netdev - Re: [PATCH 18/15] kvm: Replace vcpu->swait with rcuwait

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Date:   Sun, 22 Mar 2020 23:32:49 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Davidlohr Bueso <dave@...olabs.net>
Cc:     tglx@...utronix.de, arnd@...db.de, balbi@...nel.org,
        bhelgaas@...gle.com, bigeasy@...utronix.de, davem@...emloft.net,
        gregkh@...uxfoundation.org, joel@...lfernandes.org,
        kurt.schwemmer@...rosemi.com, kvalo@...eaurora.org,
        linux-kernel@...r.kernel.org, linux-pci@...r.kernel.org,
        linux-usb@...r.kernel.org, linux-wireless@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org, logang@...tatee.com,
        mingo@...nel.org, mpe@...erman.id.au, netdev@...r.kernel.org,
        oleg@...hat.com, paulmck@...nel.org, rdunlap@...radead.org,
        rostedt@...dmis.org, torvalds@...ux-foundation.org,
        will@...nel.org, Paolo Bonzini <pbonzini@...hat.com>,
        Davidlohr Bueso <dbueso@...e.de>
Subject: Re: [PATCH 18/15] kvm: Replace vcpu->swait with rcuwait

On Sun, Mar 22, 2020 at 09:33:17AM -0700, Davidlohr Bueso wrote:
> On Fri, 20 Mar 2020, Peter Zijlstra wrote:
> 
> > On Fri, Mar 20, 2020 at 01:55:26AM -0700, Davidlohr Bueso wrote:
> > > -	swait_event_interruptible_exclusive(*wq, ((!vcpu->arch.power_off) &&
> > > -				       (!vcpu->arch.pause)));
> > > +	rcuwait_wait_event(*wait,
> > > +			   (!vcpu->arch.power_off) && (!vcpu->arch.pause),
> > > +			   TASK_INTERRUPTIBLE);
> > 
> > > -	for (;;) {
> > > -		prepare_to_swait_exclusive(&vcpu->wq, &wait, TASK_INTERRUPTIBLE);
> > > -
> > > -		if (kvm_vcpu_check_block(vcpu) < 0)
> > > -			break;
> > > -
> > > -		waited = true;
> > > -		schedule();
> > > -	}
> > > -
> > > -	finish_swait(&vcpu->wq, &wait);
> > > +	rcuwait_wait_event(&vcpu->wait,
> > > +			   (block_check = kvm_vcpu_check_block(vcpu)) < 0,
> > > +			   TASK_INTERRUPTIBLE);
> > 
> > Are these yet more instances that really want to be TASK_IDLE ?
> 
> Hmm probably as it makes sense for a blocked vcpu not to be contributing to
> the loadavg. So if this is the only reason to use interruptible, then yes we
> ought to change it.
> 
> However, I'll make this a separate patch, given this (ab)use isn't as obvious
> as the PS3 case, which is a kthread and therefore signals are masked.

The thing that was a dead give-away was that the return value of the
interruptible wait wasn't used.