linux-kernel - Re: [PATCH] tracepoint: rcuidle: use rcu_is

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Date:   Tue, 10 Mar 2020 16:52:19 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        rostedt <rostedt@...dmis.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        "Joel Fernandes, Google" <joel@...lfernandes.org>,
        Peter Zijlstra <peterz@...radead.org>,
        fweisbec <fweisbec@...il.com>, Ingo Molnar <mingo@...nel.org>
Subject: Re: [PATCH] tracepoint: rcuidle: use rcu_is_watching() and tree-rcu

On Tue, Mar 10, 2020 at 07:44:33PM -0400, Mathieu Desnoyers wrote:
> ----- On Mar 10, 2020, at 4:53 PM, paulmck paulmck@...nel.org wrote:
> 
> > On Tue, Mar 10, 2020 at 04:20:54PM -0400, Mathieu Desnoyers wrote:
> >> commit e6753f23d961 ("tracepoint: Make rcuidle tracepoint callers use
> >> SRCU") aimed at improving performance of rcuidle tracepoints by using
> >> SRCU rather than temporarily enabling tree-rcu every time.
> >> 
> >> commit 865e63b04e9b ("tracing: Add back in rcu_irq_enter/exit_irqson()
> >> for rcuidle tracepoints") adds back the high-overhead enabling of
> >> tree-rcu because perf expects RCU to be watching when called from
> >> rcuidle tracepoints.
> >> 
> >> It turns out that by using "rcu_is_watching()" and conditionally
> >> calling the high-overhead rcu_irq_enter/exit_irqson(), the original
> >> motivation for using SRCU in the first place disappears.
> > 
> > Adding Alexei on CC for his thoughts, given that these were his
> > benchmarks.  I believe that he also has additional use cases.
> 
> Good point! Sorry I forgot to add Alexei to my CC list for that
> patch.
> 
> > But given the use cases you describe, this seems plausible.  This does
> > mean that tracepoints cannot be attached to the CPU-hotplug code that
> > runs on the incoming/outgoing CPU early/late in that process, though
> > that might be OK.
> 
> Do you mean standard tracepoints or rcuidle tracepoints ?
> 
> Are there any such tracepoints early/late in the cpu hotplug code today ?

I have no idea.  You would know better than I.  However, I would expect
that the same common-code issue that applies to exception-entry/exit
code might also apply to the CPU hotplug code.

							Thanx, Paul