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Date:   Thu, 1 Aug 2019 20:34:53 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Oleg Nesterov <oleg@redhat.com>
cc:     LKML <linux-kernel@vger.kernel.org>, x86@kernel.org,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Sebastian Siewior <bigeasy@linutronix.de>,
        Anna-Maria Gleixner <anna-maria@linutronix.de>,
        Steven Rostedt <rostedt@goodmis.org>,
        Julia Cartwright <julia@ni.com>,
        Paul McKenney <paulmck@linux.vnet.ibm.com>,
        Frederic Weisbecker <fweisbec@gmail.com>, kvm@vger.kernel.org,
        Radim Krcmar <rkrcmar@redhat.com>,
        Paolo Bonzini <pbonzini@redhat.com>,
        John Stultz <john.stultz@linaro.org>,
        Andy Lutomirski <luto@kernel.org>,
        "Paul E. McKenney" <paulmck@linux.ibm.com>
Subject: Re: [patch 2/5] x86/kvm: Handle task_work on VMENTER/EXIT
In-Reply-To: <20190801162451.GE31538@redhat.com>
Message-ID: <alpine.DEB.2.21.1908012025100.1789@nanos.tec.linutronix.de>
References: <20190801143250.370326052@linutronix.de> <20190801143657.887648487@linutronix.de> <20190801162451.GE31538@redhat.com>
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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