Re: [PATCH v1 01/11] perf/x86/core: Support KVM to assign a dedicated counter for guest PEBS

[Peter Zijlstra]

On Mon, Mar 09, 2020 at 09:12:42AM -0400, Liang, Kan wrote:

> > Suppose your KVM thing claims counter 0/2 (ICL/SKL) for some random PEBS
> > event, and then the host wants to use PREC_DIST.. Then one of them will
> > be screwed for no reason what so ever.
> > 
> 
> The multiplexing should be triggered.
> 
> For host, if both user A and user B requires PREC_DIST, the multiplexing
> should be triggered for them.
> Now, the user B is KVM. I don't think there is difference. The multiplexing
> should still be triggered. Why it is screwed?

Becuase if KVM isn't PREC_DIST we should be able to reschedule it to a
different counter.

> > How is that not destroying scheduling freedom? Any other situation we'd
> > have moved the !PREC_DIST PEBS event to another counter.
> > 
> 
> All counters are equivalent for them. It doesn't matter if we move it to
> another counter. There is no impact for the user.

But we cannot move it to another counter, because you're pinning it.

> In the new proposal, KVM user is treated the same as other host events with
> event constraint. The scheduler is free to choose whether or not to assign a
> counter for it.

That's what it does, I understand that. I'm saying that that is creating
artificial contention.


Why is this needed anyway? Can't we force the guest to flush and then
move it over to a new counter?