Re: [patch 2/5] x86/kvm: Handle task_work on VMENTER/EXIT

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
Date: Thu, 1 Aug 2019 20:34:53 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1908012025100.1789@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20190801162451.GE31538@redhat.com>

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