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

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Date:   Wed, 11 Jul 2018 20:31:55 +0200
From:   Christian Borntraeger <borntraeger@...ibm.com>
To:     David Woodhouse <dwmw2@...radead.org>, paulmck@...ux.vnet.ibm.com
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

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?



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
>   *
>