RE: [PATCH] irqchip/gic-v4.1: Optimize the delay time of the poll on the GICR_VPENDBASER.Dirty bit

From: lushenming
Date: Fri Sep 18 2020 - 08:33:42 EST

Hi, Marc,

I measured the time from vcpu_load() (include it) to __guest_enter() on Kunpeng 920. On average, It takes 2.55 microseconds (not first run && the VPT is empty). So waiting for 10 microseconds in
vcpu scheduling really hurts performance.

And I agree that delaying the execution of its_wait_vpt_parse_complete() might be a viable solution.

-----Original Message-----
From: Marc Zyngier [mailto:maz@xxxxxxxxxx]
Sent: 2020-09-16 16:40
To: lushenming <lushenming@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>; Jason Cooper <jason@xxxxxxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; Wanghaibin (D) <>; yuzenghui <yuzenghui@xxxxxxxxxx>
Subject: Re: [PATCH] irqchip/gic-v4.1: Optimize the delay time of the poll on the GICR_VPENDBASER.Dirty bit

On 2020-09-16 08:04, lushenming wrote:
> Hi,
> Our team just discussed this issue again and consulted our GIC
> hardware design team. They think the RD can afford busy waiting. So we
> still think maybe 0 is better, at least for our hardware.
> In addition, if not 0, as I said before, in our measurement, it takes
> only hundreds of nanoseconds, or 1~2 microseconds, to finish parsing
> the VPT in most cases. So maybe 1 microseconds, or smaller, is more
> appropriate.
> Anyway, 10 microseconds is too much.
> But it has to be said that it does depend on the hardware
> implementation.

Exactly. And given that the only publicly available implementation is a software model, I am reluctant to change "performance" related things based on benchmarks that can't be verified and appears to me as a micro optimization.

> Besides, I'm not sure where are the start and end point of the total
> scheduling latency of a vcpu you said, which includes many events. Is
> the parse time of the VPT not clear enough?

Measure the time it takes from kvm_vcpu_load() to the point where the vcpu enters the guest. How much, in proportion, do these 1/2/10ms represent?

Also, a better(?) course of action would maybe to consider whether we should split the its_vpe_schedule() call into two distinct operations: one that programs the VPE to be resident, and another that poll the Dirty bit *much
later* on the entry path, giving the GIC a chance to work in parallel with the CPU on the entry path.

If your HW is a quick as you say it is, it would pretty much guarantee a clear read of GICR_VPENDBASER without waiting.

Jazz is not dead. It just smells funny...