From: Christian Borntraeger <borntraeger@de.ibm.com>
To: David Woodhouse <dwmw2@infradead.org>, paulmck@linux.vnet.ibm.com
Cc: peterz@infradead.org, mhillenb@amazon.de,
linux-kernel@vger.kernel.org, kvm@vger.kernel.org
Subject: Re: [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode
Date: Thu, 12 Jul 2018 14:04:41 +0200 [thread overview]
Message-ID: <1f63cd8e-1afa-b508-ba66-751c9091fde6@de.ibm.com> (raw)
In-Reply-To: <2997e5b4-57b4-dba5-2487-6908ca1b9500@de.ibm.com>
On 07/12/2018 01:58 PM, Christian Borntraeger wrote:
>
>
> On 07/12/2018 01:10 PM, David Woodhouse wrote:
>> On Thu, 2018-07-12 at 13:00 +0200, Christian Borntraeger wrote:
>>>
>>> On s390 this seems to add about 10ns (~3%) for a guest exit/rentry
>>> microbenchmark mostly due to rcu_eqs_enter and rcu_eqs_exit now being
>>> visible in perf samples. The older interface was cheaper.
>>
>> Well, the older interface wasn't actually working, which made it
>> moderately suboptimal :)
>>
>> But that is fixed by the first patch of the two, so perhaps the second
>> ought to be conditional on CONFIG_NO_HZ_FULL since it's only really
>> fixing a real bug there?
>>
>> Unless we can contrive some way to do the rcu_eqs_enter/exit only on
>> the paths to/from userspace, and *not* when we loop in the kernel to
>> handle interrupts (etc.) and immediately re-enter the guest without
>> returning? That's somewhat non-trivial though...
>
> Assuming that patch1 really fixes the issue (and all KVM hot loops are
> supposed to have something like
> if (need_resched())
> schedule();
>
> this could work out given that patch1 does what it is supposed to do.
>
> So you are thinking of something like
>
> diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
> index d8ee9f949ce3..cac5de8a887c 100644
> --- a/include/linux/rcutree.h
> +++ b/include/linux/rcutree.h
> @@ -72,8 +72,17 @@ void cond_synchronize_sched(unsigned long oldstate);
>
> void rcu_idle_enter(void);
> void rcu_idle_exit(void);
> +#ifdef CONFIG_NO_HZ_FULL
> void rcu_kvm_enter(void);
> void rcu_kvm_exit(void);
> +#else
> +static inline void rcu_kvm_enter(void)
> +{
> +}
> +static inline void rcu_kvm_exit(void)
> +{
> +}
> +#endif
> void rcu_irq_enter(void);
> void rcu_irq_exit(void);
> void rcu_irq_enter_irqson(void);
>
> ?
>
Of course we also need to shift the ifdefs in rcu.c
and the rcu_virt_note_context_switch() on enter as you
said.
next prev parent reply other threads:[~2018-07-12 12:04 UTC|newest]
Thread overview: 93+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-06 14:53 [RFC] Make need_resched() return true when rcu_urgent_qs requested David Woodhouse
2018-07-06 16:29 ` Peter Zijlstra
2018-07-06 17:11 ` Paul E. McKenney
2018-07-06 17:14 ` David Woodhouse
2018-07-06 21:12 ` Paul E. McKenney
2018-07-09 8:58 ` Peter Zijlstra
2018-07-09 8:53 ` Peter Zijlstra
2018-07-09 9:18 ` David Woodhouse
2018-07-09 10:44 ` Peter Zijlstra
2018-07-09 10:56 ` David Woodhouse
2018-07-09 11:06 ` Peter Zijlstra
2018-07-09 11:12 ` David Woodhouse
2018-07-09 11:31 ` Peter Zijlstra
2018-07-09 12:34 ` Paul E. McKenney
