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

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Date:   Sun, 22 Mar 2020 09:33:17 -0700
From:   Davidlohr Bueso <dave@...olabs.net>
To:     Peter Zijlstra <peterz@...radead.org>
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 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.

Thanks,
Davidlohr