linux-kernel - Re: [RFC] Make need_resched() return true when rcu_urgent

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Date:   Wed, 11 Jul 2018 13:17:59 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>
Cc:     David Woodhouse <dwmw2@...radead.org>,
        Peter Zijlstra <peterz@...radead.org>, mhillenb@...zon.de,
        linux-kernel <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>
Subject: Re: [RFC] Make need_resched() return true when rcu_urgent_qs
 requested

On Wed, Jul 11, 2018 at 08:31:55PM +0200, Christian Borntraeger wrote:
> So why is the rcu_virt_note_context_switch(smp_processor_id());
> in guest_enter_irqoff not good enough? 
> 
> This was actually supposed to tell rcu that being in the guest
> is an extended quiescing period (like userspace).
> 
> What has changed?

As I understand it, they would like to have their guest run uninterrupted
for extended times.  Because rcu_virt_note_context_switch() is a
point-in-time quiescent state, it cannot tell RCU about the extended
quiescent state.

Should we replace the current calls to rcu_virt_note_context_switch()
with rcu_kvm_enter() and rcu_kvm_exit()?  Would that be better
than the below architecture-by-architecture approach?

							Thanx, Paul

> On 07/11/2018 07:03 PM, David Woodhouse wrote:
> > On Wed, 2018-07-11 at 09:49 -0700, Paul E. McKenney wrote:
> >> And here is an updated v4.15 patch with Marius's Reported-by and David's
> >> fix to my lost exclamation point.
> > 
> > Thanks. Are you sending the original version of that to Linus? It'd be
> > useful to have the commit ID so that we can watch for it landing, and
> > chase this one up to Greg.
> > 
> > As discussed on IRC, this patch reduces synchronize_sched() latency for
> > us from ~4600s to ~160ms, which is nice.
> > 
> > However, it isn't going to be sufficient in the NO_HZ_FULL case. For
> > that you want a patch like the one below, which happily reduces the
> > latency in our (!NO_HZ_FULL) case still further to ~40ms.
> > 
> > Adding kvm list for better review...
> > 
> > From: David Woodhouse <dwmw@...zon.co.uk>
> > Subject: [PATCH] kvm/x86: Inform RCU of quiescent state when entering guest mode
> > 
> > RCU can spend long periods of time waiting for a CPU which is actually in
> > KVM guest mode, entirely pointlessly. Treat it like the idle and userspace
> > modes, and don't wait for it.
> > 
> > Signed-off-by: David Woodhouse <dwmw@...zon.co.uk>
> > ---
> >  arch/x86/kvm/x86.c      |  2 ++
> >  include/linux/rcutree.h |  2 ++
> >  kernel/rcu/tree.c       | 16 ++++++++++++++++
> >  3 files changed, 20 insertions(+)
> > 
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 0046aa70205a..b0c82f70afa7 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -7458,7 +7458,9 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
> >  		vcpu->arch.switch_db_regs &= ~KVM_DEBUGREG_RELOAD;
> >  	}
> > 
> > +	rcu_kvm_enter();
> >  	kvm_x86_ops->run(vcpu);
> > +	rcu_kvm_exit();
> > 
> >  	/*
> >  	 * Do this here before restoring debug registers on the host.  And
> > diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
> > index 914655848ef6..6d07af5a50fc 100644
> > --- a/include/linux/rcutree.h
> > +++ b/include/linux/rcutree.h
> > @@ -82,6 +82,8 @@ void cond_synchronize_sched(unsigned long oldstate);
> > 
> >  void rcu_idle_enter(void);
> >  void rcu_idle_exit(void);
> > +void rcu_kvm_enter(void);
> > +void rcu_kvm_exit(void);
> >  void rcu_irq_enter(void);
> >  void rcu_irq_exit(void);
> >  void rcu_irq_enter_irqson(void);
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index aa7cade1b9f3..df7893273939 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1019,6 +1019,22 @@ void rcu_irq_enter_irqson(void)
> >  	local_irq_restore(flags);
> >  }
> > 
> > +/*
> > + * These are currently identical to the _idle_ versions but let's
> > + * explicitly have separate copies to keep Paul honest in future.
> > + */
> > +void rcu_kvm_enter(void)
> > +{
> > +	rcu_idle_enter();
> > +}
> > +EXPORT_SYMBOL_GPL(rcu_kvm_enter);
> > +
> > +void rcu_kvm_exit(void)
> > +{
> > +	rcu_idle_exit();
> > +}
> > +EXPORT_SYMBOL_GPL(rcu_kvm_exit);
> > +
> >  /**
> >   * rcu_is_watching - see if RCU thinks that the current CPU is idle
> >   *
> >