Re: [PATCH] vfio/type1: conditional rescheduling while pinning

[Keith Busch <kbusch@kernel.org>]

On Fri, Jul 11, 2025 at 02:16:57PM -0600, Alex Williamson wrote:
> On Wed, 9 Jul 2025 14:18:58 -0600
> Keith Busch <kbusch@kernel.org> wrote:
> > I think I'm back to needing a cond_resched(). I'm finding too many user
> > space programs, including qemu, for various reasons do not utilize
> > hugepage faults, and we're ultimately locking up a cpu for long enough
> > to cause other nasty side effects, like OOM due to blocked rcu free
> > callbacks. As preferable as it is to get everything aligned to use the
> > faster faults, I don't think the kernel should depend on that to prevent
> > prolonged cpu lockups. What do you think?
> 
> I'm not opposed to adding a cond_resched, but I'll also note that Peter
> Xu has been working on a series that tries to get the right mapping
> alignment automatically.  It's still a WIP, but it'd be good to know if
> that resolves the remaining userspace issues you've seen or we're still
> susceptible to apps that aren't even trying to use THP:
> 
> https://lore.kernel.org/all/20250613134111.469884-1-peterx@redhat.com/

Yes, I saw that series and included that for testing as well.

But I'm finding many machines have a transparent_hugepage enabled policy
set to "never" due to some problems THP usage caused with other hardware
(CXL, I think). I'm trying to get these to use "madvise" instead, but
that's causing other performance regressions for some tests.