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From: Christian Borntraeger <borntraeger@de.ibm.com>
To: riel@redhat.com, kvm@vger.kernel.org
Cc: linux-kernel@vger.kernel.org, mtosatti@redhat.com,
	mingo@kernel.orgm, ak@linux.intel.com, oleg@redhat.com,
	masami.hiramatsu.pt@hitachi.com, fweisbec@gmail.com,
	paulmck@linux.vnet.ibm.com, lcapitulino@redhat.com,
	pbonzini@redhat.com, linux-s390 <linux-s390@vger.kernel.org>
Subject: Re: [PATCH v2 0/5] rcu,nohz,kvm: use RCU extended quiescent state when running KVM guest
Date: Fri, 06 Feb 2015 16:00:27 +0100	[thread overview]
Message-ID: <54D4D70B.2040300@de.ibm.com> (raw)
In-Reply-To: <1423167832-17609-1-git-send-email-riel@redhat.com>

Am 05.02.2015 um 21:23 schrieb riel@redhat.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.

Christian


  parent reply	other threads:[~2015-02-06 15:00 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-02-05 20:23 riel
2015-02-05 20:23 ` [PATCH 1/5] rcu,nohz: add state parameter to context_tracking_user_enter/exit riel
2015-02-05 23:55   ` Paul E. McKenney
2015-02-06 10:15     ` Paolo Bonzini
2015-02-06 13:41       ` Paul E. McKenney
2015-02-06 17:22   ` Frederic Weisbecker
2015-02-06 18:20     ` Rik van Riel
2015-02-06 18:23       ` Frederic Weisbecker
2015-02-06 18:51         ` Rik van Riel
2015-02-06 23:15           ` Frederic Weisbecker
2015-02-07  3:53             ` Rik van Riel
2015-02-07  6:34               ` Paul E. McKenney
2015-02-07  7:14                 ` Paul E. McKenney
2015-02-07  8:30                   ` Frederic Weisbecker
2015-02-07 11:29                     ` Rik van Riel
2015-02-07 20:06                     ` Paul E. McKenney
2015-02-09 15:42                       ` Rik van Riel
2015-02-05 20:23 ` [PATCH 2/5] rcu,nohz: run vtime_user_enter/exit only when state == IN_USER riel
2015-02-05 20:23 ` [PATCH 3/5] nohz,kvm: export context_tracking_user_enter/exit riel
2015-02-05 23:55   ` Paul E. McKenney
2015-02-05 20:23 ` [PATCH 4/5] kvm,rcu,nohz: use RCU extended quiescent state when running KVM guest riel
2015-02-05 23:56   ` Paul E. McKenney
2015-02-06 18:01   ` Frederic Weisbecker
2015-02-06 23:24   ` Frederic Weisbecker
2015-02-05 20:23 ` [PATCH 5/5] nohz: add stub context_tracking_is_enabled riel
2015-02-05 23:56   ` Paul E. McKenney
2015-02-06 13:46 ` [PATCH v2 0/5] rcu,nohz,kvm: use RCU extended quiescent state when running KVM guest Frederic Weisbecker
2015-02-06 13:50   ` Paolo Bonzini
2015-02-06 16:19     ` Paul E. McKenney
2015-02-06 18:09     ` Frederic Weisbecker
2015-02-06 14:56   ` Rik van Riel
2015-02-06 18:05     ` Frederic Weisbecker
2015-02-06 15:00 ` Christian Borntraeger [this message]
2015-02-06 18:20   ` Frederic Weisbecker

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