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Date:   Tue, 11 Feb 2020 05:03:01 -0800
From:   "Paul E. McKenney" <paulmck@kernel.org>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
        rostedt <rostedt@goodmis.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Ingo Molnar <mingo@kernel.org>,
        "Joel Fernandes, Google" <joel@joelfernandes.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        "Gustavo A. R. Silva" <gustavo@embeddedor.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Josh Triplett <josh@joshtriplett.org>,
        Lai Jiangshan <jiangshanlai@gmail.com>
Subject: Re: [PATCH] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf
 trace point hook
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On Tue, Feb 11, 2020 at 01:00:15PM +0100, Peter Zijlstra wrote:
> On Mon, Feb 10, 2020 at 07:30:32PM -0500, Mathieu Desnoyers wrote:
> 
> > > because perf only uses rcu to synchronize trace points.
> > 
> > That last part seems inaccurate. The tracepoint synchronization is two-fold:
> > one part is internal to tracepoint.c (see rcu_free_old_probes()), and the other
> > is only needed if the probes are within modules which can be unloaded (see
> > tracepoint_synchronize_unregister()). AFAIK, perf never implements probe callbacks
> > within modules, so the latter is not needed by perf.
> > 
> > The culprit of the problem here is that perf issues "rcu_read_lock()" and
> > "rcu_read_unlock()" within the probe callbacks it registers to the tracepoints,
> > including the rcuidle ones. Those require that RCU is "watching", which is
> > triggering the regression when we remove the calls to rcu_irq_enter/exit_irqson()
> > from the rcuidle tracepoint instrumentation sites.
> 
> It is not the fact that perf issues rcu_read_lock() that is the problem.
> As we established yesterday, I can probably remove most rcu_read_lock()
> calls from perf today (yay RCU flavour unification).

Glad some aspect of this unification is actually helping you.  ;-)

> The problem is that the core perf code uses RCU managed data; and we
> need an existence guarantee for it. It would be BAD (TM) if the
> ring-buffer we're writing data to were to suddenly dissapear under our
> feet etc..
> 
> > Which brings a question about handling of NMIs: in the proposed patch, if
> > a NMI nests over rcuidle context, AFAIU it will be in a state
> > !rcu_is_watching() && in_nmi(), which is handled by this patch with a simple
> > "return", meaning important NMIs doing hardware event sampling can be
> > completely lost.
> > 
> > Considering that we cannot use rcu_irq_enter/exit_irqson() from NMI context,
> > is it at all valid to use rcu_read_lock/unlock() as perf does from NMI handlers,
> 
> Again, rcu_read_lock() itself really isn't the problem. But we need
> NMIs, just like regular interrupts, to imply rcu_read_lock(). That is,
> any observable (RCU managed) pointer must stay valid during the NMI/IRQ
> execution.
> 
> > considering that those can be nested on top of rcuidle context ?
> 
> As per nmi_enter() calling rcu_nmi_enter() I've always assumed that NMIs
> are fully covered by RCU.
> 
> If this isn't so, RCU it terminally broken :-)

All RCU can do is respond to calls to rcu_nmi_enter() and rcu_nmi_exit().
It has not yet figured out how to force people to add these calls where
they are needed.  ;-)

But yes, it would be very nice if architectures arranged things so
that all NMI handlers were visible to RCU.  And we no longer have
half-interrupts, so maybe there is hope...

							Thanx, Paul