linux-kernel - Re: [Xen-devel] [RFC v3 2/2] x86/xen: allow privcmd hypercalls to be preempted

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Date:	Thu, 22 Jan 2015 13:07:31 -0800
From:	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	"Luis R. Rodriguez" <mcgrof@...e.com>,
	Andrew Cooper <andrew.cooper3@...rix.com>,
	"Luis R. Rodriguez" <mcgrof@...not-panic.com>,
	david.vrabel@...rix.com, konrad.wilk@...cle.com,
	boris.ostrovsky@...cle.com, xen-devel@...ts.xenproject.org,
	kvm@...r.kernel.org, x86@...nel.org, linux-kernel@...r.kernel.org,
	Andy Lutomirski <luto@...capital.net>,
	Ingo Molnar <mingo@...hat.com>,
	Jan Beulich <JBeulich@...e.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Borislav Petkov <bp@...e.de>
Subject: Re: [Xen-devel] [RFC v3 2/2] x86/xen: allow privcmd hypercalls to be
 preempted

On Thu, Jan 22, 2015 at 03:16:57PM -0500, Steven Rostedt wrote:
> 
> [ Added Paul McKenney ]
> 
> On Thu, 22 Jan 2015 19:39:13 +0100
> "Luis R. Rodriguez" <mcgrof@...e.com> wrote:
> 
> > > Why not make this a tracepoint? Then you can enable it only when you
> > > want to. As tracepoints are also hooks, you could add you own code that
> > > hooks to it and does a printk as well. The advantage of doing it via a
> > > tracepoint is that you can turn it on and off regardless of what the
> > > loglevel is set at.
> > 
> > This uses NOKPROBE_SYMBOL and notrace since based on Andy's advice
> > we are not confident that tracing and kprobes are safe to use in what
> > might be an extended RCU quiescent state (i.e. where we're outside
> > irq_enter and irq_exit).
> 
> We have trace_*_rcuidle() for such cases.
> 
> That is, you create the tracepoint just the same, and instead of having
> trace_foo(), if you are in a known area that is outside of rcu viewing,
> you use trace_foo_rcuidle() and it will tell RCU "hey, there's something
> here that may need RCU, so look at me!"

What Steve said!

Also, there is an rcu_is_watching() API member that can tell you
whether or not RCU is paying attention at a given point.  Or test with
CONFIG_PROVE_RCU, in which case lockdep will yell at you if you should
have used the _rcuidle() form of the tracing hooks.  ;-)

							Thanx, Paul

> Also, please remove the "notrace", because function tracing goes an
> extra step to not require RCU being visible. The only thing you get
> with notrace is not being able to trace an otherwise traceable function.
> 
> -- Steve
> 
> > 
> > > That is, if there is any practical use for that message. Tracing just
> > > sched_switch will give you the same info.
> > 
> > IMHO it may be more useful if we knew exactly what hypercalls were
> > being preempted but perhaps all that can be left as a secondary
> > exercise and for now I'll just nuke the print.
> 

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