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

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Date:	Sat, 7 Feb 2015 00:15:14 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Rik van Riel <riel@...hat.com>
Cc:	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, paulmck@...ux.vnet.ibm.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 01:51:56PM -0500, Rik van Riel wrote:
> -----BEGIN PGP SIGNED MESSAGE-----
> Hash: SHA1
> 
> On 02/06/2015 01:23 PM, Frederic Weisbecker wrote:
> > On Fri, Feb 06, 2015 at 01:20:21PM -0500, Rik van Riel wrote: On
> > 02/06/2015 12:22 PM, Frederic Weisbecker wrote:
> >>>> On Thu, Feb 05, 2015 at 03:23:48PM -0500, riel@...hat.com
> >>>> wrote:
> >>>>> From: Rik van Riel <riel@...hat.com>
> >>>>> 
> >>>>> Add the expected ctx_state as a parameter to 
> >>>>> context_tracking_user_enter and
> >>>>> context_tracking_user_exit, allowing the same functions to
> >>>>> not just track kernel <> user space switching, but also
> >>>>> kernel <> guest transitions.
> >>>>> 
> >>>>> Signed-off-by: Rik van Riel <riel@...hat.com>
> >>>> 
> >>>> You should consider using guest_enter() and guest_exit()
> >>>> instead. These are context tracking APIs too but specifically
> >>>> for guest.
> > 
> > What do you mean instead?  KVM already uses those.
> > 
> > I just wanted to avoid duplicating the code...
> > 
> >> I mean you can call rcu_user APIs directly from
> >> guest_enter/exit. You don't really need to call the
> >> context_tracking_user functions since guest_enter/guest_exit
> >> already handle the vtime accounting.
> 
> I would still have to modify exception_enter and exception_exit,
> and with them context_tracking_user_enter and
> context_tracking_user_exit.
> 
> We have to re-enable RCU when an exception happens.
> 
> I suspect exceptions in a guest just trigger VMEXIT, and we
> figure later why the exception happened. However, if we were
> to get an exception during the code where we transition into
> or out of guest mode, we would still need exception_enter
> and exception_exit...

Ah that's a fair point. I didn't think about that. Ok then a real
IN_GUEST mode makes sense. And context_tracking_user_enter/exit() can
be reused as is indeed.

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.

Thanks.
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