Hello Shrikanth,
On 9/11/2025 10:22 PM, Shrikanth Hegde wrote:
>>> + if (is_cpu_paravirt(cpu))
>>> + push_current_from_paravirt_cpu(rq);
>>
>> Does this mean paravirt CPU is capable of handling an interrupt but may
>> not be continuously available to run a task?
>
> When i run hackbench which involves fair bit of IRQ stuff, it moves out.
>
> For example,
>
> echo 600-710 > /sys/devices/system/cpu/paravirt
>
> 11:31:54 AM CPU %usr %nice %sys %iowait %irq %soft %steal
> %guest %gnice %idle
> 11:31:57 AM 598 2.04 0.00 77.55 0.00 18.37 0.00 1.02
> 0.00 0.00 1.02
> 11:31:57 AM 599 1.01 0.00 79.80 0.00 17.17 0.00 1.01
> 0.00 0.00 1.01
> 11:31:57 AM 600 0.00 0.00 0.00 0.00 0.00 0.00 0.99
> 0.00 0.00 99.01
> 11:31:57 AM 601 0.00 0.00 0.00 0.00 0.00 0.00 0.00
> 0.00 0.00 100.00
> 11:31:57 AM 602 0.00 0.00 0.00 0.00 0.00 0.00 0.00
> 0.00 0.00 100.00
>
>
> There could some workloads which doesn't move irq's out, for which needs
> irqbalance change.
> Looking into it.
>
> Or is the VMM expected to set
>> the CPU on the paravirt mask and give the vCPU sufficient time to move the
>> task before yanking it away from the pCPU?
>>
>
> If the vCPU is running something, it is going to run at some point on pCPU.
> hypervisor will give the cycles to this vCPU by preempting some other vCPU.
>
> It is that using this infra, there is should be nothing on that paravirt vCPU.
> That way collectively VMM gets only limited request for pCPU which it can
> satify
> without vCPU preemption.
Ack! Just wanted to understand the usage.
P.S. I remember discussions during last LPC where we could communicate
this unavailability via CPU capacity. Was that problematic for some
reason? Sorry if I didn't follow this discussion earlier.
[..snip..]
>>> + local_irq_save(flags);
>>> + preempt_disable();
>>
>> Disabling IRQs implies preemption is disabled.
>>
>
> Most of the places stop_one_cpu_nowait called with preemption & irq disabled.
> stopper runs at the next possible opportunity.
But is there any reason to do both local_irq_save() and
preempt_disable()? include/linux/preempt.h defines preemptible() as:
#define preemptible() (preempt_count() == 0 && !irqs_disabled())
so disabling IRQs should be sufficient right or am I missing something?
>
> stop_one_cpu_nowait
> ->queues the task into stopper list
> -> wake_up_process(stopper)
> -> set need_resched
> -> stopper runs as early as possible.
>
--
Thanks and Regards,
Prateek
