Re: [PATCH] Updates to Xen hypercall preemption

[Juergen Gross <jgross@xxxxxxxx>]

On 22.06.23 13:15, Peter Zijlstra wrote:
> On Thu, Jun 22, 2023 at 12:33:31PM +0200, Juergen Gross wrote:
> > On 22.06.23 10:26, Peter Zijlstra wrote:
> > > > The downside would be that some workloads might see worse performance
> > > > due to backend I/O handling might get preempted.
> > > Is that an actual concern? Mark this a legaxy inteface and anybody who
> > > wants to get away from it updates.
> > It isn't that easy. See above.
> Well, the old stuff gets to use full preemption on Dom0, then the new
> stuff gets more shiny options.
Yeah, but what about the hypercalls from non-dom0 systems needing the same
handling? This would require to run all guests which are using hypercalls
fully preemptive.

> > > > Just thinking - can full preemption be enabled per process?
> > > Nope, that's a system wide thing. Preemption is something that's driven
> > > by the requirements of the tasks that preempt, not something by the
> > > tasks that get preempted.
> > Depends. If a task in a non-preempt system could switch itself to be
> > preemptable, we could do so around hypercalls without compromising the
> > general preemption setting. Disabling preemption in a preemptable system
> > should continue to be possible for short code paths only, of course.
> So something along those lines was suggested elsewhere, and I'm still
> not entirely sure how I feel about it, but look here:
>
>    https://lkml.kernel.org/r/20230403052233.1880567-1-ankur.a.arora@xxxxxxxxxx
>
> Specifically patches 7 and 8. It is very close so that you currently
> do/want. Those patches are many moons old and i've not seen an update on
> them, so I've no idea where they are.
>
> It solves a similar problem except it is 'rep string' instructions
> that's being interrupted.
Right. I'll ping Ankur.


Juergen

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