LKML Archive on lore.kernel.org
help / color / mirror / Atom feed
From: Frederic Weisbecker <fweisbec@gmail.com>
To: Rik van Riel <riel@redhat.com>
Cc: kvm@vger.kernel.org, borntraeger@de.ibm.com,
	linux-kernel@vger.kernel.org, mtosatti@redhat.com,
	mingo@kernel.orgm, ak@linux.intel.com, oleg@redhat.com,
	masami.hiramatsu.pt@hitachi.com, paulmck@linux.vnet.ibm.com,
	lcapitulino@redhat.com, pbonzini@redhat.com
Subject: Re: [PATCH 1/5] rcu,nohz: add state parameter to context_tracking_user_enter/exit
Date: Sat, 7 Feb 2015 00:15:14 +0100	[thread overview]
Message-ID: <20150206231511.GA18934@lerouge> (raw)
In-Reply-To: <54D50D4C.8010505@redhat.com>

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@redhat.com
> >>>> wrote:
> >>>>> From: Rik van Riel <riel@redhat.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@redhat.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.

  reply	other threads:[~2015-02-06 23:15 UTC|newest]

Thread overview: 34+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2015-02-05 20:23 [PATCH v2 0/5] rcu,nohz,kvm: use RCU extended quiescent state when running KVM guest 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 [this message]
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
2015-02-06 18:20   ` Frederic Weisbecker

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20150206231511.GA18934@lerouge \
    --to=fweisbec@gmail.com \
    --cc=ak@linux.intel.com \
    --cc=borntraeger@de.ibm.com \
    --cc=kvm@vger.kernel.org \
    --cc=lcapitulino@redhat.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=masami.hiramatsu.pt@hitachi.com \
    --cc=mingo@kernel.orgm \
    --cc=mtosatti@redhat.com \
    --cc=oleg@redhat.com \
    --cc=paulmck@linux.vnet.ibm.com \
    --cc=pbonzini@redhat.com \
    --cc=riel@redhat.com \
    --subject='Re: [PATCH 1/5] rcu,nohz: add state parameter to context_tracking_user_enter/exit' \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).