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From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Rik van Riel <riel@redhat.com>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>,
	kvm@vger.kernel.org, linux-kernel@vger.kernel.org,
	mtosatti@redhat.com, mingo@kernel.org, ak@linux.intel.com,
	oleg@redhat.com, masami.hiramatsu.pt@hitachi.com,
	fweisbec@gmail.com, lcapitulino@redhat.com, pbonzini@redhat.com
Subject: Re: [PATCH 4/4] kvm,rcu: use RCU extended quiescent state when running KVM guest
Date: Thu, 5 Feb 2015 11:27:04 -0800	[thread overview]
Message-ID: <20150205192704.GM5370@linux.vnet.ibm.com> (raw)
In-Reply-To: <54D3BE4B.9090301@redhat.com>

On Thu, Feb 05, 2015 at 02:02:35PM -0500, Rik van Riel wrote:
> On 02/05/2015 01:56 PM, Paul E. McKenney wrote:
> 
> > The real danger is doing neither.
> > 
> > On tick_nohz_full_cpu() CPUs, the exit-to-userspace code should invoke
> > rcu_user_enter(), which sets some per-CPU state telling RCU to ignore
> > that CPU, since it cannot possibly do host RCU read-side critical sections
> > while running a guest.
> > 
> > In contrast, a non-tick_nohz_full_cpu() CPU doesn't let RCU
> > know that it is executing in a guest or in userspace.  So the
> > rcu_virt_note_context_switch() does the notification in that case.
> 
> Looking at context_tracking.h, I see the
> function context_tracking_cpu_is_enabled().
> 
> That looks like it should do the right thing
> in this case.

Right you are -- that same check is used to guard the
context_tracking_user_enter() function's call to rcu_user_enter().

Not sure why it open-codes the check rather than invoking
context_tracking_cpu_is_enabled().  Hmmm....  One reason is that
the context_tracking_cpu_is_enabled() function isn't available in
that context, according to my compiler.  ;-)

							Thanx, Paul


  reply	other threads:[~2015-02-05 19:27 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-02-05 16:35 [PATCH 0/4] rcu,nohz,kvm: " riel
2015-02-05 16:35 ` [PATCH 1/4] rcu,nohz: add state parameter to context_tracking_user_enter/exit riel
2015-02-05 16:35 ` [PATCH 2/4] rcu,nohz: run vtime_user_enter/exit only when state == IN_USER riel
2015-02-05 16:35 ` [PATCH 3/4] nohz,kvm: export context_tracking_user_enter/exit riel
2015-02-05 16:35 ` [PATCH 4/4] kvm,rcu: use RCU extended quiescent state when running KVM guest riel
2015-02-05 16:44   ` Christian Borntraeger
2015-02-05 16:52     ` Rik van Riel
2015-02-05 17:50       ` Paul E. McKenney
2015-02-05 18:09         ` Rik van Riel
2015-02-05 18:56           ` Paul E. McKenney
2015-02-05 18:59             ` Rik van Riel
2015-02-05 19:02             ` Rik van Riel
2015-02-05 19:27               ` Paul E. McKenney [this message]
2015-02-05 20:19                 ` Rik van Riel

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