Hello Shrikanth,

On 9/11/2025 10:22 PM, Shrikanth Hegde wrote:

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+    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

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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..]

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+    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