linux-kernel - Re: [GIT PULL] nohz: Enable full dynticks on guest mode

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Date:	Mon, 16 Mar 2015 15:53:09 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Paolo Bonzini <pbonzini@...hat.com>,
	Alexander Graf <agraf@...e.de>,
	Luiz Capitulino <lcapitulino@...hat.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	"Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
	Andy Lutomirski <luto@...capital.net>,
	Will deacon <will.deacon@....com>,
	Rik van Riel <riel@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [GIT PULL] nohz: Enable full dynticks on guest mode


* Frederic Weisbecker <fweisbec@...il.com> wrote:

> Ingo,
> 
> Please pull the nohz/guest branch that can be found at:
> 
> git://git.kernel.org/pub/scm/linux/kernel/git/frederic/linux-dynticks.git
> 	nohz/guest
> 
> This is 4.1 material.
> 
> HEAD: 126a6a542446f1a49b9f3c69237c87df3eb4e6e1
> 
> ---
> Some measurements showed that disabling the tick on the host while the
> guest is running can be interesting on some workloads. Indeed the
> host tick is irrelevant while vcpu runs, it consumes CPU time and cache
> for no good reasons.
> 
> Full dynticks already works in every context but RCU prevents it to
> be effective outside userspace because the CPU needs to take part of
> RCU grace period completion as long as RCU may be used on it, which is
> the case in kernel context. However guest is similar to userspace and idle
> in that we know RCU is unused on such context. Therefore a CPU in
> guest/userspace/idle context can let other CPUs report its own RCU
> quiescent state on its behalf and shut down the tick safely, provided it
> isn't needed for other reasons than RCU. This is called RCU extended
> quiescent state.
> 
> This was implemented for idle and userspace. This patchset now brings it
> for guest through the following steps:
> 
> * Generalize the context tracking APIs to also track guest
> * Rename/sanitize a few CPP symbols accordingly
> * Report guest entry/exit to RCU and define this context area as an RCU
>   extended quiescent state.
> ---
> 
> Thanks,
> 	Frederic
> ---
> 
> Rik van Riel (5):
>       context_tracking: Generalize context tracking APIs to support user and guest
>       context_tracking: Add stub context_tracking_is_enabled
>       context_tracking: Run vtime_user_enter/exit only when state == CONTEXT_USER
>       context_tracking: Export context_tracking_user_enter/exit
>       kvm,rcu,nohz: use RCU extended quiescent state when running KVM guest
> 
> Frederic Weisbecker (2):
>       ppc: Remove unused cpp symbols in kvm headers
>       context_tracking: Rename context symbols to prepare for transition state
> 
> 
>  arch/powerpc/include/asm/kvm_book3s.h  |  4 ---
>  arch/x86/kernel/traps.c                |  2 +-
>  include/linux/context_tracking.h       | 15 +++++++--
>  include/linux/context_tracking_state.h |  9 ++++--
>  include/linux/kvm_host.h               |  3 +-
>  kernel/context_tracking.c              | 59 ++++++++++++++++++++++------------
>  kernel/sched/core.c                    |  2 +-
>  7 files changed, 61 insertions(+), 33 deletions(-)

Pulled into tip:timers/nohz, thanks a lot Frederic!

	Ingo
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