linux-kernel - Re: [PATCH 1/3] tracepoints: Do not use call_rcu_sched() before early

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Date:	Sun, 14 Dec 2014 17:29:28 +0000 (UTC)
From:	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	linux-kernel@...r.kernel.org,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ingo Molnar <mingo@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Subject: Re: [PATCH 1/3] tracepoints: Do not use call_rcu_sched() before
 early_initcall()

----- Original Message -----
> From: "Steven Rostedt" <rostedt@...dmis.org>
> To: "Mathieu Desnoyers" <mathieu.desnoyers@...icios.com>
> Cc: linux-kernel@...r.kernel.org, "Linus Torvalds" <torvalds@...ux-foundation.org>, "Ingo Molnar" <mingo@...nel.org>,
> "Andrew Morton" <akpm@...ux-foundation.org>, "Thomas Gleixner" <tglx@...utronix.de>, "Paul E. McKenney"
> <paulmck@...ux.vnet.ibm.com>
> Sent: Sunday, December 14, 2014 12:21:36 PM
> Subject: Re: [PATCH 1/3] tracepoints: Do not use call_rcu_sched() before early_initcall()
> 
> On Sun, 14 Dec 2014 12:14:57 -0500
> Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> > If IRQ setup, or other core pieces of code can use this, then I would
> > agree that rcu should be changed. But until then, I rather not add
> > another conditional to the core RCU code.
> 
> Also, call_rcu*() is called from several hot paths. This location is
> not a hot path, and actually, it is in fact rather slow. That's another
> reason not to add the logic into the RCU code if just adding it to
> tracepoints is sufficient.

Given that your reason for having this RCU-specific logic in tracepoint.c
rather than within call_rcu*() is not slowing down a fast-path, how about
creating a new call_rcu_early() and call_rcu_sched_early() which can be
called in normal operation and at early boot ?

This would allow us to keep the RCU logic within the RCU implementation
rather than strongly coupling it with the tracepoint code.

Thanks,

Mathieu

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