Re: [PATCH v2] KVM: arm64: Add KVM_CAP to control WFx trapping

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Thanks for the feedback.

Quentin Perret <qperret@xxxxxxxxxx> writes:

On Friday 22 Mar 2024 at 14:24:35 (+0000), Quentin Perret wrote:
On Tuesday 19 Mar 2024 at 16:43:41 (+0000), Colton Lewis wrote:
> Add a KVM_CAP to control WFx (WFI or WFE) trapping based on scheduler
> runqueue depth. This is so they can be passed through if the runqueue
> is shallow or the CPU has support for direct interrupt injection. They
> may be always trapped by setting this value to 0. Technically this
> means traps will be cleared when the runqueue depth is 0, but that
> implies nothing is running anyway so there is no reason to care. The
> default value is 1 to preserve previous behavior before adding this
> option.

I recently discovered that this was enabled by default, but it's not
obvious to me everyone will want this enabled, so I'm in favour of
figuring out a way to turn it off (in fact we might want to make this
feature opt in as the status quo used to be to always trap).

Setting the introduced threshold to zero will cause it to trap whenever
something is running. Is there a problem with doing it that way?

I'd also be interested to get more input before changing the current
default behavior.

There are a few potential issues I see with having this enabled:

  - a lone vcpu thread on a CPU will completely screw up the host
    scheduler's load tracking metrics if the vCPU actually spends a
    significant amount of time in WFI (the PELT signal will no longer
    be a good proxy for "how much CPU time does this task need");

  - the scheduler's decision will impact massively the behaviour of the
    vcpu task itself. Co-scheduling a task with a vcpu task (or not) will
    impact massively the perceived behaviour of the vcpu task in a way
    that is entirely unpredictable to the scheduler;

  - while the above problems might be OK for some users, I don't think
    this will always be true, e.g. when running on big.LITTLE systems the
    above sounds nightmare-ish;

  - the guest spending long periods of time in WFI prevents the host from
    being able to enter deeper idle states, which will impact power very

And probably a whole bunch of other things.

> Think about his option as a threshold. The instruction will be trapped
> if the runqueue depth is higher than the threshold.

So talking about the exact interface, I'm not sure exposing this to
userspace is really appropriate. The current rq depth is next to
impossible for userspace to control well.

Using runqueue depth is going off of a suggestion from Oliver [1], who I've
also talked to internally at Google a few times about this.

But hearing your comment makes me lean more towards having some
enumeration of behaviors like TRAP_ALWAYS, TRAP_NEVER,

My gut feeling tells me we might want to gate all of this on
PREEMPT_FULL instead, since PREEMPT_FULL is pretty much a way to say
"I'm willing to give up scheduler tracking accuracy to gain throughput
when I've got a task running alone on a CPU". Thoughts?

And obviously I meant s/PREEMPT_FULL/NOHZ_FULL, but hopefully that was
clear :-)

Sounds good to me but I've not touched anything scheduling related before.


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