linux-kernel - Re: [PATCH] Updates to Xen hypercall preemption

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Message-ID: <20230622111552.GI4253@hirez.programming.kicks-ass.net>
Date:   Thu, 22 Jun 2023 13:15:52 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Juergen Gross <jgross@...e.com>
Cc:     Per Bilse <Per.Bilse@...rix.com>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        "H. Peter Anvin" <hpa@...or.com>,
        Stefano Stabellini <sstabellini@...nel.org>,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        "open list:X86 ENTRY CODE" <linux-kernel@...r.kernel.org>,
        "moderated list:XEN HYPERVISOR INTERFACE" 
        <xen-devel@...ts.xenproject.org>
Subject: Re: [PATCH] Updates to Xen hypercall preemption

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.

> > > 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@oracle.com

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.