From: Sean Christopherson <sean.j.christopherson@intel.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Oleg Nesterov <oleg@redhat.com>,
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 14:35:50 -0700 [thread overview]
Message-ID: <20190801213550.GE6783@linux.intel.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1908012025100.1789@nanos.tec.linutronix.de>
On Thu, Aug 01, 2019 at 08:34:53PM +0200, Thomas Gleixner wrote:
> 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?
cond_resched() is called via tracehook_notify_resume()->task_work_run(),
and "kernel code can only call cond_resched() in places where it ...
cannot hold references to any RCU-protected data structures" according to
https://lwn.net/Articles/603252/.
next prev parent reply other threads:[~2019-08-01 21:35 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-08-01 14:32 [patch 0/5] posix-cpu-timers: Move expiry into task work context Thomas Gleixner
2019-08-01 14:32 ` [patch 1/5] tracehook: Provide TIF_NOTIFY_RESUME handling for KVM Thomas Gleixner
2019-08-01 14:48 ` Peter Zijlstra
2019-08-01 15:10 ` Thomas Gleixner
2019-08-01 17:02 ` Andy Lutomirski
2019-08-01 14:32 ` [patch 2/5] x86/kvm: Handle task_work on VMENTER/EXIT Thomas Gleixner
2019-08-01 16:24 ` Oleg Nesterov
2019-08-01 18:34 ` Thomas Gleixner
2019-08-01 21:35 ` Sean Christopherson [this message]
2019-08-01 21:44 ` Peter Zijlstra
2019-08-01 21:44 ` Thomas Gleixner
2019-08-01 21:47 ` Thomas Gleixner
2019-08-02 21:35 ` Paolo Bonzini
2019-08-02 22:22 ` Thomas Gleixner
2019-08-02 22:39 ` Andy Lutomirski
2019-08-02 12:04 ` Oleg Nesterov
2019-08-01 14:32 ` [patch 3/5] posix-cpu-timers: Split run_posix_cpu_timers() Thomas Gleixner
2019-08-01 14:32 ` [patch 4/5] posix-cpu-timers: Defer timer handling to task_work Thomas Gleixner
2019-08-01 14:51 ` Peter Zijlstra
2019-08-01 15:10 ` Thomas Gleixner
2019-08-01 15:39 ` Oleg Nesterov
2019-08-01 18:41 ` Thomas Gleixner
2019-08-01 14:32 ` [patch 5/5] x86: Select POSIX_CPU_TIMERS_TASK_WORK Thomas Gleixner
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