linux-kernel - Re: [PATCH v2 08/12] sched/wait: Drop WQ_FLAG_EXCLUSIVE from add_wait_queue

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Message-ID: <20250521114233.GC39944@noisy.programming.kicks-ass.net>
Date: Wed, 21 May 2025 13:42:33 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Sean Christopherson <seanjc@...gle.com>, mhklinux@...look.com
Cc: Paolo Bonzini <pbonzini@...hat.com>, Ingo Molnar <mingo@...hat.com>,
	Juri Lelli <juri.lelli@...hat.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
	kvmarm@...ts.linux.dev, K Prateek Nayak <kprateek.nayak@....com>,
	David Matlack <dmatlack@...gle.com>,
	Juergen Gross <jgross@...e.com>,
	Stefano Stabellini <sstabellini@...nel.org>,
	Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>
Subject: Re: [PATCH v2 08/12] sched/wait: Drop WQ_FLAG_EXCLUSIVE from
 add_wait_queue_priority()

On Tue, May 20, 2025 at 03:20:00PM -0700, Sean Christopherson wrote:
> On Tue, May 20, 2025, Peter Zijlstra wrote:
> > On Mon, May 19, 2025 at 11:55:10AM -0700, Sean Christopherson wrote:
> > > Drop the setting of WQ_FLAG_EXCLUSIVE from add_wait_queue_priority() to
> > > differentiate it from add_wait_queue_priority_exclusive().  The one and
> > > only user add_wait_queue_priority(), Xen privcmd's irqfd_wakeup(),
> > > unconditionally returns '0', i.e. doesn't actually operate in exclusive
> > > mode.
> > 
> > I find:
> > 
> > drivers/hv/mshv_eventfd.c:      add_wait_queue_priority(wqh, &irqfd->irqfd_wait);
> > drivers/xen/privcmd.c:  add_wait_queue_priority(wqh, &kirqfd->wait);
> > 
> > I mean, it might still be true and all, but hyperv seems to also use
> > this now.
> 
> Oh FFS, another "heavily inspired by KVM".  I should have bribed someone to take
> this series when I had the chance.  *sigh*
> 
> Unfortunately, the Hyper-V code does actually operate in exclusive mode.  Unless
> you have a better idea, I'll tweak the series to:
> 
>   1. Drop WQ_FLAG_EXCLUSIVE from add_wait_queue_priority() and have the callers
>      explicitly set the flag, 
>   2. Add a patch to drop WQ_FLAG_EXCLUSIVE from Xen privcmd entirely.
>   3. Introduce add_wait_queue_priority_exclusive() and switch KVM to use it.
> 
> That has an added bonus of introducing the Xen change in a dedicated patch, i.e.
> is probably a sequence anyways.
> 
> Alternatively, I could rewrite the Hyper-V code a la the KVM changes, but I'm not
> feeling very charitable at the moment (the complete lack of documentation for
> their ioctl doesn't help).

Works for me. Michael is typically very responsive wrt hyperv (but you
probably know this).