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From: Andrew Morton <>
Cc: Ingo Molnar <>,
	Paul E McKenney <>,
Subject: Re: [mm PATCH 4/6] RCU: preemptible RCU
Date: Tue, 23 Jan 2007 16:32:59 -0800	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <>

On Tue, 16 Jan 2007 00:58:58 +0530
Dipankar Sarma <> wrote:

> This patch implements a new version of RCU which allows its read-side
> critical sections to be preempted.

Why is it selectable if CONFIG_PREEMPT=n?

> It uses a set of counter pairs
> to keep track of the read-side critical sections and flips them
> when all tasks exit read-side critical section. The details
> of this implementation can be found in this paper -
> This patch was developed as a part of the -rt kernel
> development and meant to provide better latencies when
> read-side critical sections of RCU don't disable preemption.

Does it succeed in that attempt?  Thus far you've given no reason for
merging this code..

This is a pile of tricky new core kernel code for us to test, maintain,
understand, debug, etc.  It needs to provide a substantial benefit.  Does

> As a consequence of keeping track of RCU readers, the readers
> have a slight overhead (optimizations in the paper).
> This implementation co-exists with the "classic" RCU
> implementations and can be switched to at compiler.

That's yet another question we need to ask people when their kernel dies,
and yet another deviation between the kernels which we all test, causing
more dilution of testing efforts.  It would be much better if we could
remove classic RCU.  You say this would incur extra cost, but the magnitude
of that cost is not clear.  Please help us make that decision.

  reply	other threads:[~2007-01-24  0:35 UTC|newest]

Thread overview: 15+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2007-01-15 19:19 [mm PATCH] RCU: various patches Dipankar Sarma
2007-01-15 19:21 ` [mm PATCH 1/6] RCU: split classic rcu Dipankar Sarma
2007-01-16 17:49   ` Paul E. McKenney
2007-01-15 19:22 ` [mm PATCH 2/6] RCU: softirq for RCU Dipankar Sarma
2007-01-16 17:49   ` Paul E. McKenney
2007-01-15 19:24 ` [mm PATCH 3/6] RCU: Fix barriers Dipankar Sarma
2007-01-16 17:53   ` Paul E. McKenney
2007-01-15 19:28 ` [mm PATCH 4/6] RCU: preemptible RCU Dipankar Sarma
2007-01-24  0:32   ` Andrew Morton [this message]
2007-01-26 21:00     ` Dipankar Sarma
2007-01-26 21:25       ` Andrew Morton
2007-01-24  0:38   ` Andrew Morton
2007-01-15 19:30 ` [mm PATCH 5/6] RCU: debug trace for RCU Dipankar Sarma
2007-01-15 19:31 ` [mm PATCH 6/6] RCU: trivial fixes Dipankar Sarma
2007-01-16 17:56   ` Paul E. McKenney

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