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

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Date:   Tue, 10 Mar 2020 19:56:17 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     paulmck <paulmck@...nel.org>
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 Mar 10, 2020, at 7:52 PM, paulmck paulmck@...nel.org wrote:

> 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.

I would also expect early/late CPU hotplug states to fall into the same
category of "low-level entry/exit" code which Thomas would like to hide
from instrumentation.

Thanks,

Mathieu

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com