS upper
> bound: Much like right now a single level of nesting is already too much,
> if the limit was doubled even a single IRQ would end up consuming too much
> stack space (together with cpumask_raise_softirq() also having such a
> variable). Yet further doubling would not allow any such stack variables
> anymore.
>
> 01: limit channel changes
> 02: disable unused channels
> 03: use single, global, low-priority vector for broadcast IRQ
> 04: ignore "stale" IRQs
> 05: avoid indirect call to event handler
> 06: make another channel flags update atomic
> 07: move legacy tick IRQ count adjustment
> 08: shrink IRQ-descriptor locked region in set_channel_irq_affinity()
> 09: reduce hpet_next_event() call sites
> 10: don't use hardcoded 0 for "long timeout"
>
> Jan
>