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

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Message-ID: <c7064b0c-5a86-4138-b8d4-a90bd7e6751c@suse.com>
Date: Wed, 21 May 2025 15:22:02 +0200
From: Jürgen Groß <jgross@...e.com>
To: Sean Christopherson <seanjc@...gle.com>,
 Peter Zijlstra <peterz@...radead.org>
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>,
 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 21.05.25 00:20, 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.

Works for me.


Juergen

> 
> 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).


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