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

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Message-Id: <20180711174843.GX3593@linux.vnet.ibm.com>
Date:   Wed, 11 Jul 2018 10:48:43 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     David Woodhouse <dwmw2@...radead.org>
Cc:     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 06:03:42PM +0100, 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.

That would be great!  The commit ID is currently 6d1b6b684e1f ("rcu: Make
need_resched() respond to urgent RCU-QS needs"), but is subject to change
given the likely need to rebase in order to fix bugs in commits preceding
that one.  Given your Reported-by, you will be CCed when it reaches -tip,
won't you?  At that point, the commit ID would be set in stone.

Either way, I would very much welcome any help with -stable.  I would
be happy to send you an email when its commit ID become set in stone,
for example, if that would help.

> As discussed on IRC, this patch reduces synchronize_sched() latency for
> us from ~4600s to ~160ms, which is nice.

Woo-hoo!!!  ;-)

> 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.

Even better, good stuff, thank you!

> Adding kvm list for better review...

And a comment below.

							Thanx, Paul

> 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);

These look good, but we also need these in include/linux/rcutiny.h:

	static inline void rcu_kvm_enter(void) { }
	static inline void rcu_kvm_exit(void) { }

Unless KVM is excluded on !SMP systems or some such.

Alternatively, you could just have a single pair of static inlines in
include/linux/rcupdate.h (after the #include of rcutree.h and rcutiny.h)
that mapped the _kvm_ functions to the _idle_ functions.  Your choice,
I am fine either way.

> +
>  /**
>   * rcu_is_watching - see if RCU thinks that the current CPU is idle
>   *
> -- 
> 2.17.1
> 
> 
> -- 
> dwmw2
>