linux-kernel - Re: [PATCH] tracing/perf: Move rcu_irq_enter/exit

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Message-ID: <20200211132313.GJ2935@paulmck-ThinkPad-P72>
Date:   Tue, 11 Feb 2020 05:23:13 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        rostedt <rostedt@...dmis.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        "Gustavo A. R. Silva" <gustavo@...eddedor.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [PATCH] tracing/perf: Move rcu_irq_enter/exit_irqson() to perf
 trace point hook

On Tue, Feb 11, 2020 at 02:16:37PM +0100, Peter Zijlstra wrote:
> On Tue, Feb 11, 2020 at 05:03:01AM -0800, Paul E. McKenney wrote:
> 
> > > 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.  ;-)
> 
> rcu_read_lock() is exceedingly cheap though, so I never really worried
> about it. But now that RCU includes RCU-sched (again) we can go and
> remove a bunch of them.
> 
> > > 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...
> 
> Well,.. you could go back to simply _always_ watching :-)

The idle loop always was unwatched, even back in DYNIX/ptx.  And watching
the idle loop requires waking up idle CPUs, which makes lots of people
quite unhappy.  But it could be done with something sort of like
synchronize_rcu_tasks(), as long as this didn't need to be used in
production on battery-powered systems.

							Thanx, Paul