Re: [PATCH v2 for-4.21 3/9] x86/HPET: replace handle_hpet_broadcast()'s on-stack cpumask_t

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Mon, Oct 20, 2025 at 01:19:20PM +0200, Jan Beulich wrote:
> With large NR_CPUS on-stack cpumask_t variables are problematic. Now that
> the IRQ handler can't be invoked in a nested manner anymore, we can
> instead use a per-CPU variable. While we can't use scratch_cpumask in code
> invoked from IRQ handlers, simply amend that one with a HPET-special form.
> (Note that only one of the two IRQ handling functions can come into play
> at any one time.)
> 
> Fixes: 996576b965cc ("xen: allow up to 16383 cpus")
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>

> ---
> While doing this I noticed that this and all pre-existing uses of
> DEFINE_PER_CPU_READ_MOSTLY() aren't quite right: When the type is
> cpumask_var_t, the variable is read-mostly only when NR_CPUS <=
> 2 * BITS_PER_LONG. That'll want sorting separately, though.
> 
> It is important for this to not be moved ahead of "x86/HPET: use single,
> global, low-priority vector for broadcast IRQ", as the original call here
> from set_channel_irq_affinity() may not use the new variable (it would
> need to use scratch_cpumask instead).
> ---
> v2: New.
> 
> --- a/xen/arch/x86/hpet.c
> +++ b/xen/arch/x86/hpet.c
> @@ -196,7 +196,7 @@ static void evt_do_broadcast(cpumask_t *
>  
>  static void cf_check handle_hpet_broadcast(struct hpet_event_channel *ch)
>  {
> -    cpumask_t mask;
> +    cpumask_t *scratch = this_cpu(hpet_scratch_cpumask);
>      s_time_t now, next_event;
>      unsigned int cpu;
>      unsigned long flags;
> @@ -209,7 +209,7 @@ again:
>      spin_unlock_irqrestore(&ch->lock, flags);
>  
>      next_event = STIME_MAX;
> -    cpumask_clear(&mask);
> +    cpumask_clear(scratch);
>      now = NOW();
>  
>      /* find all expired events */
> @@ -218,13 +218,13 @@ again:
>          s_time_t deadline = ACCESS_ONCE(per_cpu(timer_deadline, cpu));
>  
>          if ( deadline <= now )
> -            __cpumask_set_cpu(cpu, &mask);
> +            __cpumask_set_cpu(cpu, scratch);
>          else if ( deadline < next_event )
>              next_event = deadline;
>      }
>  
>      /* wakeup the cpus which have an expired event. */
> -    evt_do_broadcast(&mask);
> +    evt_do_broadcast(scratch);
>  
>      if ( next_event != STIME_MAX )
>      {
> --- a/xen/arch/x86/include/asm/smp.h
> +++ b/xen/arch/x86/include/asm/smp.h
> @@ -22,6 +22,7 @@
>  DECLARE_PER_CPU(cpumask_var_t, cpu_sibling_mask);
>  DECLARE_PER_CPU(cpumask_var_t, cpu_core_mask);
>  DECLARE_PER_CPU(cpumask_var_t, scratch_cpumask);
> +DECLARE_PER_CPU(cpumask_var_t, hpet_scratch_cpumask);

Should this be declared in the hpet.h header?

>  DECLARE_PER_CPU(cpumask_var_t, send_ipi_cpumask);
>  
>  /*
> --- a/xen/arch/x86/smpboot.c
> +++ b/xen/arch/x86/smpboot.c
> @@ -55,6 +55,9 @@ DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t
>  DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, scratch_cpumask);
>  static cpumask_t scratch_cpu0mask;
>  
> +DEFINE_PER_CPU_READ_MOSTLY(cpumask_var_t, hpet_scratch_cpumask);
> +static cpumask_t hpet_scratch_cpu0mask;

And then this defined in hpet.c.

Just thinking whether someone would like to introduce support for
build time disabling HPET in the future.

Can always be moved at a later time anyway.

Thanks, Roger.