Re: [PATCH v2] KVM: x86: Rate-limit global clock updates on vCPU load

[Thorsten Leemhuis]

On 5/6/26 14:55, Sean Christopherson wrote:
> On Wed, May 06, 2026, Thorsten Leemhuis wrote:
>> On 4/9/26 21:21, Sean Christopherson wrote:
>>> On Thu, Apr 09, 2026, Lei Chen wrote:
>>>> commit 446fcce2a52b ("Revert "x86: kvm: rate-limit global clock updates"")
>>>> dropped the rate limiting for KVM_REQ_GLOBAL_CLOCK_UPDATE.
>>>>
>>>> As a result, kvm_arch_vcpu_load() can queue global clock update requests
>>>> every time a vCPU is scheduled when the master clock is disabled or when
>>>> the vCPU is loaded for the first time.
>>>>
>>>> Restore the throttling with a per-VM ratelimit state and gate
>>>> KVM_REQ_GLOBAL_CLOCK_UPDATE through __ratelimit(), so frequent vCPU
>>>> scheduling does not generate a steady stream of redundant clock update
>>>> requests.
>>>>
>>>> Fixes: 446fcce2a52b ("Revert "x86: kvm: rate-limit global clock updates"")
>>>> Signed-off-by: Lei Chen <lei.chen@xxxxxxxxxx>
>>>> Reported-by: Jaroslav Pulchart <jaroslav.pulchart@xxxxxxxxxxxx>
>>>> Closes: https://lore.kernel.org/all/CAK8fFZ5gY8_Mw2A=iZVFNVKQNrXQzVsn-HTd+Me9K6ZfmdgA+Q@xxxxxxxxxxxxxx/
>>
>> Was this performance regression ever addressed?
> Nope, not yet.
> 
>> Looks like this fall through the cracks, but it's easy to miss something.
> 
> It's in my list of patches to apply (probably for 7.2?).  I didn't want to squeeze
> it into the initial 7.1 pull request for a variety of reasons.

Hmmm. CCing Linus so he can speak up if he wants to about the following:

Given that this is a fix for a performance regression[1] I'd say it's
not as urgent as a "something stopped working" case -- so I guess it's
something where the "[fix] "within a week", preferably before the next
rc" approach Linus recently mentioned does not need to be applied strictly.

But Jaroslav OTOH reported it more than 7 weeks ago already and back
then called it something that "severely impacts KVM hosts running many
Firecracker microVMs."[1]; and a potential fix exists for 4 weeks
already. Due to that, 7.2 feels a bit too far away for me, as that is
still ~15 weeks away. But maybe that's just me.

Ciao, Thorsten

[1]
https://lore.kernel.org/all/CAK8fFZ5gY8_Mw2A=iZVFNVKQNrXQzVsn-HTd+Me9K6ZfmdgA+Q@xxxxxxxxxxxxxx/