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

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Message-ID: <561555431.24628.1583883873699.JavaMail.zimbra@efficios.com>
Date:   Tue, 10 Mar 2020 19:44:33 -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 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 ?

Thanks,

Mathieu

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