linux-kernel - Re: [patch 2/5] x86/kvm: Handle task

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Message-ID: <alpine.DEB.2.21.1908012025100.1789@nanos.tec.linutronix.de>
Date:   Thu, 1 Aug 2019 20:34:53 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Oleg Nesterov <oleg@...hat.com>
cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Sebastian Siewior <bigeasy@...utronix.de>,
        Anna-Maria Gleixner <anna-maria@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Julia Cartwright <julia@...com>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@...il.com>, kvm@...r.kernel.org,
        Radim Krcmar <rkrcmar@...hat.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        John Stultz <john.stultz@...aro.org>,
        Andy Lutomirski <luto@...nel.org>,
        "Paul E. McKenney" <paulmck@...ux.ibm.com>
Subject: Re: [patch 2/5] x86/kvm: Handle task_work on VMENTER/EXIT

On Thu, 1 Aug 2019, Oleg Nesterov wrote:
> On 08/01, Thomas Gleixner wrote:
> >
> > @@ -8172,6 +8174,10 @@ static int vcpu_run(struct kvm_vcpu *vcp
> >  			++vcpu->stat.signal_exits;
> >  			break;
> >  		}
> > +
> > +		if (notify_resume_pending())
> > +			tracehook_handle_notify_resume();
> 
> shouldn't you drop kvm->srcu before tracehook_handle_notify_resume() ?
> 
> I don't understand this code at all, but vcpu_run() does this even before
> cond_resched().

Yeah, I noticed that it's dropped around cond_resched().

My understanding is that for voluntary giving up the CPU via cond_resched()
it needs to be dropped.

For involuntary preemption (CONFIG_PREEMPT=y) it's not required as the
whole code section after preempt_enable() is fully preemptible.

Now the 1Mio$ question is whether any of the notify functions invokes
cond_resched() and whether that really matters. Paolo?

Thanks,

	tglx