linux-kernel - Re: [PATCH 7/7 v3] tracing: Make sure rcu_irq_enter() can work for trace_*

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Message-Id: <20170410183624.GA1600@linux.vnet.ibm.com>
Date:   Mon, 10 Apr 2017 11:36:24 -0700
From:   "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: [PATCH 7/7 v3] tracing: Make sure rcu_irq_enter() can work for
 trace_*_rcuidle() trace events

On Mon, Apr 10, 2017 at 02:32:40PM -0400, Steven Rostedt wrote:
> On Mon, 10 Apr 2017 11:29:01 -0700
> "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com> wrote:
> 
> > On Mon, Apr 10, 2017 at 02:10:12PM -0400, Steven Rostedt wrote:
> > > From: "Steven Rostedt (VMware)" <rostedt@...dmis.org>
> > > 
> > > Stack tracing discovered that there's a small location inside the RCU
> > > infrastructure where calling rcu_irq_enter() does not work. As trace events
> > > use rcu_irq_enter() it must make sure that it is functionable. A check
> > > against rcu_irq_enter_disabled() is added with a WARN_ON_ONCE() as no trace
> > > event should ever be used in that part of RCU. If the warning is triggered,
> > > then the trace event is ignored.
> > > 
> > > Restructure the __DO_TRACE() a bit to get rid of the prercu and postrcu,
> > > and just have an rcucheck that does the work from within the _DO_TRACE()
> > > macro. gcc optimization will compile out the rcucheck=0 case.
> > > 
> > > Link: http://lkml.kernel.org/r/20170405093207.404f8deb@gandalf.local.home
> > > 
> > > Acked-by: Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
> > > Signed-off-by: Steven Rostedt (VMware) <rostedt@...dmis.org>  
> > 
> > Acked-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > 
> > As an aside, it looks like the rcu_irq_enter_disabled() settings
> > in the RCU idle-entry code could be placed under CONFIG_PROVE_RCU=y
> > should my idle-entry-overhead concerns prove to be well-founded.
> > The errors would be caught during testing, but no production-side
> > overhead.
> 
> Even with the underscored __this_cpu_*() calls?

That change did not completely address my concerns, but it did make me
willing to take a wait-and-see attitude.  ;-)

> > Again, I am not necessarily agitating for this change now, just getting
> > this possibility on the record.  ;-)
> 
> That would be easy to add :-) But we can do that at a later time.

Agreed, if it is actually needed.  The usual suspects will no doubt be
quick to let us know if it is needed.

							Thanx, Pau