linux-kernel - Re: [PATCH v2 0/5] rcu,nohz,kvm: use RCU extended quiescent state when running KVM guest

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Date:	Fri, 6 Feb 2015 19:20:58 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	riel@...hat.com, kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	mtosatti@...hat.com, mingo@...nel.orgm, ak@...ux.intel.com,
	oleg@...hat.com, masami.hiramatsu.pt@...achi.com,
	paulmck@...ux.vnet.ibm.com, lcapitulino@...hat.com,
	pbonzini@...hat.com, linux-s390 <linux-s390@...r.kernel.org>
Subject: Re: [PATCH v2 0/5] rcu,nohz,kvm: use RCU extended quiescent state
 when running KVM guest

On Fri, Feb 06, 2015 at 04:00:27PM +0100, Christian Borntraeger wrote:
> Am 05.02.2015 um 21:23 schrieb riel@...hat.com:
> > When running a KVM guest on a system with NOHZ_FULL enabled, and the
> > KVM guest running with idle=poll mode, we still get wakeups of the
> > rcuos/N threads.
> > 
> > This problem has already been solved for user space by telling the
> > RCU subsystem that the CPU is in an extended quiescent state while
> > running user space code.
> > 
> > This patch series extends that code a little bit to make it usable
> > to track KVM guest space, too.
> > 
> > I tested the code by booting a KVM guest with idle=poll, on a system
> > with NOHZ_FULL enabled on most CPUs, and a VCPU thread bound to a
> > CPU. In a 10 second interval, rcuos/N threads on other CPUs got woken
> > up several times, while the rcuos thread on the CPU running the bound
> > and alwasy running VCPU thread never got woken up once.
> > 
> > Thanks to Christian Borntraeger and Paul McKenney for reviewing the
> > first version of this patch series, and helping optimize patch 4/5.
> 
> I gave it a quick run on s390/kvm and everything still seem to be 
> running fine. A also I like the idea of this patch set.
> 
> We have seen several cases were the fact that we are in guest context
> a full tick for cpu bound guests (10ms on s390) caused significant
> latencies for host synchronize-rcu heavy workload - e.g. getting rid
> of macvtap devices on guest shutdown, adding hundreds of irq routes
> for many guest devices....
> 
> s390 has no context tracking infrastructure yet (no nohz_full), but
> this series looks like that the current case (nohz_idle) still works.
> With this in place, having hohz==full on s390 now even makes more
> sense, as KVM hosts with cpu bound guests should have get much quicker
> rcu response times when most host CPUs are in an extended quiescant
> state.

Sure, if you need any help for context tracking, don't hesitate to ask,
it can be a bit tricky to implement sometimes. Perhaps x86 isn't the
best example because it does quite some weird dances to minimize fast path
overhead. ARM is perhaps clearer.

> 
> Christian
> 
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