From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Vojtech Pavlik <vojtech@suse.cz>
Cc: Jiri Slaby <jslaby@suse.cz>,
linux-kernel@vger.kernel.org, jirislaby@gmail.com,
Michael Matz <matz@suse.de>, Jiri Kosina <jkosina@suse.cz>,
Steven Rostedt <rostedt@goodmis.org>,
Frederic Weisbecker <fweisbec@gmail.com>,
Ingo Molnar <mingo@redhat.com>,
Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
"Theodore Ts'o" <tytso@mit.edu>,
Dipankar Sarma <dipankar@in.ibm.com>, Tejun Heo <tj@kernel.org>
Subject: Re: [RFC 09/16] kgr: mark task_safe in some kthreads
Date: Wed, 30 Apr 2014 12:07:39 -0700 [thread overview]
Message-ID: <20140430190739.GT8754@linux.vnet.ibm.com> (raw)
In-Reply-To: <20140430183327.GA3926@suse.cz>
On Wed, Apr 30, 2014 at 08:33:27PM +0200, Vojtech Pavlik wrote:
> On Wed, Apr 30, 2014 at 09:55:32AM -0700, Paul E. McKenney wrote:
> > On Wed, Apr 30, 2014 at 04:30:42PM +0200, Jiri Slaby wrote:
> > > Some threads do not use kthread_should_stop. Before we enable a
> > > kthread support in kgr, we must make sure all those mark themselves
> > > safe explicitly.
> >
> > Would it make sense to bury kgr_task_safe() in wait_event_interruptible()
> > and friends? The kgr_task_safe() implementation looks pretty lightweight,
> > so it should not be a performance problem.
>
> For userspace tasks, the kGraft in progress flag is cleared when
> entering or exiting userspace. At that point it is safe to switch the
> task to a post-patch world view.
>
> For kernel threads, it's a bit more complicated: They never exit the
> kernel, they keep executing within the kernel continuously. The
> kgr_task_safe() call is thus inserted at a location within the main loop
> where a 'new loop' begins - where there are no dependencies on results
> of calls of functions from the previous loop.
>
> Hence, putting kgr_task_safe() into every wait_event_interruptible()
> wouldn't work, only a few of them are at that strategic spot where a
> 'new loop' can be indicated to kGraft.
>
> The reason kgr_task_safe() is called from within the condition
> evaluation statement in wait_event_interruptible() in this patch is
> because we want it to be called as soon as a new loop begins - even if
> that loop is empty because the condition to stop waiting has not been
> met.
>
> This also means that kGraft currently cannot patch the main loops of
> kernel threads themselves as the thread of execution never exits them.
>
> Jiří (Slabý) has some ideas about how to do without calling
> kgr_task_safe() from within the kernel thread main loops, but for now,
> the goal is to keep things simple and easy to understand.
OK, from an RCU perspective:
Acked-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> > One reason might this might be a bad idea is that there are calls to
> > wait_event_interruptible() all over the place, which might therefore
> > constrain where grafting could be safely done. That would be fair enough,
> > but does that also imply new constraints on where kthread_should_stop()
> > can be invoked? Any new constraints might not be a big deal given that
> > a very large fraction of the kthreads (and maybe all of them) invoke
> > kthread_should_stop() from their top-level function, but would be good
> > to call out.
>
> > So, what is the story?
>
> kGraft currently assumes that kthread_should_stop() is always in a part
> of the main loop which doesn't carry over effect dependencies from the
> previous iteration. This is currently true for all the uses of
> kthread_should_stop(), but indeed it is an additional constraint for the
> future.
