From: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: linux-kernel@vger.kernel.org, Ingo Molnar <mingo@kernel.org>,
Andrew Morton <akpm@linux-foundation.org>
Subject: Re: [PATCH 6/5]rcu/tracing: Add rcu_disabled to denote when rcu_irq_enter() will not work
Date: Fri, 7 Apr 2017 09:53:29 -0700 [thread overview]
Message-ID: <20170407165329.GZ1600@linux.vnet.ibm.com> (raw)
In-Reply-To: <20170407124440.7697e14f@gandalf.local.home>
On Fri, Apr 07, 2017 at 12:44:40PM -0400, Steven Rostedt wrote:
> On Fri, 7 Apr 2017 09:42:20 -0700
> "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
>
> > On Fri, Apr 07, 2017 at 12:35:16PM -0400, Steven Rostedt wrote:
> > > From: "Steven Rostedt (VMware)" <rostedt@goodmis.org>
> > >
> > > Tracing uses rcu_irq_enter() as a way to make sure that RCU is watching when
> > > it needs to use rcu_read_lock() and friends. This is because tracing can
> > > happen as RCU is about to enter user space, or about to go idle, and RCU
> > > does not watch for RCU read side critical sections as it makes the
> > > transition.
> > >
> > > There is a small location within the RCU infrastructure that rcu_irq_enter()
> > > itself will not work. If tracing were to occur in that section it will break
> > > if it tries to use rcu_irq_enter().
> > >
> > > Originally, this happens with the stack_tracer, because it will call
> > > save_stack_trace when it encounters stack usage that is greater than any
> > > stack usage it had encountered previously. There was a case where that
> > > happened in the RCU section where rcu_irq_enter() did not work, and lockdep
> > > complained loudly about it. To fix it, stack tracing added a call to be
> > > disabled and RCU would disable stack tracing during the critical section
> > > that rcu_irq_enter() was inoperable. This solution worked, but there are
> > > other cases that use rcu_irq_enter() and it would be a good idea to let RCU
> > > give a way to let others know that rcu_irq_enter() will not work. For
> > > example, in trace events.
> > >
> > > Another helpful aspect of this change is that it also moves the per cpu
> > > variable called in the RCU critical section into a cache locale along with
> > > other RCU per cpu variables used in that same location.
> > >
> > > I'm keeping the stack_trace_disable() code, as that still could be used in
> > > the future by places that really need to disable it. And since it's only a
> > > static inline, it wont take up any kernel text if it is not used.
> > >
> > > Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>
> >
> > Looks better, especially __this_cpu_inc() and __this_cpu_dec().
> >
> > How about rcu_irq_enter_disabled instead of rcu_disabled? We aren't
> > really disabling all of RCU. ;-)
>
> OK, I'll make the update and send a 6.1 (and also a new 7/5)!
So the ex-Borg member was really 9 of 7 rather than 7 of 9, then? ;-)
Thanx, Paul
next prev parent reply other threads:[~2017-04-07 16:53 UTC|newest]
Thread overview: 39+ messages / expand[flat|nested] mbox.gz Atom feed top
2017-04-07 14:01 [PATCH 0/5 v2] tracing: Add usecase of synchronize_rcu_tasks() and stack_tracer_disable() Steven Rostedt
2017-04-07 14:01 ` [PATCH 1/5 v2] ftrace: Add use of synchronize_rcu_tasks() with dynamic trampolines Steven Rostedt
2017-04-07 14:01 ` [PATCH 2/5 v2] tracing: Replace the per_cpu() with this_cpu() in trace_stack.c Steven Rostedt
2017-04-07 14:36 ` Paul E. McKenney
2017-04-07 14:48 ` Steven Rostedt
2017-04-07 15:08 ` Paul E. McKenney
2017-04-07 14:01 ` [PATCH 3/5 v2] tracing: Add stack_tracer_disable/enable() functions Steven Rostedt
2017-04-07 14:22 ` Steven Rostedt
2017-04-07 14:25 ` [PATCH 3/5 v2.1] " Steven Rostedt
2017-04-07 14:01 ` [PATCH 4/5 v2] tracing: Rename trace_active to disable_stack_tracer and inline its modification Steven Rostedt
2017-04-07 14:01 ` [PATCH 5/5 v2] rcu: Fix dyntick-idle tracing Steven Rostedt
2017-04-07 14:40 ` Paul E. McKenney
2017-04-07 14:53 ` Steven Rostedt
2017-04-07 15:09 ` Paul E. McKenney
2017-04-07 15:29 ` Steven Rostedt
2017-04-07 14:43 ` [PATCH 0/5 v2] tracing: Add usecase of synchronize_rcu_tasks() and stack_tracer_disable() Paul E. McKenney
2017-04-07 14:58 ` Steven Rostedt
2017-04-07 15:11 ` Paul E. McKenney
2017-04-07 15:28 ` Steven Rostedt
2017-04-07 16:35 ` [PATCH 6/5]rcu/tracing: Add rcu_disabled to denote when rcu_irq_enter() will not work Steven Rostedt
2017-04-07 16:42 ` Paul E. McKenney
2017-04-07 16:44 ` Steven Rostedt
2017-04-07 16:53 ` Paul E. McKenney [this message]
2017-04-07 17:03 ` [PATCH 6/5 v2] rcu/tracing: " Steven Rostedt
2017-04-07 17:15 ` Paul E. McKenney
2017-04-07 17:06 ` [PATCH 7/5] tracing: Make sure rcu_irq_enter() can work for trace_*_rcuidle() trace events Steven Rostedt
2017-04-07 17:15 ` Paul E. McKenney
2017-04-07 17:19 ` Mathieu Desnoyers
2017-04-07 17:26 ` Steven Rostedt
2017-04-07 17:32 ` Steven Rostedt
2017-04-07 17:49 ` Mathieu Desnoyers
2017-04-07 17:55 ` Steven Rostedt
2017-04-07 18:10 ` [PATCH 7/5 v3] " Steven Rostedt
2017-04-07 18:17 ` Mathieu Desnoyers
2017-04-07 19:41 ` [PATCH 7/5 v4] " Steven Rostedt
2017-04-07 19:43 ` Steven Rostedt
2017-04-10 17:11 ` Mathieu Desnoyers
2017-04-07 17:28 ` [PATCH 7/5] " Steven Rostedt
2017-04-07 17:48 ` [PATCH 7/5 v2] " Steven Rostedt
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