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Date: Tue, 5 May 2015 12:53:46 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
Cc: Rik van Riel <riel@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>,
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        Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
Subject: Re: question about RCU dynticks_nesting
Message-ID: <20150505105346.GJ21418@twins.programming.kicks-ass.net>
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On Mon, May 04, 2015 at 12:39:23PM -0700, Paul E. McKenney wrote:
> But in non-preemptible RCU, we have PREEMPT=n, so there is no preempt
> counter in production kernels.  Even if there was, we have to sample this
> on other CPUs, so the overhead of preempt_disable() and preempt_enable()
> would be where kernel entry/exit is, so I expect that this would be a
> net loss in overall performance.

We unconditionally have the preempt_count, its just not used much for
PREEMPT_COUNT=n kernels.
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