Got it. It would be good to document this. ;-)
Thanx, Paul
> Vojtech
>
next prev parent reply other threads:[~2014-04-30 19:07 UTC|newest]
Thread overview: 59+ messages / expand[flat|nested] mbox.gz Atom feed top
2014-04-30 14:30 [RFC 00/16] kGraft Jiri Slaby
2014-04-30 14:30 ` [RFC 01/16] ftrace: Add function to find fentry of function Jiri Slaby
2014-04-30 14:48 ` Steven Rostedt
2014-04-30 14:58 ` Jiri Slaby
2014-04-30 14:30 ` [RFC 02/16] ftrace: Make ftrace_is_dead available globally Jiri Slaby
2014-04-30 14:30 ` [RFC 03/16] kgr: initial code Jiri Slaby
2014-04-30 14:56 ` Steven Rostedt
2014-04-30 14:57 ` Jiri Slaby
2014-05-01 20:20 ` Andi Kleen
2014-05-01 20:37 ` Jiri Kosina
2014-05-14 9:28 ` Aravinda Prasad
2014-05-14 10:12 ` Jiri Slaby
2014-05-14 10:41 ` Aravinda Prasad
2014-05-14 10:44 ` Jiri Slaby
2014-05-14 11:19 ` Aravinda Prasad
2014-05-20 11:36 ` Jiri Slaby
2014-05-21 18:28 ` Aravinda Prasad
2014-05-26 8:50 ` Jiri Kosina
2014-04-30 14:30 ` [RFC 04/16] kgr: add testing kgraft patch Jiri Slaby
2014-05-06 11:03 ` Pavel Machek
2014-05-12 12:50 ` Jiri Slaby
2014-04-30 14:30 ` [RFC 05/16] kgr: update Kconfig documentation Jiri Slaby
2014-05-03 14:32 ` Randy Dunlap
2014-04-30 14:30 ` [RFC 06/16] kgr: add Documentation Jiri Slaby
2014-05-06 11:03 ` Pavel Machek
2014-05-09 9:31 ` kgr: dealing with optimalizations? (was Re: [RFC 06/16] kgr: add Documentat)ion Pavel Machek
2014-05-09 12:22 ` Michael Matz
2014-04-30 14:30 ` [RFC 07/16] kgr: trigger the first check earlier Jiri Slaby
2014-04-30 14:30 ` [RFC 08/16] kgr: sched.h, introduce kgr_task_safe helper Jiri Slaby
2014-04-30 14:30 ` [RFC 09/16] kgr: mark task_safe in some kthreads Jiri Slaby
2014-04-30 15:49 ` Greg Kroah-Hartman
2014-04-30 16:55 ` Paul E. McKenney
2014-04-30 18:33 ` Vojtech Pavlik
2014-04-30 19:07 ` Paul E. McKenney [this message]
2014-05-01 14:24 ` Tejun Heo
2014-05-01 20:17 ` Jiri Kosina
2014-05-01 21:02 ` Tejun Heo
2014-05-01 21:09 ` Tejun Heo
2014-05-14 14:59 ` Jiri Slaby
2014-05-14 15:15 ` Vojtech Pavlik
2014-05-14 15:30 ` Paul E. McKenney
2014-05-14 16:32 ` Tejun Heo
2014-05-15 3:53 ` Mike Galbraith
2014-05-15 4:06 ` Tejun Heo
2014-05-15 4:46 ` Mike Galbraith
2014-05-15 4:50 ` Tejun Heo
2014-05-15 5:04 ` Mike Galbraith
2014-05-15 5:09 ` Tejun Heo
2014-05-15 5:32 ` Mike Galbraith
2014-05-15 6:05 ` Tejun Heo
2014-05-15 6:32 ` Mike Galbraith
2014-04-30 14:30 ` [RFC 10/16] kgr: kthreads support Jiri Slaby
2014-04-30 14:30 ` [RFC 11/16] kgr: handle irqs Jiri Slaby
2014-04-30 14:30 ` [RFC 12/16] kgr: add tools Jiri Slaby
2014-05-06 11:03 ` Pavel Machek
2014-04-30 14:30 ` [RFC 13/16] kgr: add MAINTAINERS entry Jiri Slaby
2014-04-30 14:30 ` [RFC 14/16] kgr: x86: refuse to build without fentry support Jiri Slaby
2014-04-30 14:30 ` [RFC 15/16] kgr: add procfs interface for per-process 'kgr_in_progress' Jiri Slaby
2014-04-30 14:30 ` [RFC 16/16] kgr: make a per-process 'in progress' flag a single bit Jiri Slaby
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