Re: [PATCH v3] drivers: hv: vmbus: Use kthread for vmbus interrupts on PREEMPT_RT

[Jan Kiszka <jan.kiszka@siemens.com>]

On 18.02.26 00:03, Bezdeka, Florian (FT RPD CED OES-DE) wrote:
> On Mon, 2026-02-16 at 17:24 +0100, Jan Kiszka wrote:
>> From: Jan Kiszka <jan.kiszka@siemens.com>
>>
>> Resolves the following lockdep report when booting PREEMPT_RT on Hyper-V
>> with related guest support enabled:
>>
>> [    1.127941] hv_vmbus: registering driver hyperv_drm
>>
>> [    1.132518] =============================
>> [    1.132519] [ BUG: Invalid wait context ]
>> [    1.132521] 6.19.0-rc8+ #9 Not tainted
>> [    1.132524] -----------------------------
>> [    1.132525] swapper/0/0 is trying to lock:
>> [    1.132526] ffff8b9381bb3c90 (&channel->sched_lock){....}-{3:3}, at: vmbus_chan_sched+0xc4/0x2b0
>> [    1.132543] other info that might help us debug this:
>> [    1.132544] context-{2:2}
>> [    1.132545] 1 lock held by swapper/0/0:
>> [    1.132547]  #0: ffffffffa010c4c0 (rcu_read_lock){....}-{1:3}, at: vmbus_chan_sched+0x31/0x2b0
>> [    1.132557] stack backtrace:
>> [    1.132560] CPU: 0 UID: 0 PID: 0 Comm: swapper/0 Not tainted 6.19.0-rc8+ #9 PREEMPT_{RT,(lazy)}
>> [    1.132565] Hardware name: Microsoft Corporation Virtual Machine/Virtual Machine, BIOS Hyper-V UEFI Release v4.1 09/25/2025
>> [    1.132567] Call Trace:
>> [    1.132570]  <IRQ>
>> [    1.132573]  dump_stack_lvl+0x6e/0xa0
>> [    1.132581]  __lock_acquire+0xee0/0x21b0
>> [    1.132592]  lock_acquire+0xd5/0x2d0
>> [    1.132598]  ? vmbus_chan_sched+0xc4/0x2b0
>> [    1.132606]  ? lock_acquire+0xd5/0x2d0
>> [    1.132613]  ? vmbus_chan_sched+0x31/0x2b0
>> [    1.132619]  rt_spin_lock+0x3f/0x1f0
>> [    1.132623]  ? vmbus_chan_sched+0xc4/0x2b0
>> [    1.132629]  ? vmbus_chan_sched+0x31/0x2b0
>> [    1.132634]  vmbus_chan_sched+0xc4/0x2b0
>> [    1.132641]  vmbus_isr+0x2c/0x150
>> [    1.132648]  __sysvec_hyperv_callback+0x5f/0xa0
>> [    1.132654]  sysvec_hyperv_callback+0x88/0xb0
>> [    1.132658]  </IRQ>
>> [    1.132659]  <TASK>
>> [    1.132660]  asm_sysvec_hyperv_callback+0x1a/0x20
>>
>> As code paths that handle vmbus IRQs use sleepy locks under PREEMPT_RT,
>> the vmbus_isr execution needs to be moved into thread context. Open-
>> coding this allows to skip the IPI that irq_work would additionally
>> bring and which we do not need, being an IRQ, never an NMI.
>>
>> This affects both x86 and arm64, therefore hook into the common driver
>> logic.
> 
> I tested this patch in combination with the related SCSI driver patch.
> The tests were done on x86 with both VM generations provided by Hyper-v.
> 
> Lockdep was enabled and there were no splat reports within 24 hours of
> massive load produced by stress-ng.
> 
> With that:
> 
> Reviewed-by: Florian Bezdeka <florian.bezdeka@siemens.com>
> Tested-by: Florian Bezdeka <florian.bezdeka@siemens.com>
> 
> 
> Side note: We did some backports down to 6.1 already, just in case
> someone is interested. We recognized a massive network performance drop
> in 6.1. The root cause has been identified and is not related to this
> patch. It's simply another RT regression caused by a missing stable-rt
> backport. Upstreaming in progress...
> 

Submitted:
https://lore.kernel.org/stable/05ae6b87-0b53-4948-a1ed-2a3235a5f82b@siemens.com/T/#u

Jan

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