2018-07-09 12:47 ` David Woodhouse
2018-07-09 14:30 ` Paul E. McKenney
2018-07-09 12:55 ` Peter Zijlstra
2018-07-09 12:57 ` David Woodhouse
2018-07-09 13:02 ` Peter Zijlstra
2018-07-09 14:29 ` Paul E. McKenney
2018-07-09 14:43 ` Peter Zijlstra
2018-07-09 14:54 ` Paul E. McKenney
2018-07-09 15:26 ` Peter Zijlstra
2018-07-09 16:34 ` Paul E. McKenney
2018-07-09 16:44 ` Paul E. McKenney
2018-07-09 18:50 ` David Woodhouse
2018-07-09 20:34 ` Paul E. McKenney
2018-07-09 20:35 ` David Woodhouse
2018-07-09 20:42 ` Paul E. McKenney
2018-07-09 20:45 ` David Woodhouse
2018-07-09 21:05 ` Paul E. McKenney
2018-07-09 22:08 ` Paul E. McKenney
2018-07-11 10:57 ` David Woodhouse
2018-07-11 12:51 ` Paul E. McKenney
2018-07-11 12:58 ` David Woodhouse
2018-07-11 14:25 ` Paul E. McKenney
2018-07-11 14:23 ` David Woodhouse
2018-07-11 14:43 ` Paul E. McKenney
2018-07-11 16:49 ` Paul E. McKenney
2018-07-11 17:03 ` David Woodhouse
2018-07-11 17:48 ` Paul E. McKenney
2018-07-11 18:01 ` [PATCH v2] kvm/x86: Inform RCU of quiescent state when entering guest mode David Woodhouse
2018-07-11 18:20 ` Paul E. McKenney
2018-07-11 18:36 ` Paul E. McKenney
2018-07-11 18:39 ` Christian Borntraeger
2018-07-11 20:27 ` Paul E. McKenney
2018-07-11 20:54 ` David Woodhouse
2018-07-11 21:09 ` Paul E. McKenney
2018-07-11 21:11 ` Christian Borntraeger
2018-07-11 21:32 ` Paul E. McKenney
2018-07-11 21:39 ` Christian Borntraeger
2018-07-11 23:47 ` Paul E. McKenney
2018-07-12 8:31 ` David Woodhouse
2018-07-12 11:00 ` Christian Borntraeger
2018-07-12 11:10 ` David Woodhouse
2018-07-12 11:58 ` Christian Borntraeger
2018-07-12 12:04 ` Christian Borntraeger [this message]
2018-07-11 23:37 ` Paul E. McKenney
2018-07-12 2:15 ` Paul E. McKenney
2018-07-12 6:21 ` Christian Borntraeger
2018-07-12 9:52 ` David Woodhouse
2018-07-11 18:31 ` [RFC] Make need_resched() return true when rcu_urgent_qs requested Christian Borntraeger
2018-07-11 20:17 ` Paul E. McKenney
2018-07-11 20:19 ` David Woodhouse
2018-07-11 21:08 ` Paul E. McKenney
2018-07-12 12:00 ` David Woodhouse
2018-07-12 12:53 ` Paul E. McKenney
2018-07-12 16:17 ` Paul E. McKenney
2018-07-16 15:40 ` Paul E. McKenney
2018-07-17 8:19 ` David Woodhouse
2018-07-17 12:56 ` Paul E. McKenney
2018-07-18 15:36 ` Paul E. McKenney
2018-07-18 16:01 ` David Woodhouse
2018-07-18 16:37 ` Paul E. McKenney
2018-07-18 19:41 ` David Woodhouse
2018-07-18 20:17 ` Paul E. McKenney
2018-07-19 0:26 ` Frederic Weisbecker
2018-07-19 6:45 ` Christian Borntraeger
2018-07-19 7:20 ` David Woodhouse
2018-07-19 10:23 ` Christian Borntraeger
2018-07-19 12:55 ` Paul E. McKenney
2018-07-19 13:14 ` Frederic Weisbecker
2018-07-19 13:36 ` David Woodhouse
2018-07-19 17:09 ` Paul E. McKenney
2018-07-23 8:08 ` David Woodhouse
2018-07-23 12:22 ` Paul E. McKenney
2018-07-19 0:32 ` Frederic Weisbecker
2018-07-19 3:11 ` Paul E. McKenney
2018-07-19 6:16 ` David Woodhouse
2018-07-19 13:17 ` Frederic Weisbecker
2018-07-19 13:15 ` Frederic Weisbecker
2018-07-10 9:24 ` Peter Zijlstra
2018-07-10 16:26 ` Paul E. McKenney
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