Re: [RFC 00/60] Coscheduling for Linux

[Peter Zijlstra]

On Fri, Sep 14, 2018 at 06:25:44PM +0200, Jan H. Schönherr wrote:

> Assuming, there is a cgroup-less solution that can prevent simultaneous
> execution of tasks on a core, when they're not supposed to. How would you
> tell the scheduler, which tasks these are?

Specifically for L1TF I hooked into/extended KVM's preempt_notifier
registration interface, which tells us which tasks are VCPUs and to
which VM they belong.

But if we want to actually expose this to userspace, we can either do a
prctl() or extend struct sched_attr.

> >> 1. Execute parallel applications that rely on active waiting or synchronous
> >>    execution concurrently with other applications.
> >>
> >>    The prime example in this class are probably virtual machines. Here,
> >>    coscheduling is an alternative to paravirtualized spinlocks, pause loop
> >>    exiting, and other techniques with its own set of advantages and
> >>    disadvantages over the other approaches.
> > 
> > Note that in order to avoid PLE and paravirt spinlocks and paravirt
> > tlb-invalidate you have to gang-schedule the _entire_ VM, not just SMT
> > siblings.
> > 
> > Now explain to me how you're going to gang-schedule a VM with a good
> > number of vCPU threads (say spanning a number of nodes) and preserving
> > the rest of CFS without it turning into a massive trainwreck?
> 
> You probably don't -- for the same reason, why it is a bad idea to give
> an endless loop realtime priority. It's just a bad idea. As I said in the
> text you quoted: coscheduling comes with its own set of advantages and
> disadvantages. Just because you find one example, where it is a bad idea,
> doesn't make it a bad thing in general.

Well, you mentioned it as an alternative to paravirt spinlocks -- I'm
saying that co-scheduling cannot do that, you need full featured
gang-scheduling for that.