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From: "Paul E. McKenney" <>
To: Steven Rostedt <>
Cc: Mathieu Desnoyers <>,,
	Linus Torvalds <>,
	Ingo Molnar <>,
	Andrew Morton <>,
	Thomas Gleixner <>
Subject: Re: [PATCH 1/3] tracepoints: Do not use call_rcu_sched() before early_initcall()
Date: Sun, 14 Dec 2014 10:13:00 -0800
Message-ID: <> (raw)
In-Reply-To: <20141214124431.48689d64@gandalf.local.home>

On Sun, Dec 14, 2014 at 12:44:31PM -0500, Steven Rostedt wrote:
> On Sun, 14 Dec 2014 17:29:28 +0000 (UTC)
> Mathieu Desnoyers <> wrote:
> > 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 ?
> That's a possibility.
> > 
> > This would allow us to keep the RCU logic within the RCU implementation
> > rather than strongly coupling it with the tracepoint code.
> > 
> It's not that strong of a coupling to RCU. It's more coupled to being
> called really early (which needs special care).
> It just happened that RCU failed for being called that early. Other
> things could possible fail too (if added to the tracepoint logic).
> Maybe I should rename the variable to "tracepoint_earlyboot" instead.

But you do have to call this quite early.  After rcu_init() is invoked,
things should work fine.

> But as RCU is the only thing that failed (so far in my testing), I'll
> think about adding a call_rcu_sched_early(). But then, this does make
> things more complex, and me more nervous about adding it.

I really am nervous about a call_rcu_sched_early() that immediately
invokes the specified callback.  That is just begging for someone to
invoke it while traversing a list in an RCU read-side critical section,
which will break.

My thought is to make the compiler initialize the pieces of RCU that
are needed.  That said, this initialization includes per-CPU variables,
so the question then becomes "when do per-CPU variables get initialized?"

							Thanx, Paul

  reply index

Thread overview: 20+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2014-12-14 16:41 [PATCH 0/3] tracing: Enable tracepoints early and allow printk to use them Steven Rostedt
2014-12-14 16:41 ` [PATCH 1/3] tracepoints: Do not use call_rcu_sched() before early_initcall() Steven Rostedt
2014-12-14 16:53   ` Steven Rostedt
2014-12-14 18:08     ` Paul E. McKenney
2014-12-14 18:15       ` Steven Rostedt
2014-12-14 18:18         ` Paul E. McKenney
2014-12-14 18:25           ` Steven Rostedt
2014-12-14 20:23             ` Paul E. McKenney
2014-12-14 17:05   ` Mathieu Desnoyers
2014-12-14 17:14     ` Steven Rostedt
2014-12-14 17:21       ` Steven Rostedt
2014-12-14 17:29         ` Mathieu Desnoyers
2014-12-14 17:44           ` Steven Rostedt
2014-12-14 18:13             ` Paul E. McKenney [this message]
2014-12-14 16:41 ` [PATCH 2/3] tracing: Move enabling tracepoints to just after mm_init() Steven Rostedt
2014-12-14 18:14   ` Paul E. McKenney
2014-12-14 18:20     ` Steven Rostedt
2014-12-14 20:26       ` Paul E. McKenney
2014-12-14 16:41 ` [PATCH 3/3] tracing: Add tp_printk cmdline to have tracepoints go to printk() Steven Rostedt
2014-12-14 17:07 ` [PATCH 0/3] tracing: Enable tracepoints early and allow printk to use them Steven Rostedt

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