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Re: [PATCH for-4.21 01/10] x86/HPET: limit channel changes
- To: Roger Pau Monné <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Fri, 17 Oct 2025 11:55:07 +0200
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- Cc: "xen-devel@xxxxxxxxxxxxxxxxxxxx" <xen-devel@xxxxxxxxxxxxxxxxxxxx>, Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, Oleksii Kurochko <oleksii.kurochko@xxxxxxxxx>
- Delivery-date: Fri, 17 Oct 2025 09:55:20 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
On 17.10.2025 11:23, Roger Pau Monné wrote:
> On Thu, Oct 16, 2025 at 09:31:21AM +0200, Jan Beulich wrote:
>> Despite 1db7829e5657 ("x86/hpet: do local APIC EOI after interrupt
>> processing") we can still observe nested invocations of
>> hpet_interrupt_handler(). This is, afaict, a result of previously used
>> channels retaining their IRQ affinity until some other CPU re-uses them.
>> Such nesting is increasingly problematic with higher CPU counts, as both
>> handle_hpet_broadcast() and cpumask_raise_softirq() have a cpumask_t local
>> variable. IOW already a single level of nesting may require more stack
>> space (2 times above 4k) than we have available (8k), when NR_CPUS=16383
>> (the maximum value presently possible).
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>> ---
>> Whether this is still worthwhile with "x86/HPET: use single, global, low-
>> priority vector for broadcast IRQ" isn't quite clear to me.
>
> Seeing the rest of the series, I don't think this is necessary
> anymore? Also the comment you here is made stale by the patch that
> uses a global vector.
Right now I'm not quite sure, hence the remark and the patch being part of
the series. If I re-work patch 3 to avoid the mask/unmask upon affinity
changes, I think the one here can indeed be dropped.
Jan
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