linux-kernel - Re: [PATCH 1/5] rcu,nohz: add state parameter to context_tracking_user

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Message-ID: <20150207083036.GA24538@lerouge>
Date:	Sat, 7 Feb 2015 09:30:41 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:	Rik van Riel <riel@...hat.com>, kvm@...r.kernel.org,
	borntraeger@...ibm.com, linux-kernel@...r.kernel.org,
	mtosatti@...hat.com, mingo@...nel.orgm, ak@...ux.intel.com,
	oleg@...hat.com, masami.hiramatsu.pt@...achi.com,
	lcapitulino@...hat.com, pbonzini@...hat.com
Subject: Re: [PATCH 1/5] rcu,nohz: add state parameter to
 context_tracking_user_enter/exit

On Fri, Feb 06, 2015 at 11:14:53PM -0800, Paul E. McKenney wrote:
> On Fri, Feb 06, 2015 at 10:34:21PM -0800, Paul E. McKenney wrote:
> > On Fri, Feb 06, 2015 at 10:53:34PM -0500, Rik van Riel wrote:
> > > -----BEGIN PGP SIGNED MESSAGE-----
> > > Hash: SHA1
> > > 
> > > On 02/06/2015 06:15 PM, Frederic Weisbecker wrote:
> > > 
> > > > Just a few things then:
> > > > 
> > > > 1) In this case rename context_tracking_user_enter/exit() to 
> > > > context_tracking_enter() and context_tracking_exit(), since it's
> > > > not anymore about user only but about any generic context.
> > > > 
> > > > 2) We have the "WARN_ON_ONCE(!current->mm);" condition that is a
> > > > debug check specific to userspace transitions because kernel
> > > > threads aren't expected to resume to userspace. Can we also expect
> > > > that we never switch to/from guest from a kernel thread? AFAICS
> > > > this happens from an ioctl (thus user task) in x86 for kvm. But I
> > > > only know this case.
> > > > 
> > > > 3) You might want to update a few comments that assume we only deal
> > > > with userspace transitions.
> > > > 
> > > > 4) trace_user_enter/exit() should stay user-transitions specific.
> > > 
> > > Paul, would you like me to send follow-up patches with the cleanups
> > > suggested by Frederic, or would you prefer me to send a new series
> > > with the cleanups integrated?
> > 
> > I would prefer a new series, in order to prevent possible future
> > confusion.
> 
> Of course, if Frederic would rather push them himself, I am fine with
> that.  And in that case, you should ask him for his preferences, which
> just might differ from mine.  ;-)

I prefer a new series too. Now whether you or me take the patches, I don't mind
either way :-)

Also I wonder how this feature is going to be enabled. Will it be enabled on
full dynticks or should it be a seperate feature depending on full dynticks?
Or even just CONFIG_RCU_USER_EQS? Because I'm still unclear about how and what
this is used, if it involves full dynticks or only RCU extended quiescent states.

Thanks.
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